Associate Professor Schmittner notes that many previous studies only looked at periods spanning from 1850 to today, thus not taking into account a fully integrated palaeoclimate data on a global scale.
The authors of the study stress that global warming is real and that increases in atmospheric CO2, which has doubled from pre-industrial standards, will have multiple serious impacts.
But more severe estimates that predict temperatures could rise up to an average of 10 degrees Celsius are unlikely, the researchers report in the journal Science.
The new study suggests temperatures will rise on average 2.3 degrees under the same conditions ::::
“When you reconstruct sea and land surface temperatures from the peak of the last ice age 21,000 years ago – which is referred to as the Last Glacial Maximum – and compare it with climate model simulations of that period, you get a much different picture,” said lead author Andreas Schmittner, from Oregon State University.
“If these palaeoclimatic constraints apply to the future, as predicted by our model, the results imply less probability of extreme climatic change than previously thought.”
Scientists have long struggled to quantify climate sensitivity, or how the Earth will respond to projected increases in carbon dioxide, the main greenhouse gas. Associate Professor Schmittner notes that many previous studies only looked at periods spanning from 1850 to today, thus not taking into account a fully integrated palaeoclimate data on a global scale. The researchers based their study on ice age land and ocean surface temperature obtained by examining ices cores, bore holes, seafloor sediments and other factors. When they first looked at the palaeoclimatic data, the researchers only found very small differences in ocean temperatures then compared to now.
“Yet the planet was completely different, huge ice sheets over North America and northern Europe, more sea ice and snow, different vegetation, lower sea levels and more dust in the air,” Associate Professor Schmittner said. ”It shows that even very small changes in the ocean’s surface temperature can have an enormous impact elsewhere, particularly over land areas at mid to high latitudes.”
Schmittner warned that continued, unabated use of fossil fuels could lead to similar warming of sea surfaces today.
Professor Colin Prentice from Macquarie University says he is not surprised by the results. Prentice, who was not involved in the study, says the new paper is based on a careful compilation of data and addresses an issue that is “absolutely central”.
“What it means is we can be a bit more sure about the sort of range of temperature changes that will result from the given change in the amount of fossil fuel and CO2 and other greenhouse gases,” said Prentice. ”The key point is that there has been ongoing buzz about the possibility that the climate sensitivity may be way, way higher than in mainstream climate models.
“So for very technical reasons with data just from contemporary observations and observations from the recent historical period, you just haven’t got enough information to really rule out those numbers. What [this study] has shown is that those very high values are ruled out, we still have a major issue about climate change, but it is much better quantified, much better pinned down.” said Prentice.