DRAMATIC forecasts of global warming resulting from a doubling of atmospheric carbon dioxide have been exaggerated, according to a peer-reviewed study by a team of international researchers.
In the study, published today in the leading journal Science, the researchers found that while rising levels of CO2 would cause climate change, the most severe predictions – some of which were adopted by the UN’s peak climate body in its seminal 2007 report – had been significantly overstated.
The authors used a novel approach based on modelling the effects of reduced CO2 levels on climate, which they compared with proxy-records of conditions during the last glaciation, to infer the effects of doubling CO2 levels.
They concluded that current worst-case scenarios for global warming were exaggerated.
“Now these very large changes (predicted for the coming decades) can be ruled out, and we have some room to breathe and time to figure out solutions to the problem,” the study’s lead author, Andreas Schmittner, an associate professor at Oregon State University, said.
Scientists have struggled for many years to understand how to quantify “climate sensitivity” – how Earth will respond to projected increases in atmospheric carbon dioxide.
In 2007, the UN’s peak climate body, the Intergovernmental Panel on Climate Change, warned that a doubling of CO2 from pre-industrial levels would warm the Earth’s surface by an average of 2C to 4.5C, although some studies have claimed the impact could be 10C or higher.
Professor Schmittner said it had been very difficult to rule out these extreme “high-sensitivity” scenarios, which were very important for understanding risks associated with climate change.
The study found high-sensitivity models led to a “runaway effect” under which the Earth would have been covered in ice during the last glacial maximum, about 20,000 years ago, when CO2 levels were much lower.
“Clearly that didn’t happen, and that’s why we are pretty confident that these high climate sensitivities can be ruled out,” he said.
Professor Schmittner said taking his results literally, the IPCC’s average or “expected” value of a 3C average temperature increase for a doubling of CO2 ought to be regarded as an upper limit.
“Many previous climate-sensitivity studies have looked at the past only from 1850 through to today, and not fully integrated paleoclimate data, especially on a global scale,” he said. “If these paleoclimatic constraints apply to the future, as predicted by our model, the results imply less probability of extreme climatic change than previously thought.”
However, he cautioned that extreme climate change could still occur in some areas.