by Fred Pearce

From NewScientist, March 22, 2008

CLIMATE sceptics are right. Temperature increases do precede rises in atmospheric carbon dioxide - the opposite of what you would expect if changes in CO₂ levels were really driving climate change. That's the verdict of leading atmospheric modeller Peter Cox, a climate expert at the University of Exeter, UK. Yet far from dismissing the threat of global warming, Cox says this means things are worse than we thought. Events in the Little Ice Age, 400 years ago, prove the point, he says.

One of the most important pieces of evidence linking climate change to greenhouse gas emissions is that for tens of thousands of years, temperature changes have been in lockstep with atmospheric levels of carbon dioxide. But sceptics keep pointing out that temperature changes seem to come first.

That charge is true, says Cox. "In climate predictions, we have been in denial about how temperature changes CO₂." But that certainly does not mean we don't need to worry about rising CO₂ levels, he stresses. "People on both sides want a one-way link, but the historical record shows that causality goes both ways." Rising - or falling - temperatures and CO₂ concentrations reinforce each other. Embarrassingly for climate modellers, Cox added: "Actually, CO₂ is more sensitive to temperature than the other way round." This is supported by a study of the Little Ice Age by Cox and colleagues (Geophysical Research Letters, vol 33, p L10702).

The cool period began with reduced solar radiation reaching the Earth due to natural variation in sunspots. But after about 50 years, CO2 levels fell and this amplified the cooling. This is not surprising, says Cox, because in colder conditions oceans absorb more CO2, and the carbon cycle on land slows, absorbing yet more.

"There seems to be a change of about 40 parts per million (ppm) in CO2 levels for every 1 °C change in temperature," says Cox, who has revisited the Little Ice Age data. Since further global warming is inevitable in the near future, it means we're heading for big natural increases in CO2 on top of human-made emissions.

This extra increase will boost global warming in the coming century to about 50 per cent above mainstream climate projections, says Cox, because they only include the effect of CO2 on temperature, and not temperature's effect on CO2.

"The system turns out to be more sensitive than we thought. If we get 4 °C of warming in the coming century, that by itself will raise CO2 levels by an extra 160 ppm. And that may be rather conservative." Current levels are 380 ppm, compared with pre-industrial levels of 270 ppm. Many scientists believe anything above 450 ppm will create a devastating global climate.

Cox's findings were among many unnerving observations about past climate change presented at the meeting suggesting existing climate models are too optimistic.

"We are headed into unknown territory and the only things we have to guide us are physics and our knowledge of the past," says Gavin Schmidt, a climate modeller at NASA's Goddard Institute for Space Studies in New York City.

Link for subscribers: http://environment.newscientist.com/channel/earth/mg19726483.800
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