RSS Feed for the unit Environment: Following the flows

Introduction

The Open University — Wed, 04 May 2011 11:18:29 GMT

The scientific theory of plate tectonics suggests that at least some of these Arctic lands were once tropical. Since then the continents have moved and ice has changed the landscape. This unit will concentrate on evidence from the last 800,000 years using information collected from ice cores from Greenland and Antarctica, and will use this evidence to discuss current and possible future climate. The cores show that there have been nine periods in the recent past when large areas of the Earth were covered by ice. During the last 10,000 years – called the Holocene – there has been an unusually stable climate compared with the rest of the record, and the Holocene encompasses the entire development of human civilisation.

The Arctic, like any region, has always undergone climate change but there is evidence, for example in the decreasing sea ice cover, that suggests that the changes are happening faster. I intend to show how evidence from the ice cores suggests that flows of chemicals and energy dominate natural systems and cause these changes. I will discuss flows of water, heat and even pollution around the planet and show how, through positive feedback processes, the flows that are affecting the Arctic are already changing the whole planet. There will be further changes, with an impact on us all. The Arctic is often considered a victim of climate change – and it certainly is – but I hope to show that Sheila Watt-Cloutier was also right when she described the Arctic as a planetary barometer.

To discover the evidence that the Earth is dominated by flows I will start with that most famous Arctic animal of all – the polar bear.

This unit is an adapted extract from the Open University course Environment: journeys through a changing world (U116).

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Learning outcomes

The Open University — Wed, 04 May 2011 11:18:29 GMT

By the end of this unit you should be able to:

appreciate how chemical processes in the rest of the world affect the Arctic environment and the species inhabiting it;
recognise the physical processes that determine atmosphere and oceanic flows in the Arctic;
appreciate the scientific research process and the use of scientific evidence;
use quantitative scientific evidence to examine the link between atmospheric carbon dioxide levels and global temperatures;
recognise how scientific data is used to predict global climate change;
recognise the role (and limitations?) of scientific data in attempting to predict global climatic change.

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1 A climate change icon

The Open University — Wed, 04 May 2011 11:18:29 GMT

The polar bear has become an international climate change icon. But how much is known about this bear, its habitat and life? This unit will talk about the role of language, but by way of introduction how about the name of this bear? To me it is the polar bear; to a German it is an Eisbär (ice bear) and to a French person it is an ours blanc (white bear). In these three examples the bear is referred to as polar, white, or an ice bear – eminently sensible. The Latin name for this bear is Ursus maritimus, made up of Ursus, which is bear, and maritimus, which means ‘of the sea’. To us it is polar bear, but the Latin name says sea bear. The reason for this is given by the writer Barry Lopez:

The polar bear is a creature of arctic edges: he hunts the ice margins, the surface of the water, and the continental shore. … He dives to the ocean floor for mussels and kelp, and soundlessly breaks the water's glassy surface on his return to study a sleeping seal. Twenty miles from shore he treads water amid schooling fish. In winter, while the grizzly hibernates, the polar bear is out on the sea ice, hunting. In summer his tracks turn up a hundred miles inland, where he has feasted on crowberries and blueberries.
(Lopez, 1986)

Figure 1 shows the movements of one satellite-tracked female between 1994 and 1998 (Wiig, 2003). Over four years this bear travelled more than 14 500 km, covering an area of almost 500 000 square kilometres in search of its main prey species – the seal. The state of the seas and ice of the region will therefore directly affect the bears, but it turns out that there are also effects from much further afield.

Figure 1 (a) The travels of a female polar bear tracked over 1415 days between 1994 and 1998; also shown are the minimum (9 September 1996) and maximum (7 January 1997) ice extent (Source: adapted from Wiig et al., 2003) (b) A drugged polar bear with a satellite collar attached to its neck. Although it looks a tight fit the collar is carefully designed to expand as the bear grows, whilst not falling off as it moves about (Source: http://alaska.usgs.gov/science/biology/polar_bears/distribution.html, accessed November 2008)

Long description

Attaching a satellite tracking device to polar bears is not easy and they have to be drugged. This gives an opportunity for them to be weighed, measured, tagged, and have various samples such as hair, fat and teeth removed for later chemical analysis. The amount of body fat on a bear indicates whether it has been eating well or is starving. But a chemical analysis of this body fat gave a surprise: polar bears have measurable amounts of a family of chemicals called polybrominated diphenyl ethers (PBDEs) in their fat (Figure 2).

Figure 2 The concentration of PBDE in the fat of polar bears and the ringed seal (one of their main prey species) at different sites across the Arctic

Long description

The same discovery was made in Arctic ringed seals. PBDEs are a group of synthetic chemicals developed over the twentieth century as fire retardants. Fabrics and furniture are impregnated with them with the sole aim of slowing the rate at which they burn, and they have been very successful. However, once created, PBDEs are very difficult to destroy and will not break down into their elements over time. For this reason they are considered a persistent organic pollutant (POP) (Box 1).

Box 1 Pollution and bioaccumulation

The term ‘pollutant’ came up several times in Block 1, and it is a very wide-ranging term. When the introduction or action of something into our environment causes harm it is considered a pollutant. This could be a harmful chemical such as smoke from a chimney, or it could be a more subtle and transient effect such as floodlights at an evening football match preventing stargazing.

There are many examples of how society has responded to pollution, such as the removal of lead in petrol which affected human health, or the banning of chlorofluorocarbons (CFCs), which damaged the ozone layer. In both these cases, when the pollution source was removed the levels of them in the environment reduced and consequently so have the effects – albeit with a time delay. By definition, persistent pollutants (POPs) such as PBDEs do not break down, so continued introduction of even minute levels of them into an environment leads to accumulation and perhaps magnification of potential harm. For example, at a landfill site the PBDE level is likely to increase with time. Animals around that landfill may ingest PBDEs directly, but the level that accumulates in their tissues may be so small that it does not cause problems to any particular animal. However, a predator such as a cat might eat dozens of rats that live around the landfill, so it would receive the combined dose that each of these rats had within it. If this dose were subsequently absorbed by the cat then the resulting accumulated level could be significantly more harmful.

This concentration of pollutants at higher levels in the food chain is called bioaccumulation, and the result is that higher predators can be poisoned and suffer harm while animals at lower levels in the food chain are apparently unaffected.

In the early 1980s scientists began to detect POPs in the tissues of fish and shellfish close to populated areas. Concentrations were then detected in human breast milk, and the levels were shown to be increasing with time – perhaps through direct exposure to PBDEs or through bioaccumulation. The scale in Figure 2 is given in nanograms per gram. So, in every gram of the sample of bear and seal fat in East Greenland there is about 50 nanograms of PBDE. This is 0.000000005 grams of PBDE in every gram of sample. This may seem an extremely small amount, but PBDEs are potentially very toxic to liver and thyroid function and have been shown to hinder development of nervous tissue in mammals. For this reason the European Union banned several of them in 2004 and then more in 2008. The migration of PBDEs into humans and shellfish was explained by proximity to where they were used. While it is relatively simple to see how PBDEs can get into subjects close to their source, it is not so clear how they end up in polar bears and ringed seals in the Arctic.

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2 The atmospheric and ocean flows

The Open University — Wed, 04 May 2011 11:18:29 GMT

We now know that PBDEs end up in the Arctic through their physical transport by the winds, the ocean and the rivers of the world. All three mechanisms are important, but the most rapid carrier is the wind. The basic principle of global atmospheric circulation is simple: warm air rises and cold air sinks. The warming effect of the Sun is much greater at the equator than at higher latitudes and so the air is much warmer and it rises. At high latitudes the air cools and it sinks. This drives a horizontal wind. To help picture this, imagine a room with a radiator on one wall, and at the other end of the room an open fridge (Figure 3).

Figure 3 (a) A room with a radiator on one wall and an open fridge on the other will cause air to rise and sink at opposite ends; (b) horizontal winds are set up to replace this ascending and descending air

Long description

The radiator heats up the air around it, and the air rises in what is called a convection current all the way the to the ceiling and starts to spread. At the other end the fridge is doing the opposite and cooling the air, which sinks and spreads across the floor. To replace the air that has risen, the air beneath the radiator is pulled upwards and then heated and rises while the opposite is happening at the other end of the room.

At the most basic level, on Earth the same process is happening, with warm air rising from lower latitudes and sinking at higher, colder latitudes, so high-level winds tend to blow from the hotter regions to the colder. This general pattern is modified by the rotation of the Earth, which deflects the wind flow away from the apparently direct path.

These wind flows are further complicated by the distribution of continents across the globe and their mountain ranges. Winds are modified as they move around and over mountain ranges. They are also affected as they travel over land and sea surfaces where the air is warmed to different extents. This is because of two additional processes that affect the heating and cooling of the air. Land and sea surfaces reflect different amounts of solar energy falling on them and materials such as rocks and water need different amounts of heat to warm them up.

When solar energy reaches the Earth's surface a proportion of it is reflected straight back out into space and only the fraction which is not reflected heats the terrain. Different materials have a different albedo and so reflect a different amount of solar energy. If you put your hand on a black car on a warm sunny day, and then on a white car, you will notice that the black car feels warmer. This is because it reflects less energy so it heats up more. The black car has a lower albedo than the white car. Table 1 shows the albedo of some typical surfaces. For example, the surface of the ocean has an albedo of 3%, which means that 100% − 3% = 97% or almost all of the incoming energy from the sun actually heats the water. Fresh snow, on the other hand, reflects away most solar energy, a property that has important consequences for the climate of the Arctic.

Table 1 The albedo of typical features on Earth

Surface	Albedo
Ocean surface	3%
Conifer forest in summer	9%
Grassy fields	25%
Sea ice	40%
Desert sand	40%
Fresh snow	80–90%

SAQ 1 The importance of albedo

If the same amount of energy reaches a desert and sea ice on a frozen sea, what proportion of the energy is available to heat up the material? If snow then falls to cover the sea ice, what will be the amount of energy available to heat up the ice?

Answer

If the same amount of energy reaches a desert and a frozen sea, the amount of energy available to heat up the material will be the same, because Table 1 shows us that the two substances have the same albedo − 40%. In both cases the amount of energy available to heat up the material will be:

So 60% of the incoming energy will be available to heat up the material.

If snow falls on the sea ice, then its albedo will increase from 40% to 80–90% and so the amount of energy available to heat up the ice will be:

Only 10% of the incoming energy is now available to heat up the ice, and almost all of the incident energy is reflected away. Clearly the albedo is extremely important for the polar regions.

When energy reaches the surface of an object the amount the object heats up is determined by its specific heat capacity. This is a measure of how much energy it takes to raise the temperature of 1 kg of a particular substance by 1 °C. A lower specific heat capacity means that it takes less energy to heat up something, and vice versa. Although the term may be unfamiliar, the concept most likely is not.

Activity 1 The effect of specific heat capacity

On a very hot sunny day on a table outside in the sun there is a glass containing 1 kg of water (i.e. 1 litre), a 1 kg piece of cork and a 1 kg piece of iron. Ignore albedo effects and assume that all three items absorb the same amount of energy from the sun. How hot will each become after 1 hour? (Ignore all sources of heat except that directly received from the Sun.)

Discussion

You probably recognised that the 1 kg of iron would be the hottest. It does not take very much heat energy to change the temperature of the iron because it has a low specific heat capacity. The other two items are harder to place, but the cork will be cooler than the iron and finally the water, which has the highest specific heat capacity, will be the coolest item on the table.

Water has an extremely high specific heat capacity and it takes a vast amount of energy to heat it. This is why virtually all car engines use water in their cooling systems. Taking into account the combined effects of the albedo and specific heat capacity, even two adjacent areas, such as a beach and the sea lapping on it, will heat up by different amounts on a sunny day.

Areas with lower heat capacities and lower albedo heat up more. This heat is transferred to the air above so in these areas it will rise at a faster rate, and in cooler areas the air sinks. The rising and sinking air drives horizontal winds much as in Figure 3, although on a planetary scale.

Sea ice cover is also constantly moving. It is pushed by the winds and ocean currents and drifts in the pattern shown in Figure 4. The Arctic is defined as being north of the treeline, so how do tree trunks get there? They are mostly Siberian fir trees (Abies sibirica), a native of the great forests of northern Russia. Tree trunks are carried out to sea in summer by rivers such as the Lena, the Ob, the Yenisei and Volga. Then they are frozen into the sea ice and travel in two ocean currents called the Transpolar Drift Stream and the Beaufort Gyre. Eventually they reach the shores of Svalbard and Greenland. Dating of these tree trunks using carbon dating shows that some are several thousand years old.

Figure 4 The mean ice drift across the Arctic Ocean. The ice is trapped in two major circulation features, the Beaufort Gyre and the Transpolar Drift Stream. White arrows show the general movement of the ocean currents; blue arrows show the general drift of the sea ice.

Long description

Box 2 Nansen and the voyage of the Fram

Wood on the shores of Svalbard and East Greenland caused confusion to the first explorers. But when wreckage from a ship called the Jeanette was found on the coast of East Greenland, the best environmental scientist of the age, the Norwegian Fridtjof Nansen (Figure 5), had a eureka moment. Nansen knew that the Jeanette had sunk off Alaska on the other side of the Arctic Ocean and deduced that the wreckage must have been carried across the frozen sea by the sea ice. He decided to try to use the ice drift to reach the North Pole and study the Arctic environment on the journey. He had the ship Fram (Norwegian for ‘forward’) built (Figure 5). The ship had a round hull so that it would not get crushed like the Jeanette, and Nansen left Norway in 1893 for the Arctic and the North Pole. It was over three years before he and his colleagues returned.

Figure 5 (a) Fridtjof Nansen; (b) his ship the Fram frozen into the Arctic Ocean and drifting in the Transpolar Drift Stream

Long description

They followed the Russian coast and the Fram froze into the sea ice off Siberia. As they drifted northwards Nansen realised that the Fram was going to miss the pole so he and Hjalmar Johansen left the ship to make for the pole on foot. This was incredible. They knew the ship was drifting and they must have been certain that they would never find her again. The Fram survived the Arctic drift and reached Svalbard in the summer of 1896. Nansen and Johansen turned back just north of 86° N, having reached the highest latitude then attained. After an epic journey across the sea ice they endured the winter of 1895 on the island of Franz Josef Land (Zemlya Frantsa Iosifa) and then caught a ship back to Norway. After the long separation they arrived three days before the Fram in August 1896 (Figure 6).

Figure 6 The voyage of the Fram (solid line) and route of Nansen and Johansen (dashed line) during their expedition of 1893–96

Long description

After the expedition Nansen became the Norwegian ambassador to Britain, a leader in the League of Nations (the ideological forerunner to the United Nations), and won the Nobel Peace prize in 1922. The other hero, Hjalmar Johansen, joined Roald Amundsen on his expedition to the South Pole (1910–12). During that trip he fell out with Amundsen and committed suicide on his return to Norway in 1913. The sea channel between the Svalbard archipelago and Greenland was named the Fram Strait in honour of the famous polar research ship.

Winds, ocean currents and flow from rivers can all carry pollutants from their source to the Arctic. On a stereographic plot, the routes of wind-borne contaminants from the warmer, populated areas of Earth to the cooler, Arctic are clear (Figure 7). These winds can transport contaminants to the poles, where they are removed from the atmosphere most likely through snowfall and are then absorbed by animals, perhaps through direct contact. The North Atlantic Current shown in Figure 7 flows directly past the waters off western Europe, likely to be a major source of PBDEs. For top predators such as polar bears, there is also likely to be bioaccumulation from the high levels of PBDEs in their prey, the seals.

Figure 7 Transportation pathways for persistent organic pollutants (POPs) to the Arctic. Note the curving path of the wind currents caused by the rotation of the Earth. (Source: adapted from Macdonald et al., 2005)

Long description

Overall, the toxicity of POPs to the polar wildlife is not clear, but the fact that they are manufactured only in populated regions and yet can be detected in Arctic wildlife is striking. POPs give us a graphic demonstration that a region once thought of as remote is clearly physically connected to the rest of the planet.

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3.1 Greenland's snowfall

The Open University — Wed, 04 May 2011 11:18:29 GMT

Greenland snowfall differs depending on whether it falls in summer (when snow is comparatively warm and moist) or winter (when snow is cold and dry). These differences mean that as the snow is turned to ice, annual layers are formed that are in many ways similar to tree rings: thick annual layers mean high snowfall and thin annual layers low snowfall. The accumulation of snowfall on the summit of Greenland – and most importantly what is trapped within the crystals as it turns to ice – can provide a record of the past. Digging down into the ice cap is equivalent to going back in time through the snowfall of previous years. One would have to go down a long way (equivalent perhaps to 300 years of snowfall) before reaching the ice. To make the digging back in time easier a drilling rig that extracts ice cores about 13 cm in diameter (Figure 8 (a) is used to get to very deep levels. Once extracted, the annual layers in the cores are clear (Figure 8(b).

Figure 8 (a) The GISP ice camp on the summit of the Greenland ice cap; (b) snow layers from a Greenland ice core. The arrows show successive years of snowfall.

Long description

As well as looking at snowfall, the use of different chemical and physical techniques on ice cores can tell us about dust in the atmosphere, past volcanic activity – and even the industrial production of civilisations long past. For example, Figure 9 shows the concentrations of lead in the ice of different ages, and compares it with the recorded production of lead.

Figure 9 (a) Global lead production; (b) the concentration of lead in a Greenland ice core (Source: adapted from Hong et al., 1994)

Long description

Study note: powers of ten and scientific notation

Figure 9(a) shows the production of lead in tonnes multiplied by different powers of ten (10⁰, 10², etc.). When you see numbers written down it is quite easy to read and understand them when they have few digits, for example 0.01, 0.5, 4, 15, or 132. But when numbers have a lot of digits, for example a small number like 0.0000067, or a very large number like 1,700,000,000, they are less easy to read, and consequently it is harder to understand what they are telling you.

For example, if I ask you to say ‘75 kg’ you would probably respond immediately ‘seventy-five kilograms’. But if I asked you to say the mass 330,000,000 tonnes I am sure you would have to start counting the zeros. To make large and small numbers easier to comprehend we use a system based on what we call powers of ten. In this case the power is the number of tens that are multiplied together. For example, 10², which we would say as ‘ten to the power of 2’, means that two tens are multiplied together. So:

Similarly,

And so on; for example:

10⁷ is easier to understand than 10000000. Note that 10¹ implies just one ten, that is, 10¹ = 10, so we do not add the power 1 in this case. When dealing with powers of 10 you could also just say that the power is the number of zeros after the 1, so 10⁰ is just the number 1.

That covers numbers greater than 1, but what about numbers less than 1 such as 0.1? In powers of ten it would be written as 1 divided by 10, so

and this is written as 10⁻¹.

Similarly, 10⁻⁴ is 1 divided by 10 four times:

So how would you write the number 150 as a power of 10? The number 150 is 1.5 × 10 × 10, so would be written 1.5 × 10². This form of writing numbers is known as scientific notation. A number written in scientific notation always looks like this: (number between 1 and 10) × 10^{some power}.

SAQ 2 Taking readings from a graph

From the graph in Figure 9, what was the maximum global lead production in tonnes per year before the Industrial Revolution?

When did this occur, and what was the lead concentration in the Greenland ice core at this time?

Graphs can both reveal and conceal information. It is important that you look closely at the axes to make sure that you understand what is being plotted, and on what scale. Some graphs just show what the author believes is the general trend in changes in value; others may show individual points, with or without connecting lines. Where connecting lines are drawn, as in Figure 9(b), the effect may be to lead your eyes to think that an isolated point is more important than it really is. The visual impact of a graph is both a strength and a weakness!

Answer

The peak in global lead production before the industrial revolution was approximately 2000 years before the present (BP). At this point the global lead production was about 10⁵ tonnes per year. The concentration of lead in the greenland ice core at this time was approximately 3 × 10⁻¹² grams of lead per gram of ice.

Extracting lead from its ores, and to a lesser extent working the lead into pipes, etc. (the word ‘plumbing’ derives directly from the Latin for lead, plumbum, as does its chemical symbol, Pb) results in discharge of lead-rich dust to the atmosphere. Given the pattern of wind movements shown in Figure 7, it is therefore not surprising that lead should appear in the precipitation over the Arctic for the corresponding period.

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3.2 The past temperature of the planet

The Open University — Wed, 04 May 2011 11:18:29 GMT

Measuring the concentration of lead in the ice is called a direct measurement: the ice sample is melted and the water produced contains a very small but readily measured quantity of lead dust. A very accurate set of scales would be needed to measure it, but it is a directly measured quantity. There are also many indirect measurements that can be made using proxy data. The concept for using proxies is both simple and brilliant: one measured property allows inference about other states of the system (Box 3).

Box 3 Proxies

Proxy data occur when scientists measure one, two or even several direct quantities and use these values to infer some other quantity. For example, if I measure my waistline, my weight and my height every week for a year there would be a data set consisting of three variables measured 52 times over the course of a year. They are called variables because they are varying quantities; in this case they vary with time. Typical results would be like those shown in Figure 10.

Because I have stopped growing my height does not change throughout the year so, as in the top panel of Figure 10, the graph would be a flat line. However, both my waistline and weight do vary. With my body shape, when my weight goes up it all goes on to my waistline, so both the graph of my waistline and the graph of my weight vary in the same way. As my waistline gets bigger I get heavier. The opposite also applies – when my weight goes down my waistline reduces. Because my waistline and weight seem to vary together we say the two variables are correlated. In this case they are positively correlated because when my waistline gets bigger, so does my weight. If, for some strange reason, as my waistline got bigger my weight decreased (not a likely scenario!) then the two variables are said to be negatively correlated.

Figure 10 Schematic measurements of height, waistline and weight for the author throughout a year

Long description

Because my waistline is positively correlated to my weight, there is a mathematical relation between the two variables. So for example, it might be that when my waistline increased by 2 cm I was 1 kg heavier. If I just gave you the data for my waistline over a year, and my starting weight, you could derive values for my weight over the whole year. This makes my waistline a proxy for my weight. If I then told you that I tended to eat more over Christmas and exercised a lot in the summer, then you could think it reasonable to add dates to the graphs in Figure 10. My weight and waistline would then be a proxy for the time of year as well. It is important to understand that correlated variables do not tell us anything about the cause of the observation – they only tell us that the items vary in a particular way. In the example above, clearly the expansion of my waistline is not the cause of my weight changing – it is the result of it. A more extreme example of this is that the number of people in the British armed forces has decreased since the First World War, and at the same time global atmospheric temperatures have risen. Whilst these two variables are negatively correlated there is no physical mechanism for one influencing or controlling the other.

I noted above that because two things are correlated it does not necessarily mean that one causes the other, although in the case of the lead data there is an obvious causal link. What is perhaps not so obvious is that we cannot be sure just by looking at a graph whether two variables are correlated. To be sure that the observations do show correlation, scientists use formal statistical tests. The details of these are beyond the scope of this course, but they are essential in scientific investigation. In principle, statistical tests use mathematics to tell us the likelihood that the results we see occur just by chance. If the mathematics suggest that the results are indeed just chance, we cannot draw any conclusions from them. If, however, the likelihood of it being just a chance relationship is very small, then we can assume that there really is some repeatable relationship between the two. To use one item as a proxy for others, we therefore need first to be sure that there really is a correlation, according to accepted scientific standards. Observing a correlation should also lead us to look for a plausible mechanism whereby one item could affect the other. In the example of temperature and service personnel given above, such a mechanism is almost totally implausible. Even if the correlation were statistically acceptable, its implausibility would lead a scientist to reject it as being due to chance.

SAQ 3 Proxy variables

Do the data in Figure 9 suggest that lead production and the concentration of lead in ice cores are correlated, so that one could be used as a proxy for the other?

Answer

Yes, they do appear to be correlated, as the values rise and fall together. There is also a direct physical link between the two items, so it might be acceptable to use one as a proxy for the other.

This process of analysis and checking for plausible mechanisms using proxy data has revolutionised the study of past climates.

For example, a simple ice core proxy for moisture in the atmosphere at the time of deposition would be the thickness of the annual snow layer. A thicker layer would mean more snowfall, so the atmosphere must have been wetter to hold the increased snow before it fell. A thinner annual snow layer would imply the opposite. The temperature record was constructed entirely from proxy data using the relative amounts of oxygen-16 and oxygen-18 isotopes. The water molecules in the ice have a proportion of all three isotopes of oxygen in them (see the Note below) and it has been shown that the relative amounts of the different isotopes vary depending on the temperature of the oceans at the time the ice was deposited. This fact has led to a most useful proxy technique, which uses the amount of oxygen-16 compared with the amount of oxygen-18 in the sample to derive the temperature of past climates. In this case the ratio of the oxygen isotopes is a proxy for the temperature of the planet.

The central part of an atom, which makes up most of its mass, is called the nucleus; this is surrounded by an ‘electron cloud’, which largely determines how the atom reacts with other atoms or molecules. The nucleus of an atom is made of building blocks called protons and neutrons. The number of protons determines what element the atom actually is. An atom with one proton is hydrogen and an atom with eight protons is oxygen. However, the number of neutrons at the centre of an atom can vary. Oxygen exists in its natural state with eight protons and either eight, nine or ten neutrons. Atoms with the same number of protons but different numbers of neutrons are called isotopes. The most abundant oxygen isotope, with eight protons and eight neutrons, is called oxygen-16 (8 protons + 8 neutrons), the oxygen isotope which has eight protons and nine neutrons is oxygen-17 (8 protons + 9 neutrons), and the oxygen isotope which has eight protons and ten neutrons is called oxygen-18 (8 protons + 10 neutrons).

Throughout this unit the focus has been on the Arctic, but because some data from the ice cores tell us about conditions over the entire planet (such as -->Figure 9) I am going to show data from another core, this time from Antarctica. The only reason for this is that the core goes back much further in time than any Greenland one. The particular core I am going to use is called the EPICA (European Project for Ice Coring in Antarctica) – Dome C core. Dome C is currently the longest ice core and it has snow layers going back almost 800 000 years throughout the Quaternary and includes the period when Homo sapiens evolved. In fact the EPICA core can be used to reconstruct the temperature more than half a million years before Homo sapiens ever walked the Earth (Figure 11).

Figure 11 The temperature from the EPICA ice core going back to 800 000 years before the present (thousands of years before present or BP). The vertical temperature scale has 0 as the present mean temperature, and goes from −12 °C to +5 °C relative to this.

Long description

Figure 11 shows that global temperatures have varied considerably, but there also appear to be regular cyclical patterns. At the low points the temperature shown by the core was as much as 10 °C colder than today: colder periods happen about every 100 000 years, with warmer periods between. Four times in the last 450 000 years the intervening warm periods have been warmer than today (up to 4 °C warmer 120 000 years ago). During the nine cold periods shown in Figure 11 the snow that fell in winter did not melt in the following summer heat, and the ice sheets grew. What did these temperature variations mean for the rest of the Earth? Other proxy data, such as from sediments found at the bottom of the oceans and lake beds, and the dating of rocks and analysis of ice cores from high-altitude mountain glaciers, show that during the cold periods a large proportion of the northern hemisphere was covered by an ice sheet that was in places several kilometres thick. Glaciers advanced, eroding valleys and mountains, and in the northern hemisphere wildlife moved south to more temperate regions. At the lowest temperatures (Figure 12) the ice sheets covered about 10% of the entire planet – up to 30% of all the land.

Figure 12 The maximum extent of the ice sheets of the northern hemisphere during the 800 000 years of EPICA ice core data. Oceans are coloured dark blue and continents yellow. Ice is shown as lighter shades of blue.

Long description

The sea froze as far south as the northern Spanish coast and almost all of Britain was buried beneath the ice. These periods are called the ice ages. A vast quantity of water was locked in these ice sheets, so sea level was as much as 120 m lower than today and there was dry land between Britain and the rest of Europe. During times between these cold periods the ice sheets melted and the water from land ice meant that sea levels rose. These are called interglacials. Note that it is only the melting of land ice that changes sea levels. As climate change deniers never tire of reminding us, melting sea ice does not change the sea level. Remember that ice is less dense than water, so it floats. As sea ice melts, it forms a smaller volume of water than the volume of ice. In fact, the volume of water formed is exactly the same as the volume of ice that was below the water surface when it was floating, so no change in sea level occurs. Of course, when ice on the land melts and flows into the seas, this does raise sea levels.

SAQ 4 The Arctic defined during an ice age

What would happen to the size of the Arctic, as we have defined it, during an ice age?

Answer

During an ice age, because the planet was colder and ice covered so much land, the treeline – our proxy for the Arctic definition – was much further south than today. This means that the area of the Arctic would have been much larger than at present.

Activity 2 Rates of change of temperature

Look carefully at the temperature record in Figure 11. Are there any general observations you can make about the rates of change of temperature between the relatively warm and the relatively cold periods?

Discussion

The record in Figure 11 shows that the temperatures fall relatively slowly but rise relatively quickly – particularly in the most recent 450 000 years.

Assuming (correctly) that the temperature is a proxy for the amount of ice on the planet, the ice sheets in Figure 12 took about 100 000 years to grow, and yet they rapidly disappeared – typically in only approximately 10 000 years. Consequently sea levels fall slowly as the ice sheets grow, and rise relatively quickly as they decay again. The obvious question from Figure 11 is what causes these regular fluctuations in temperature and ice cover. One of the most influential is the Milankovitch cycles of the Earth's orbit.

The amount of energy that the Earth receives from the Sun depends on its distance from the Sun. We tend to assume that this is constant, but in fact the orbit of the Earth around the Sun is an ellipse, with the Sun at one of its foci (Figure 13), so the distance from the Earth to the Sun varies over the course of an orbit (one year). If the Sun emits a constant amount of energy, then when the Earth is closer it will receive more than when it is further away. However, the shape of the ellipse also varies with time, and the Earth's axis of rotation also wobbles, like a gyroscope. The Serbian geophysicist Milutin Milanković realised in 1920 that the varying energy received by the Earth as a result of these two factors could be the cause of the ice ages. He showed that the ellipse changes shape over periods of about 100 000 years. The timing of these changes, combined with the wobble in the Earth's rotation, matched up with data he had for the times and durations of the ice ages. He showed that the incoming energy would be at a minimum when there was an ice age and at a maximum during an interglacial. Unfortunately, modern records go back much further than the data to which Milanković had access, and further back in time the match is not so good. Earlier ice ages can be earlier and later than the predictions from Milanković's model. Clearly there are other factors affecting the climate. This story illustrates another aspect of the way that science develops. Milanković's model was tested against new data, and found not to be fully consistent with it. The challenge was then for scientists either to completely reject that model, or to look for other effects that could be combined with the basic model to provide a better explanation of the observations. Scientific models are always subject to revision as new data is found.

Figure 13 The orbit of the Earth around the Sun is an ellipse, so throughout a year the Earth–Sun distance, and consequently the amount of solar energy received at the surface of the Earth, varies. Note that this picture shows the orbit shape greatly exaggerated.

Long description

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3.3 The Keeling curve

The Open University — Wed, 04 May 2011 11:18:29 GMT

The Keeling curve is the plot showing the trend in rising atmospheric CO₂ concentrations since 1958 recorded at Mauna Loa in Hawaii. The story of atmospheric CO₂ in the last 50 years is a relentless rise derived from human use of hydrocarbons and, as I write this in 2008, the annual mean concentration is 383 parts per million (ppm). When Keeling first collected his CO₂ data he travelled around making the measurements at widely spaced locations – but he saw that apart from the daily and seasonal variation caused by local plant photosynthesis and respiration the concentration was virtually the same wherever he measured it. Keeling quickly realised that this meant it was possible to measure the CO₂ in one location, such as Mauna Loa, and it would be a reference point for the whole planet.

Activity 3 How representative is the Keeling curve?

Is Keeling's contention that the Mauna loa data is a good reference for the whole planet consistent with what you have learned about atmospheric movements?

Discussion

Recall from the discussion of the spread of pollutants by wind (and from your own experience if you live in an exposed area!) that there are constant air movements around the planet. These movements stir up the air and mix it constantly. This constant mixing means that the concentration of CO₂ is likely to be similar all over the globe. This sort of questioning as to whether methods and data are plausible is another good example of scientific method.

After a few years of measurement Keeling must have been astonished to see CO₂ levels rising so rapidly. The problem of course with the Keeling CO₂ data is that it extends back only to 1958. However, ice-core researchers realised that the air bubbles trapped when the ice was formed would contain atmospheric gas samples. As well as giving a proxy record of past temperatures, ice cores can give the exact atmospheric CO₂ concentration for the last 800 000 years.

SAQ 5 Direct and proxy measurements

Is measurement of gas trapped in a bubble in an ice core a direct or proxy measurement?

Answer

Measurement of the CO₂ contained in a trapped gas is a direct measurement, not a proxy. This means it is very accurate.

It takes a certain period of time for the bubbles to be closed off and air to be isolated. As a result this method cannot provide a concentration until this has happened. In the case of the Dome C core the most recent atmospheric CO₂ concentration available is from 130 or so years ago. Figure 14 shows that over the last nine glacial cycles the CO₂ and temperature appear to be positively and very closely correlated, showing the same patterns of change.

Figure 14 Past atmospheric CO₂ concentrations and temperatures going back through nine ice ages, taken from the EPICA ice core

Long description

SAQ 6 Temperature and CO₂ values

According to Figure 14, what were the typical CO₂ levels during the extreme low-temperature periods (ice ages) and at the height of the warmer interglacials?
How does the value of the atmospheric CO₂ concentration for 2008 quoted earlier compare with that in the interglacials of the previous nine cycles of the EPICA Dome C ice core?

Answer

In an ice age, when the temperature is low, the CO₂ is also low, typically 180 to 200 ppm. When the temperature is highest, in the interglacials, the CO₂ is also high, at about 280 ppm for the most recent four interglacials and slightly lower at 260 ppm for the earliest five interglacials.
The 2008 atmospheric CO₂ concentration is 383 ppm. This is about 100 ppm higher than what it was in the most recent four interglacials, and it is about 120 ppm higher than the earliest five interglacials in the EPICA Dome C record.

We now have data that we could possibly use to predict what might happen as a result of the increasing CO₂ concentration that Keeling detected. We could theoretically plot a graph of temperature against CO₂ concentration and, from this, read off what is the temperature for any given CO₂ concentration. Unfortunately, there is a problem with this. The current atmospheric CO₂ concentration is higher than at any time in the previous 800,000 years, so even if we had a graph of the mathematical relationship between temperature and CO₂ concentration from the earlier data, it would not include the current (and much less, any possible future increased) CO₂ concentration. We would have to extrapolate (that is, extend) the graph beyond the available set of values, and we cannot know for sure that the relationship will hold outside these limits. This means that it is difficult to use information from these earlier periods to predict what may happen in the near future. We are fairly sure that Milankovitch cycles amplified by greenhouse gases are responsible for the coming and going of ice ages; it is our best theory and one to which almost all climate scientists subscribe. But as we have seen, it is not a complete explanation, and some of the earlier cycles do not conform to this theory. To make useful predictions for the near future, and hence to suggest actions to protect our environment, we need to look for some more detailed information and scientific models.

Box 4 Prediction, extrapolation and falsification

Much of science is concerned with gathering data, but according to the philosopher of science Karl Popper (1902–94) a key part of scientific method is making testable predictions from the data. According to his model of scientific method, a scientist makes observations about the world, and from these observations constructs theories about the causes of the observed phenomena. Using those theories, the scientist is then able to make predictions as to what might occur in a new, but similar situation to the ones previously observed. The scientist should then set up such a situation, and test whether the observed behaviour does indeed occur. If it does, then the theory is supported. But if the observations do not accord with the theory, then the theory is either inadequate or possibly completely wrong.

The story of Nansen's expedition in the drifting ice is a spectacular example of this. From the observation that trees from Siberia turned up in Svalbard, he predicted that a ship trapped in the ice would follow the same path. He then proceeded to test this theory in a very practical, but dangerous way.

This continual attempt to test, and potentially falsify, theories is regarded as the essential feature of scientific method that distinguishes it from other approaches. An artist or a journalist may want to present their interpretation of a situation, but this interpretation is only descriptive, not predictive. Some religions and similar codes make predictions and suggestions about what should happen, but these are rarely tested.

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4 The end of the last ice age: the Holocene

The Open University — Wed, 04 May 2011 11:18:29 GMT

I have already noted that the great ice sheets took about 100,000 years to form and only about 10,000 years to decay. So what happened at the end of the last ice age? Figure 15 shows the EPICA ice core CO₂ concentration and air temperature for the most recent 20 000 years, which is within the last ice age. The temperature scale shows the difference from the average temperature of the last 1000 years, so 0 °C is no change from today's climate.

Figure 15 The atmospheric CO₂ concentration (relatively smooth line) and air temperature anomaly (spiky line) at the EPICA ice core in Antarctica over the last 20 000 years

Long description

Figure 15 shows again the high correlation between the two variables: 20 000 years ago it was up to 10 °C colder and CO₂ concentration was about 200 ppm lower than today. Over the most recent 10 000 years atmospheric temperature has been within about 2 °C of current temperatures and this climatically stable time period is called the Holocene. Figure 14 shows that such a warm, stable period has been very unusual in the last 800 000 years, yet it is only during the Holocene that agriculture and civilisations have developed. Homo sapiens has flourished in the stable climate era. Figure 15 shows that up to approximately 14 000 years ago the planet appeared to be leaving the ice age, and the temperature rose to within 1 °C of the 0 °C line. But then there was a very rapid cooling of 4–5 °C (and most of this in just a couple of decades) and lower temperatures resumed from 12 900 to 11 600 years before the present. This cold period affected most of the planet and is called the Younger Dryas, after a pretty Arctic alpine flowering plant called the white dryas (Figure 16). This species spread its geographical range as temperatures fell and the tundra biome expanded in area.

Figure 16 The white dryas. The Latin name of this pretty flower is Dryas octopetala (meaning dryas flower with eight petals – although it can have up to 16 petals).

Long description

Another interesting event shown in Figure 15 happened just before 8000 years ago (called the ‘8.2 ka event’ where ka is an abbreviation meaning 1000 years), when there was a clear but relatively small temperature and CO₂ decrease which was associated with drier conditions in some parts of the world. This represents the largest climatic variation that civilisation has currently had to cope with. So what happened in the Younger Dryas and 8000 years ago to make the planet suddenly colder? The changes occurred too fast for the Milankovitch cycle to be responsible. We now believe that the only way to cause that much cooling is by a sudden change in part of the global ocean circulation. Just as there are global patterns of air circulation, so there are also much slower, but enormous, movements of water around the oceans, driven by changes in water temperature and salinity (Box 5).

Box 5 Wally Broecker's great ocean conveyor belt

The density of fresh water decreases as its temperature rises above 4 °C. The density of salt water in the oceans likewise depends on temperature, but also on the amount of salt within it. In the seas of the North Atlantic Ocean the surface waters are cooler than the lower layers and so they sink. In contrast, in places like the central Pacific Ocean the relatively dry, warm air increases evaporation and the surface waters are both warm and salty. All around the planet different regional climatic conditions create surface waters with different densities. Because the denser waters sink, over time horizontal currents are set up similar to the processes for the winds. The result is a vast, three-dimensional circulation across the entire ocean. In the 1980s the American climate scientist Wallace Broecker suggested that the global ocean circulation could be viewed as analogous to a conveyor belt that moved heat and salt around the planet. Broecker's schematic picture (Figure 17) has become one of the iconic images of climate science.

Figure 17 A schematic of the great ocean conveyor that moves both heat and salt around the planet

Long description

It is a huge simplification, but on a global scale Broecker's conveyor belt is excellent at helping us understand planetary processes such as the Younger Dryas and the 8.2 ka event.

Heat that is carried in the ocean conveyor past Britain and up the coast of Norway towards Svalbard both keeps the UK climate warmer and moister than it would otherwise be and also means that the ice edge is a long way north compared with similar latitudes in North America (Figure 18).

Figure 18 Sea ice concentration (the amount of the ocean covered by ice measured as a percentage) measured by satellite on 9 January 2008. Purple colours are almost continuous ice cover and the blue colours represent open water. Land is green.

Long description

One way to cool the planet, as occurred in the Younger Dryas or the 8.2 ka event, is to stop the ocean conveyor carrying the heat northwards. It is believed that this indeed happened as a result of large quantities of melt water from the North American continental ice sheets flooding into the north Atlantic and changing the surface density of the ocean. Once the conveyor was stopped, the climate was plunged into a cold period. Although similar events seem to have occurred further back in time, the Younger Dryas and the 8.2 ka events may have been particularly significant for human civilisation. The earliest dated human settlements are in the Mediterranean about 13,000 years ago – in the middle of the Younger Dryas. It is interesting to compare the spread of human civilisation across the Middle East and Europe (Figure 19) with the temperature data of Figure 15. During the first 5000 years of human civilisation, from 13 ka BP to 8.4 ka BP, settlements are concentrated on the shores of the Mediterranean and Black Sea. However, after the 8.2 ka event and the collapse of the North American ice sheets, the flooding of fresh water into the Atlantic that stopped the conveyor also caused a rapid sea-level rise of over 1.4 m and large-scale flooding. After this date the settlements rapidly spread northwards.

Figure 19 Locations and dates of sites of Neolithic settlements across the Middle East and Europe. The coloured dots indicate new sites that were established during each time period; grey dots represent pre-existing sites established during earlier time periods. (Source: Turney and Brown, 2007)

Long description

The exact driving factor for this human migration is impossible to determine, but it is interesting that it seemed to begin immediately after the 8.2 ka climate event. This event may even have been the main factor in the migration and an example of the effects of climate change that our society will have to cope with. Unfortunately we do not have available the empty land of 8000 years ago.

In recent decades our understanding of the reality of climate change has moved from one of slow and gradual change over deep time to clear evidence that there have been naturally occurring climate changes of several degrees Celsius and sea-level jumps of over 1 m within timescales of a decade or so. In fact, the very latest research on the Younger Dryas using Greenland ice core data has revealed that central Greenland cooled by a staggering 2–4 °C in just 1–3 years!

While the Younger Dryas and the 8.2 ka event were entirely natural, Block 1 established that today there is an additional human contribution to consider. But when exactly did the human contribution begin? Often the phrase ‘pre-industrial levels’ is used to mean ‘before significant anthropogenic changes started’, but it is not specific. Could humans have influenced the climate before the Industrial Revolution of the eighteenth century? Block 1 noted that another very significant greenhouse gas is methane (CH₄), and 1 cubic metre of methane in the atmosphere can be over 25 times more effective at trapping heat than the same amount of CO₂. Past atmospheric methane concentrations can also be directly measured from ice cores. Over the last quarter of a million years, CH₄ concentration and the variation of solar radiation reaching the Earth attributed to the Milankovitch cycle are positively correlated (Figure 20). But this correlation dramatically breaks down in the most recent data of the Holocene. The latest 5000 years of methane data show that the atmospheric concentration has risen dramatically out of synchrony with the solar radiation. The most recent ice core data has a concentration as high as any period in the entire ice core record, at over 700 ppb (parts per billion). Carbon dioxide has a similar break from the expected downward trend although starting earlier at about 8000 years ago.

If the ‘normal’ trend of methane and carbon dioxide was downwards, along with the Milankovitch cycle, then where have the extra gases come from?

Figure 20 (a) The atmospheric concentration of methane and solar radiation reaching the Earth's surface, from the Milankovitch cycle;(b) observed and expected atmospheric methane levels over the last 11 000 years

Long description

Carbon dioxide and methane are by-products of our civilisation, and in the words of the climate scientist William Ruddiman,

Human activities tied to farming – primarily agricultural deforestation and crop irrigation – must have added the extra CO₂ and methane to the atmosphere.
(Ruddiman, 2005)

As with CO₂, since the Industrial Revolution the atmospheric concentration of methane has more than doubled and currently is over 1700 ppb. Virtually all of that rise has been from anthropogenic sources, including major food production activities such as rice and cattle production. Not only were humans possibly affected by climate change during the Holocene, but we had also started our impact on the planet thousands of years before the Egyptian pyramids were built. How then are these changes being seen today?

SAQ 7 Recent climates

How does the earth's climate over the last 10 000 years compare with that of previous times, and what does this mean for humans in the future?

Answer

Over the last 10 000 years the Earth's climate appears to have remained in a warm, stable state for longer than was normal in the preceding climate cycles. This has probably been important for humans in that they have been able to develop agriculture and other aspects of civilisation without the major disruption that would be caused by the major rapid cooling associated with ice ages.

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5 The contemporary Arctic climate

The Open University — Wed, 04 May 2011 11:18:29 GMT

There is a remarkable seasonality in the Arctic climate. For example, the flow in some of the great rivers of Russia and North America that empty into the Arctic Ocean almost stops in winter (Figure 21). During May, ice in the rivers starts to break and in June there is a rapid flood of fresh water followed by a fall in flow until November, when it freezes.

Figure 21 The monthly discharge on the Lena River (Russia). Each individual bar in the graph represents a monthly value for each year during 1935–1999.

Long description

A similar huge seasonal signal is seen in the Arctic sea ice cover. Most people are surprised to realise that the sea ice of the frozen Arctic Ocean is only a few metres thick. Beneath this is a few kilometres of water. In winter as much as 16 million square kilometres of the ocean freezes, and as this melts in summer only about 6 million square kilometres remains frozen (Figure 22). The seasonal variation of almost 10 million square kilometres is equivalent to about 45 times the area of the United Kingdom.

Figure 22 The ice cover of the Arctic Ocean in September 2005. The average ice extent for September (when ice area is at a minimum) for the period 1979–2000 is shown as a red line and the average ice extent for March (when the ice area is at its maximum) is shown as a blue line.

Long description

The contemporary Arctic climate appears to be changing. However, average global temperatures mask regional variations and the Arctic has been warming faster than the global mean (Figure 23).

Figure 23 The annual average near surface temperature from all weather stations on land relative to the average for 1961–1990 for all regions from 60° N to 90° N (Source: Arctic Climate Impact Assessment, 2005).

Long description

Activity 4 Recent climate change in the Arctic

Describe the changes in Arctic temperature that are shown in Figure 23.

Discussion

With the exception of a period in the 1960s and 70s the Arctic temperature has been above the 1961–1990 average in most years since 1920. Currently the temperature is about 1 °C above the mean temperature for 1961–1990. This data came from the Arctic Climate Impact Assessment. They appear to use the latitude of 60° N as their definition of the Arctic, so Figure 23 must include meteorological stations that are not in the Arctic as we have defined it, and are less likely to be affected directly by changing ice and snow cover. For this reason Figure 23 most likely underestimates the temperature increase – but what is its impact?

Figure 24 compares the surface melting on the Greenland ice cap in 1992 and 2005 as measured by satellite. For ice to form, the snow has to survive the following summer. But an increasing area of the Greenland ice cap is melting in summer so annual snow layers are not being converted to ice in these regions.

Figure 24 A comparison of the surface melt of the Greenland ice cap in 1992 and 2005

Long description

It is an extremely complex process to estimate the melt of the whole ice cap, and the current best value is that Greenland is melting in the range 90–250 billion tonnes of ice per year. All of this melt is contributing to the predicted sea-level rise of at least 1–2 m by 2100, and a rise of only 1 m would affect well over 100 million people worldwide. The fresh water from the ice cap could also slow Broecker's conveyor (Box 5), causing other climate impacts.

For the Arctic sea ice the signal of climate change is clear: it is getting thinner and the amount of it that survives the summer is reducing. Figure 25 shows the trend in extent of sea ice in September (the summer minimum, the mean of which is shown as the red line in Figure 22). The sea ice minimum is decreasing at a rate of almost 9% per decade. This means that approximately 100 000 square kilometres less of the ocean is covered by sea ice each year. In 2007 the September ice area was only about 4 million square kilometres (off the bottom of the scale in Figure 25!); by September 2008 the minimum was slightly higher but still off the scale. I would be interested in what the latest September value is as you are reading this.

Figure 25 The minimum extent of Arctic sea ice in September of each year from 1979 to 2006. There is a consistent trend downwards. More recent data for 2007 and 2008 is off the scale.

Long description

Activity 5 The changing mean albedo

What is likely to be the effect of these changes in ice cover on the albedo of the Arctic region?

Discussion

Recall from Table 1 that the albedo of open water is 3% and that of sea ice is 40%. So the increased thawing during summer will decrease the albedo, so that less energy will be reflected back into space, and more energy will be absorbed.

The effect of this on the albedo is actually more complex than suggested by Activity 5, but this ice–albedo feedback loop (Figure 26) is potentially very important. Table 1 gives the average albedo of sea ice as approximately 40%. Sea ice is complex and it could consist of a mixture of bare ice, ice with snow on (the snow could be either wet or dry) or even ponds of freshwater on the ice as it melts, and each one of these types has a different albedo. As temperatures rise there will be more bare ice, melt ponds and open water and the overall albedo will decrease. This means that less energy will be reflected, so more solar energy is absorbed by the ocean, causing further warming and ice melting. The ice–ocean system is in a positive feedback loop and changes such as melting ice naturally lead to more melting ice.

Figure 26 The ice–albedo feedback loop: (a) graph of the albedo of various ice categories and open water; (b) an increase in absorbed sunlight leads ice melting which lowers the albedo, causing more sunlight to be absorbed

Long description

Box 6 Positive and negative feedback

Feedback is the term that is used to describe the situation where the output from a process affects the input to that process. You may have encountered the ‘howl’ that can occur when a microphone is placed too near a loudspeaker; the sound from the loudspeaker feeds back to the microphone, gets amplified and fed back again so that the volume of sound keeps on increasing until the amplifier overloads. This is an example of positive feedback. Populations of organisms can exhibit the same effect.

If one generation produces more than one surviving offspring per adult, there are more organisms to produce young in the next generation, who produce more young in the next, and so on. This leads to a population explosion. Economic growth is supposed to work the same way – increased wealth this year allows us to spend and invest to produce more wealth next year, to produce more wealth the next year, etc. Of course, the sound from the loudspeaker cannot get louder and louder forever, populations of organisms don't actually go on expanding forever and, whatever economists may tell us, economic growth is unlikely to continue unchecked. The sound from the speaker is limited by the power available to the amplifier and populations can be limited by their food supply. These limits can either have an effect like running into a brick wall, or they can be more subtle. The subtler version is the phenomenon of negative feedback, where an increase in the output from the process causes the process itself to ‘slow down’, so that output returns to a lower level. Populations are a classic example. As there are more organisms present, there is likely to be less food available per individual (or the increased population may attract more predators), so that the rate of production of young decreases and the population tends to stabilise. Negative feedback is a fundamental concept in the control of machinery and electronic devices, and there are many other examples from ecosystems. Maybe economists should spend some time studying control engineering!

Many climate models suggest that, given the predictions of Arctic warming, the sea ice may disappear completely in summer sometime around 2060. But given the observations of the last few years, I could not put the current situation any better than this recent article:

With sharply rising atmospheric greenhouse gas concentrations, the change to a seasonally ice-free Arctic Ocean seems inevitable. The only question is how fast we get there. The emerging view is that if we're still waiting for the rapid slide towards this ice-free state, we won't be waiting much longer.
(Serreze and Stroeve, 2008)

The extent of snow cover in the northern hemisphere is decreasing in a similar way in another positive feedback loop, but what about the frozen ground beneath the snow that is called permafrost? Most of the global permafrost is in the Arctic and high mountain areas (Figure 27) and many cities use the frozen ground as foundations for building – and even for temporary roads in winter.

Figure 27 The permafrost distribution in the northern hemisphere. The largest area of continuous permafrost is in the Arctic and high mountain areas.

Long description

It should be expected that the area of permafrost will decrease, but it is difficult to measure. Virtually all boreholes into the permafrost show that Arctic warming (Figure 23) is penetrating into the ground. While frozen, permafrost provides a solid surface – a vehicle will leave no trace. As it melts the situation is different. The State of Alaska has strict rules for vehicle travel on permafrost to prevent environmental damage. When it is too warm, travel is not allowed. The duration of allowed permafrost travel set by the Alaska Department of Natural Resources is an interesting climate change proxy! (Figure 28). In the last 25 years the number of days on which oil exploration is allowed on the tundra has more than halved.

Figure 28 The annual duration of allowed tundra travel for oil exploration activities set by the Alaska Department of Natural Resources

Long description

The retreat of the permafrost is serious. Building foundations are collapsing and there are ‘drunken forests’ as land beneath trees melts, subsides and slumps. Building should only be done on carefully built foundations and the Trans-Alaska Pipeline was even built on refrigerated pillars to prevent pipe fracture through permafrost thaw subsidence.

There is, however, another more worrying problem as the permafrost retreats. As the ground subsides the depressions usually form lakes because the melt water cannot flow through the frozen ground beneath. Thawing of the permafrost at the lake bottom releases organic matter perhaps 30–40,000 years old into the water. The organic matter decomposes, giving off methane – a potent global warming gas. The permafrost–methane feedback cycle is another positive feedback in the system (Figure 29).

Figure 29 While permafrost retreat could be perceived as a less glamorous area of climate research, the positive methane feedback cycle means that it is a profoundly important climate issue

Long description

Another potentially significant source of methane in the Arctic is trapped in the shallow seabed of the Arctic Ocean and is called methane clathrate. This is ice with methane trapped within the crystal matrix (Figure 30). As the ocean warms, the release of large quantities of methane into the atmosphere from clathrates would be yet another positive feedback, but it is currently unquantifiable as there is not even a basic understanding of the distribution and amount in the Arctic. This has been called the clathrate gun hypothesis and it could lead to a runaway greenhouse effect that may have happened before in deep time. It could even have been responsible for mass animal extinctions.

Figure 30 Burning methane released from methane clathrate. Inset is the crystal structure: the methane molecule (green) is trapped within the ice molecules (red).

Long description

SAQ 8 Arctic feedbacks

What is the particular importance of feedback processes in the context of climate, particularly with respect to the Arctic?

Answer

There are probably many positive and negative feedback processes associated with climate, but in the Arctic changes in ice cover are a particularly good example of positive feedback, as is the role of methane. Reduction of ice cover changes the albedo so that more heat is absorbed, warming the water and reducing ice cover still further. As the permafrost melts it may release methane, a powerful greenhouse gas, potentially raising global temperature and causing further melting of permafrost and release of methane. Paradoxically, the possible effect of ice melt on the ocean currents could provide a form of negative feedback. If the warm current flowing north past north-west Europe was to cease, then this would produce a major cooling effect. Whether this would be enough to restrict further ice melt is an interesting question.

It is ironic that anthropogenic climate change driving sea ice and permafrost retreat means that more oil, coal and gas fields are becoming accessible.

I shall leave the last word at the same point as I started this part: Ursus maritimus – the sea bear. Figure 1 showed the area used by one polar bear and computer models can predict the effects of anthropogenic climate change on these areas. The story is complex but the message is stark and clear. The bear habitats will decrease in extent in the near future. It may soon be possible to see bears only in northern Greenland and the Canadian archipelago. Whether you believe this is an issue of concern depends on both your moral and political opinions. The evidence of change is too clear to ignore. If you decide that as a remote region the Arctic is not relevant then it could be that you are missing the point. The positive feedbacks and global environmental flows mean that the Arctic will not only be a victim of climate change but a source for some of the changes that we may have to adapt to, such as rising sea levels from the melting Greenland ice cap. It is therefore possibly more than just a barometer of global change.

To be more literate, the Jacobean poet John Donne wrote in the seventeenth century, before the Arctic was mapped,

No man is an island, entire of itself; every man is a piece of the continent, a part of the main … never send to know for whom the bell tolls; it tolls for thee.
(Donne, Meditation XVII, 1624)

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Summary

The Open University — Wed, 04 May 2011 11:18:29 GMT

In this part I have presented evidence showing that even apparently remote regions of our planet are intimately connected through physical processes. For example, once an organic POP is transported to the poles, then biological processes can take over and through bioaccumulation perhaps cause harm. But this physical connection has allowed the ice to preserve unique proxy records of the past climate of our planet. Directly measuring the gases trapped in the ice has enabled histories of past atmospheric CO₂ and methane concentrations to be compiled, and we now know that the current atmospheric CO₂ concentration is higher than at any time in the last million years. It is stunning to think that human civilisation has only happened over the last 10,000 years, when the ice cores show that the climate has been uncharacteristically stable. However, humans have been affecting the climate for at least half of that time and the Arctic is now warming at a higher rate than almost all of the rest of the planet. Observations show that there are already significant regional changes that humans and animals will have to adapt to. Through feedback processes these regional changes will affect us all.

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Next steps

The Open University — Wed, 04 May 2011 11:18:29 GMT

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References

The Open University — Wed, 04 May 2011 11:18:29 GMT

Alley, R. B. (2000) The Two Mile Time Machine, Princeton, Princeton University Press.

Arctic Climate Impact Assessment (2005) ACIA Scientific Report, (Accessed December 2008).

Arnakak, J. (2000) ‘What is Inuit Qaujimajatuqangit?’, Nunatsiaq News, 25 August, p. 11.

Briggs, J. (1998) Inuit Morality Play: The Emotional Education of a Three-Year-Old, New Haven, Yale University Press.

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Acknowledgements

The Open University — Wed, 04 May 2011 11:18:29 GMT

The material acknowledged below is Proprietary and used under licence, see terms and conditions). This content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Licence

Grateful acknowledgement is made to the following:

Figures

Figure 3.1a Taken from Wiig, O., Born, Erik W. and Pedersen, Leif Toudal (2003) Polar Biology, 1-8-2003, 26, Julius Springer Verlag GmbH & co Kg;

Figure 3.1b George Durner/US Geological Survey, Alaska Science Center;

Figure 3.2 Courtesy of Arctic Monitoring and Assessment Programme (AMAP);

Figure 3.4 MacDonald et al. (2005) Recent climate change in the Arctic, Science of the Total Environment, vol. 342, issue1-3, 15 April 2005, Reprinted with permission from Elsevier Inc.;

Figure 3.5a Taken from wikipedia – public domain;

Figures 3.5b and 3.6 Fram Museum, Olso;

Figure 3.7 Courtesy of AMAP;

Figure 3.8a NOAA/Michael Morrison, GISP2 SMO, University of New Hampshire;

Figure 3.8b NOAA/Anthony Gow, US Army Corps of Engineers, Cold Regions Research and Engineering Laboratory;

Figure 3.9 Hong, S. et al. (1994) Greenland ice evidence of hemispheric lead pollution two millennia ago by Greek and Roman civilisations, Science, vol. 265, 23 September 1994;

Figure 3.12 Taken from wikipediaand used under Creative Commons Attribution 2.5 Licence. Redrawn by Hannes Grobe from a USGS graphic;

Figure 3.16 Taken from wikipedia (Opiola jerzy) and used under Creative Commons Attrubution 2.5 Licence;

Figure 3.18 Spreen, G., Kaleschke, L. and Heygster, G. (2008) Sea ice remote sensing using AMSR-E 89-GHz channels, Journal of Geophysics Research, doi:10.1029/2005JC003384. © Georg Heygster, University of Bremen;

Figure 3.19 Reprinted from: Turney, C.S.M. and Brown, H. (2007) Quaternary Science Reviews, with permission from Elsevier Inc.;

Figure 3.20 From Ruddiman, W.F. (2005) How did humans first alter global climate? Scientific American/Lucy Reading-Ikkanda;

Figure 3.21 Yang, D. et al (2002) Siberian Lena River hydrologic regime and recent change, Journal of Geophysical Research, vol. 107, 7 December, © American Geophysical Union;

Figure 3.23 Taken from Climate Impact As Cooperative Institute for Research in Environmental Sciences (CIRES),187 PART 4 Ways of knowing the arctic Arctic;

Figure 3.26 Global Outlook for Ice and Snow/United Nations Environment Programme 2007/Courtesy of UNEP;

Figure 3.27 MacDonald et al. (2005) Recent climate change in the Arctic, Science of the Total Environment, vol. 342, issue 1–3, 15 April 2005. Reprinted with permission of Elsevier Inc;

Figure 3.28 Taken from Arctic Climate Impact Assessment (ACIA);

Figure 3.29 Courtesy of Randall Munroe. Used u Creative Commons Licence 2.5;

Figure 3.30 From wikipedia (Joan Earner), UNEP/GRID-Arendal, Norway.

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RSS Feed for the unit Environment: Following the flows

Introduction

Learning outcomes

1 A climate change icon

Box 1 Pollution and bioaccumulation

2 The atmospheric and ocean flows

Table 1 The albedo of typical features on Earth

SAQ 1 The importance of albedo

Answer

Activity 1 The effect of specific heat capacity

Discussion

Box 2 Nansen and the voyage of the Fram

3.1 Greenland's snowfall

Study note: powers of ten and scientific notation

SAQ 2 Taking readings from a graph

Answer

3.2 The past temperature of the planet

Box 3 Proxies

SAQ 3 Proxy variables

Answer

SAQ 4 The Arctic defined during an ice age

Answer

Activity 2 Rates of change of temperature

Discussion

3.3 The Keeling curve

Activity 3 How representative is the Keeling curve?

Discussion

SAQ 5 Direct and proxy measurements

Answer

SAQ 6 Temperature and CO2 values

Answer

Box 4 Prediction, extrapolation and falsification

4 The end of the last ice age: the Holocene

Box 5 Wally Broecker's great ocean conveyor belt

SAQ 7 Recent climates

Answer

5 The contemporary Arctic climate

Activity 4 Recent climate change in the Arctic

Discussion

Activity 5 The changing mean albedo

Discussion

Box 6 Positive and negative feedback

SAQ 8 Arctic feedbacks

Answer

Summary

Next steps

References

Acknowledgements

Figures

Unit image

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SAQ 6 Temperature and CO₂ values