Fifty million years ago, the planet had little ice, yet global temperatures were on average no more than five degrees centigrade warmer than today - slightly less than the upper bound of temperature rise that the UN Intergovernmental Panel on Climate Change (IPCC) projects for 2100. Equally relevant, CO2 levels in the atmosphere were probably no higher than the IPCC has projected as a strong possibility for 2100.
For those still unconvinced about the severity of the problems we would encounter if temperatures and CO2 levels rise in this way, it is worth remembering that 50 million years ago sea-levels were several hundred feet higher than they are today. Most of that water is now ice in Antractica and Greenland - six metres of sea level equivalent is currently locked away in the Greenland ice sheet, six metres in the relatively small and unstable West Antractic ice sheet, and 60 m. in the huge East Antractic ice sheet.
Were these ice sheets to melt, vast areas of the planet - including Denmark and large parts of eastern Britain and Holland - would vanish in their entirety. It would be world in which we would have considerable difficulty surviving, not least because of extreme weather conditions, which, combined with the loss of huge areas of cropland, would play havoc with food production.
But, given that we only have records of surface temperatures and precipitation patterns going back a few centuries at best, how can we know what the Earth's climate was like hundreds of thousands, if not millions of years ago?
That is where the Antarctic comes in, not only because of its 2,400-metre-thick cap of ice, which covers 14 million sq. km., but also because of sediments off the land mass at Cape Roberts in the Ross Sea. The ice, like that drilled at the Russian Base, Vostok, yields information going back 400,000 years on temperature, CO2 content and sea-level. One of the revelations about those 400 millennia from the analysis of gas bubbles in these ice cores is that throughout this period there is a strong association between temperature and CO2 levels - they rise and fall together - confirming that the ‘greenhouse effect' of rising levels of heat trapping gases causing temperatures to climb is not just theory.
Analysis of Antarctic sediments allow us to look even further back into the past. The sedimens overlying Beacon sandstone of the Devonian age are 1,500m thick and date from 34-17 million years ago until the present. Drilling 100m into the underlying sandstone, meanwhile, takes one back still further, to beyond 100 million years ago.
Peter Barrett, from New Zealand's Antarctic Research Centre at Victoria University, has been part of a team of some 55 scientists from Australia, Britain, Germany, Italy, Netherlands, New Zealand and the US, who investigated the sediments. As he points out in New Zealand Science Review, sediments are important because ice-core records can take you back only so far, and nothing in the ice-core history shows CO2 levels in the atmosphere as high as we are likely to reach in a few decades. ‘Global climate, even in 50 years' time,' he says, ‘may be warmer than the Earth has experienced in the past 12 million years.' From fossils in sediments, as revealed by tree stumps, leaves and coal seams, we know that 200 million years ago, Antarctica was covered in forests and swamps. Antarctic temperatures then were at least 15°C warmer than today and, consistent with that, average global temperatures were some seven to eight degrees centigrade warmer than now.
Two distinct factors may have been responsible, then, for a warmer, vegetation-covered Antarctica. During the Cretaceous and Early Cenozoic periods, between 136 and 54 million years ago, we know that atmospheric levels of CO2 were high, and certainly responsible for part of the warming. Also, at that time, and certainly responsible for part of the warming. Also, at that time, Antarctica was still part of the supercontinent of Gandwanaland. Once that continent began to beak up, Antarctica became increasingly cut off by a strong polar air circulation system and consequently a cold circumpolar ocean current.
As a result, the first ice-sheets formed over Antarctica 34 million years ago, and then, as the Earth cooled still more, some 2.5 million years ago, the ice sheet formed for the first time over Greenland in the northern hemisphere. From then on, we have had ice ages affecting both poles.
Since the Arctic has never been as isolated as Antarctica, greenhouse gas concentration was probably a critical factor in its impact on surface temperature. An additional factor was the degree to which the warm waters of the Gulf Stream penetrated into the Arctic Circle. At the other pole, in all its probability, the cold circumpolar current contributed most to the chilling of Antarctica, in which case it might take more than elevated CO2 levels to bring about a complete melting of the Antarctic ice-sheet.
Yet, as the ice-core data shows, the expansion and retreat of the ice-sheet during the glacial and inter-glacial periods have always been associated with swings in temperature that themselves closely correlate closely with levels of CO2 in the atmosphere, and changes in sea-level. Some 18,000 years ago, when the last ice-age was at its most intense, CO2 levels were 30 per cent below 1900 levels and sea-level was 120 m. below present sea-level. That should warn us that whatever regulates greenhouse gas concentrations in the atmosphere could have profound effects on climate.
In fact, we do not know which triggered which in the past: whether higher CO2 concentrations in the atmosphere triggered temperature rise or whether temperature rise triggered higher CO2 concentrations. In all likelihood, one affected the other. But we do know that the initial cause of changes in temperature and greenhouse gas concentrations in the past was almost certainly the changing obliquity and eccentricity of the Earth's orbit and the movement of the Earth from side to side (known as the Malenkovitch Wobble) which together determine changes in the pattern of solar energy reaching the Earth.
The Antarctica data are the best records we have showing correspondence between the retreat and then re-establishment of the ice sheet in relation to the Earth's orbit around the sun. Until 800,000 years ago, the glacial cycle lasted some 40,000 years, but then lengthened into the current 100,000 cycle. The two periodicities, 40,000 years and then 100,000 years happen to conform respectively to orbital changes.
But orbital changes take place over millennia and cannot explain the sudden changes in climate we are now experiencing in one century. Today, it is human activity - with the burning of fossil fuels and the destruction of carbon-absorbing soils and forests - which is filling the atmosphere with greenhouse gases and causing temperatures to rise and the climate to change. As Peter Barrett says: ‘Changes in Earth's climate of this speed and magnitude are unprecedented to our knowledge, aside from large meteorite impacts.'
Nevertheless, the impact of orbital changes in the past are still relevant because of how ice-core records tell us life responded to them. In the past, life seems to have taken advantage of the changing conditions, either embarking on a spate of photosynthetic activity, which deposits organic carbon, or, suddenly embarking on a feverish burning up of the surplus carbon store. These clues from the past suggest that we should expect even more dramatic changes in climate than suggested by the forecasts of the IPCC.
Of particular importance is the climate-modulating role of certain plankton - namely coccolithophores - which contribute significantly to the cooling of the Earth's surface, especially over the oceans, by forming clouds which reflect sunlight and by drawing down CO2 in photosynthesis, depositing it as calcium carbonate on the ocean floor.
As the ice-core record shows us, the periods when plankton are thriving correlate with the occurrence of ice ages and with falls in atmospheric CO2 concentrations. When phytoplankton activity is depressed, which appears to be correlated with warm inter-glacial periods, then CO2 levels rise.
The lessons for us now are instructive. If, as ice core records reveal, plankton activity is at a peak when the climate is colder, we can expect rising temperatures today to trigger a decline in phytoplankton activity, causing temperatures to rise still further.
The evidence from polar ice is of sudden spurts in the emissions of greenhouse gases from purely natural sources, with temperature increasing in tandem. Positive feedback mechanisms are clearly at work, yet they are nowhere to be seen in the current general climate models that the IPCC uses to predict future climate change.
If we are to learn anything from the distant past it is that we should take all precautions not to perturb a system, which at some unknown critical point, jumps violently into a very different state. The fate of Antarctica, that vast polar continent, sheared off from the rest of the living world, has warned us of that.
By Peter Bunyard
© Ecologist Asia, Vol. 10, No. 1
