Tracking temperatures in Earth’s past

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In January, 2001, the Intergovernmental Panel on Climate Change (IPCC) released its four-yearly update on what the climate has been doing and what is likely to happen next. It found that over the past hundred years the global average temperature has risen by 0.6°C

This increase may not sound like much, but remember that at the peak of the last ice age, about 18,000 years ago, the global average temperature was about 4.5 degrees lower than it is now—cold enough to cause giant ice sheets to form over large parts of North America and northern Europe, locking up so much water that the level of the oceans was more than 100 m lower than the current sea level.

More importantly, the rise in temperature appears to be accelerating. The rise of 0.6 degrees is up 20 per cent on that found in the period up to 1994, because of recent warm years. Globally, it is very likely that the 1990s was the warmest decade and 1998 the warmest year in the instru­mental record since 1861. To what extent this unusu­ally warm period can be attributed to human activi­ties such as deforestation and the burning of fossil fuels has been the subject of intense debate.

To gain some insight into how rare the current warm conditions are, a great deal of research has gone into trying to work out what temperatures prevailed before thermometers became widespread.

The methods used to study past climates are as varied as they are ingenious. One of the most important involves the ratio of two isotopes of oxygen: oxygen-18 and oxygen-16.

Oxygen-16 is normally about 500 times more abundant in seawater than oxygen-18. However, oxygen-18 is heavier (because it has two more neutrons in the nucleus of each atom) and so water containing oxygen-18 does not evaporate as readily as water containing oxygen-16. Accordingly, the ratio of oxygen-18 to oxygen-16 in water vapour in the atmos­phere is slightly less than the ratio found in seawater. The extent of this difference depends on the temperature of the sea when the water evaporates.

Some of the water vapour in the atmosphere is carried by the wind to polar regions, where it falls as snow and becomes incorporated into the ice sheets. Ice cores drilled down through the icecaps of Greenland and Antarctica show layering for each year’s snow, enabling accurate dates to be assigned to when the snow fell, going back hundreds of thousands of years.

The ratio of oxygen-18 to oxygen-16 found in the annual layers enables climatologists to work out the temperature of the tropical oceans at the time the water evaporated.

The same approach can be used with oxygen extracted from the carbon­ates in fossil coral or from the skeletons of surface-dwelling plankton found in seabed cores. The oxygen isotope ratio indicates the temperature of the water the organism lived in. A variety of dating techniques can then be used to establish the age of the skeleton as far back as a million years.

On land, fossil pollen is one of the most important indicators of past climate, as it is extremely long-lasting and readily identified.

Pollen grains preserved in sediments thousands of years old show which plants were growing at that time. Plants have different sensitivities to rainfall, temperature and frost, so a record of what was growing enables a recon­struction of past climate to be made.

Tree rings are another important source of informa­tion about rainfall and temperature. Roughly speaking, trees growing on the edge of an arid region will have a large growth ring in a year of above-average rainfall, and trees growing near the edge of their temperature tolerance—say, halfway up a mountain—will have a large growth ring in a warmer-than-average year.

Tree rings can be dated to an actual year by counting back from the present. Performing such a count is easy to do if the tree is still alive. For trees that have died and been buried underground it is sometimes possible to match part of their sequence of thick and thin growth rings with those of a living tree, and so find out the age of the buried tree. The ring sequences of older buried tree trunks may then be found to overlap with part of the tree ring sequence of the dated tree. This process can be used to extend the sequence thou­sands of years into the past.

In some parts of the world, lake sediments are laid down regularly in spring, when rivers are at their highest. These sediments form distinct layers, called varves, the thickness and composition of which can be used to estimate the spring rainfall.

One lake in Sweden has layers which can be counted back 15,000 years, to around the finish of the last ice age.

Using proxy data, a temperature curve has been put together for the north­ern hemisphere, covering the past 1000 years. This graph shows that the recent warming and current high temperatures have no precedent during this period. It is therefore considered likely that part of the warming is due to an enhanced greenhouse effect caused by human activities.

The greenhouse effect in the atmosphere works as follows. Everything with a temperature above minus 273°C radiates energy. The characteristic wavelength of the energy depends on the temperature of the radiating body. The Sun’s surface has a temperature of around 5000°C, and radiates most of its energy as visible light. The Earth’s surface has an average temperature of around 15°C and radiates most of its energy in infrared wavelengths.

Most of the visible light from the Sun passes through the atmosphere and warms the land and oceans at Earth’s surface. However, a significant proportion of the Earth’s infrared radiation is absorbed by gases in the atmosphere. They re-radiate this infrared energy in all directions. A portion of this energy comes back down to the Earth’s surface where it is absorbed, thus further heating the Earth.

The natural action of the atmosphere as a global greenhouse is estimated to keep the Earth’s surface temperature about 30°C warmer than it would otherwise be, and it is this that makes the Earth habitable.

The most important greenhouse gas is water vapour, followed by carbon dioxide and methane. The burning of fossil fuels, such as coal and oil, has acceler­ated since the Industrial Revolution, greatly increas­ing the amount of carbon dioxide in the atmosphere.

Bubbles of gas trapped in ice laid down hundreds of years ago in the Greenland and Antarctic ice sheets have been analysed and show a level of carbon dioxide of about 280 parts per million (ppm) before 1750. The level has now reached 360 ppm and is projected to reach between 540 and 970 ppm by the end of this century, depending on how much effort is make to cut back on the burning of fossil fuels worldwide.

The amount of methane in the atmosphere is also increasing, up from 750 parts per billion before the Industrial Revolution to 1760 ppb now, and the level may double by the end of the century.

Air bubbles in the ice cores show that the current levels of both carbon dioxide and methane have not been exceeded in the past 420,000 years, and it is likely carbon dioxide has not been this high for the past 20 million years.

Further increases in the concentration of greenhouse gases is expected to intensify the greenhouse effect, causing further rises in atmospheric temperature. Elevated temperatures will, in turn, cause more water to evaporate, further fueling the greenhouse effect—a positive feedback system. However, increased water vapour is also likely to lead to in­creased cloud cover, which will reflect a greater propor­tion of incoming sunlight back out to space, thus ameliorating the greenhouse effect—a negative feedback system.

By juggling these and other feedback systems and using complex computer models of the atmosphere, the IPCC estimates that the global average temperature will rise by between 1.4 and 5.8°C by the end of the century. These increases are projected to cause a rise in sea level of between 9 and 88 cm, resulting mainly from the thermal expansion of seawater, with a smaller contribution from partial melting of glaciers and ice caps. The wide range in this estimate is a measure of uncertainties in the predic­tive models as well as in the projections of future carbon dioxide and methane emissions.

Just how much impact these changes will have on the way we live has opened up a rich field of speculation. Increased coastal erosion is on the cards, as well as major changes to agriculture, some of which may be beneficial. Pests and diseases are likely to spread into new areas, while biodiversity will continue to decline as species become extinct when their favoured climatic zones migrate up mountain slopes or towards the poles faster than they can.

An interesting perspective is offered by our increasing knowledge of the effects of past climate change on ancient societies. For example, the collapse of the Akkadian empire in Meso­potamia around 2200 BA:. coincided with a drought lasting a century or more. In Mesoamerica, sediment composition and isotope analysis of tiny ostracod crustacean shells in lake-bed cores show that the rapid decline of the Maya civilisa­tion occurred after the onset of a 200-year arid period.

In South America, ice cores taken from alpine glaciers reveal a centuries-long dry spell which coincided with the collapse of the Tiwanaku civilisation based around Lake Titicaca.

An estimated half a million people were sustained in urban centres by an agricul­tural system that exploited the use of raised fields. The technique of elevation promoted efficient nutrient recycling, and irrigation canals were used to ther­mally buffer crops against killing frosts.

The area was abruptly abandoned around 1100 A.D. Sediment cores from Lake Titicaca show a 10-metre drop in lake level as the dry spell progressed.

Coincidental observations do not, of course, prove that climate change was the cause of the collapse of these civilisations, nor is it likely to have been the only factor. However, the increasing number of examples of climate change and major disruption to society occurring at the same time make it highly likely that climate change has played a significant role in history.

I low much of a role it will play in the future we are about to find out.

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