The basis for our understanding and attribution of contemporary global warming is that carbon dioxide is a major greenhouse gas whose addition to the atmosphere would, in the absence of cooling drivers, be expected to lead to planetary energy imbalances and therefore warming.
With atmospheric carbon dioxide at levels not seen for at least 600,000 and possibly 25 million years, the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4) argued that the observed warming was very likely (greater than a 90 per cent probability) the result of human emissions of carbon dioxide and other greenhouse gases.
However, an important claim in the Channel 4 Great Global Warming Swindle was that at the termination of glaciations in the past, atmospheric levels of carbon dioxide lag rather than lead temperature change, and that this invalidates the idea that carbon dioxide is a major driver of climate change. Rather, the programme’s argument was that atmospheric carbon dioxide responds to temperature change and it used the geological record of changes in carbon dioxide and global temperature in support of this. It is therefore worth assessing the nature of the change in these variables over time in some detail.
Over the last two million years or so, the Earth has undergone repeated periods of glaciations, interspersed by periods of relative warmth called interglacials (like the present Holocene interglacial). The reason why these dramatic climate changes occurred in the past was unknown until work by the 19th century Scottish scientist James Croll developed theories based upon variations in the Earth’s orbit to account for the build up and decay of major ice sheets. This work was further refined by the Serbian geophysicist Milutin Milankoviæ in the 1930s and 1940s and the Milankoviæ-Croll hypothesis is now used to show how small changes in the amount of sunlight reaching earth, and changes in the location of maximum and minimum insolation, can drive continental-scale glaciations. These changes occur at regular intervals (cycling around 23,000, 41,000 and 100,000 year timescales) and the pattern and timing of glaciations/interglacials fits extremely well with these.
However, at the end of glacial periods it must take a lot of energy to melt ice sheets covering much of North America, and Northern Europe to a depth of a kilometre or so, and the global increase in energy produced by the suns rays falling on the Earth (probably less than 0.5 Wm-2) is insufficient to do this.
However, as the Earth warms up at the end of glaciations enormous amounts of carbon dioxide stored in permafrost and the oceans are released into the atmosphere and amplify the warming derived from insolation [solar energy] changes. This added greenhouse effect forms a crucial positive feedback on the warming, driving rapid melting of the ice sheets. Evidence from ice cores drilled into the Greenland and Antarctic ice sheets suggests that there is a lag between temperature rise caused by insolation changes and the degassing of carbon dioxide from the oceans and other carbon stores which amplifies this warming. Despite the argument put forward in the Great Global Warming Swindle this lag is not accurately known, but may be up to 800 years in length.
What is different and alarming today, is that we are depositing huge amounts of carbon dioxide into the atmosphere and this is driving global warming without any significant impact from changes in the sun’s activity. To use arguments about the role of carbon dioxide in the past to explain the very different conditions operating today is disingenuous at best and can be seen as a cynical attempt to sow confusion.
This article first appeared in the Ecologist March 2007