Causes of climate fluctuation

To be able to see the current changes in climate in context, it will be necessary to consider the causes of dramatic changes in the past.

A major cause of climate fluctuation has been the variation in the Earth's axial tilt and the path of its orbit round the sun. The Earth is subject to the influence of neighbouring planets. Their orbits produce a fluctuating gravitational pull on the Earth, affecting the angle of its axis. As the Earth wobbles, vast ice sheets wax and wane over a cycle called a Milankovitch cycle. However, thanks to the stabilising pull of the moon, the variation in tilt is contained within limits which preserves the integrity of the seasons. Without the moon, the axis could move to 90 degrees from the vertical meaning that half the planet would have permanent summer and the other endless winter.

It has been calculated that the current orbital configuration is similar to that of the warm interglacial period 400 000 years ago. We may indeed be in the early stages of an interglacial episode and the accompanying natural warming which is being augmented by human induced warming. (For more information on climate fluctuations over the past million years see Houghton J. (2004) Global Warming, 3rd edn, Cambridge University Press.)

A second factor forcing climate change is the movement of tectonic plates and the resultant formation of volcanic mountains. In themselves mountains add to the stirring effect on the atmosphere in concert with the rotation of the Earth. They also generate fluctuations in atmospheric pressure, all of which affect climate.

But it is volcanic activity which can cause dramatic changes. The surface of the Earth is constantly shifting. The collision of plates accounts for the formation of mountains. A feature of plate tectonics is that, when plates collide, one plate slides under the other; this is called subduction. In the process rocks are heated and forced through the surface as volcanoes, releasing vast quantities of debris and CO2 in the process. In the short term this can lead to a cooling as the dust cuts out solar radiation. In the longer term, large injections of CO2 lead to warming, since CO2 has a relatively long life in the atmosphere.

A third factor may be a consequence of the second. Paleoclimate data show that there have been periodic surges of ice flows into the north Atlantic which, in turn, affect the deep ocean currents, notably the Gulf Stream. To understand why the ice flows affect the Gulf Stream we need to look at what drives this rather special current.

Particularly salty and warm surface water migrates from the tropics towards the north Atlantic. As it moves north it gradually becomes cold and dense, and, as a consequence, near Greenland it plunges to the ocean floor. This, in turn, draws warmer water from the tropics which is why it is also called the conveyor belt or deep ocean pump. It accounts for 25 per cent of the heat budget of northwest Europe. So, what is the relevance of the icebergs?

As these armadas of icebergs melted as they came south they produced huge amounts of fresh water which lowered the density of surface water undermining its ability to descend to the ocean floor. The effect was to shut down the conveyor belt. As a result northern Europe was periodically plunged into arctic conditions and scientists are concerned that there is now evidence that this process is beginning to happen due to melting ice in the southern tip of Greenland. After the melted iceberg water had dispersed, the conveyor started up again leading to rapid warming. This cycle occurred 20 times in 60 000 years, and the evidence indicates that cooling was relatively slow whilst warming was rapid - 10-12°C in a lifetime. For some reason these forays of icebergs stopped about 8000 years ago, creating relatively stable conditions which facilitated the development of agriculture and ultimately the emergence of urban civilisations.

A fourth factor may seem ironic, because ice ages can be triggered by warm spells leading to the rapid expansion of forests. This, in turn, leads to huge demands for CO2 which is drawn from the atmosphere. The result of this stripping of atmospheric CO2 is a weakening of the greenhouse shield, resulting in sharply dropping temperatures.

Changes in energy levels emitted by the sun are also implicated in global fluctuations. In June 1999 the journal Nature (vol. 399, p. 437) published research evidence from the Rutherford Appleton Laboratory in Didcot, Oxfordshire which suggests that half the global warming over the last 160 years has been due to the increasing brightness of the sun. However, since 1970 the sun has become less responsible for the warming, yet the rate of warming has been increasing, indicating that increased greenhouse gases are the culprit. Some of the best evidence for the climatic effects of varying levels of radiative output from the sun comes from Africa. Sediment in Lake Naivasha in the Kenya Rift Valley reveals the levels of lake water over the past 1000 years. Periods of high water have higher concentrations of algae on the lake floor which translates to a higher carbon content in the annual layers of sediment. There were long periods of intense drought leading to famine and mass migrations, the worst being from 1000 to 1270 (Nature, vol. 403, p. 410).

Finally, we cannot ignore wider cosmic effects. The dinosaurs will testify to the effect on climate of meteor strikes creating perpetual night. New sites of catastrophic impacts are still being discovered on the Earth, but if we want a true picture of the historic record of meteor impact we can see it on Venus. The stability of that planet - no plate movement or vegetation to hide the evidence - ensures that we have a picture of meteor bombardment over hundreds of millennia. The Earth will have been no different.

There is strong historic evidence that life on Earth has a precarious foothold.

The palaeontological record shows that there have been five mass extinctions in the recorded history of the planet. The most widely known on the popular level is the final one which occurred at the end of the Cretacious period 65 million years ago. It is widely attributed to one or more massive meteorites that struck the Earth propelling huge quantities of debris into the atmosphere masking the sun probably for years. Photosynthesising plants were deprived of their energy source and food chains collapsed resulting in the extinction of 75-80 per cent of species, notably the dinosaurs.

However, of all the other mass extinctions, it is the third in the sequence that warrants most attention because it has contemporary relevance. At the end of the Permian period, 251 million years ago, a catastrophic chain of events caused the extinction of 95 per cent of all species on Earth. The prime cause was a massive and prolonged period of volcanic eruptions, not from mountains but from extensive fissures in the ground in the region which ultimately became Siberia. A chain of events caused massive expulsions of CO2 into the atmosphere which led to rapid warming and plant growth. This had the effect of stripping much of the oxygen from the atmosphere leading to a collapse of much of the biosphere. Plants and animals literally suffocated. For the next 5 million years the remaining 5 per cent of species clung to a precarious existence. It took 50 million years for the planet to return to anything like the previous rate of biodiversity (New Scientist, 26 April 2003, 'Wipeout').

The importance of this evidence lies in the fact that this mass extinction occurred because the planet warmed by a mere 6°C over a relatively short period in the paleoclimate timescale. Why this should concern us now is because the world's top climate scientists on the United Nations Inter-Governmental Panel on Climate Change (IPCC 2002) estimated that the Earth could warm to around 6°C by the latter part of the century unless global CO2 emissions are reduced by 60 per cent by 2050 against the emissions of 1990.

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