(An edited version of this post appeared in The Conversation on June 23 2014)
Dire warnings of imminent man-made climate disaster are constantly in the news but predictions of the end of the world have been made throughout history and have never yet come true. Natural climate change has always been with us, whether it’s the volcanically induced crop failures that helped precipitate the French Revolution, or the Mediaeval Warm Period that allowed Vikings to colonise Greenland. And how can we trust that computer models are reliable facsimiles of the true climate rather than bug-ridden cludges tuned to give the politically correct answer? We have a healthy reluctance to take experts’ words for anything and so we’d like to be shown the evidence. Unfortunately, that’s difficult when the details are buried under hundreds of thousands of lines of computer code which implement mathematical algorithms of mind-numbing complexity.
This is where paleoclimatology, the study of Earth’s past climates, can come in. Fossils show links between global-temperatures and carbon-dioxide levels that are literally written in stone. Fossil plant-leaves from fifty-five million years ago have a microscopic structure accurately reproduced in modern plants only when grown in a carbon-dioxide-rich atmosphere. Is it a coincidence that, at that time, it was so warm that crocodiles were living within the Arctic Circle? A back of an envelope calculation suggests that carbon dioxide concentrations were about four times present-day levels whilst global-mean temperatures were around 7°C higher. For comparison, the largely computer based predictions published by IPCC imply that quadrupling CO2 should boost temperatures by between 3°C and 9°C. I’m not arguing that this nails the case for a worryingly strong link between carbon dioxide and temperature, but it is good supporting data. Furthermore, this evidence is particularly easy to understand and particularly hard to refute.
More generally, a sedimentary-record covering half a billion years shows us exactly what we’d expect to see if climate modellers have done their sums right. Fossil and chemical traces in rocks indicate that warm periods in Earth’s history are associated with higher concentrations of carbon dioxide. Quantitative studies show that this correlation is, if anything, even stronger than predicted.
There are, of course, counter-arguments. Some important processes, such as ice-sheet disintegration, only affect climate very slowly and this explains why the long-term climate sensitivity of our planet seems to be even worse than predicted by computer models. The existence of potential complications, like these, is one of the key reasons why multiple approaches are needed but, if different researchers using different methods nevertheless come up with more or less the same answer, perhaps they’re onto something.
Another objection might be that ancient climate change is really evidence for varying solar luminosity, or varying cloud-cover caused by cosmic-ray fluctuations, or any one of a number of other exotic proposals. Fluctuating carbon dioxide levels are then a response to climate variation rather than the cause. However, solar physics suggests that the Sun was 0.5% fainter, 55 million years ago, rather than the 4% brighter needed to explain the warmer climate and there is no experimental or theoretical justification for any of the other unconventional explanations either. Why prefer completely unsubstantiated theories to the fact, confirmed experimentally one hundred and fifty years ago, that carbon dioxide absorbs infra-red radiation?
One final argument is that there’s nothing we can do anyway, because China and other rapidly developing countries will dominate CO2 output in the 21st century. However, that is completely irrelevant if we’re simply asking: “Will increased CO2 dangerously change our climate?” The fact that political and technical problems are massively more complex than anything in climatology is not a reason to stick our heads in the sand. Wide-spread agreement, that man-made global-warming is highly likely, would be progress.