How much will average temperatures rise by 2100 if humanity does not rein in the use of fossil fuels? The scientific consensus, published by the Intergovernmental Panel on Climate Change (IPCC), estimates this to be between 2.6 and 4.8 degrees Celsius above pre-industrial levels. However, a new study published in Science Advances finds that the climate has historically responded more aggressively to greenhouse gases during warm phases and concludes that extreme temperature rises going above the IPCC estimate are possible.
To understand how climate sensitivity to greenhouse gases has changed over long time spans, the paper uses “paleoclimate data”: temperature projections at the sea surface spanning the last 784,000 years. These historic temperature records can be estimated from particular molecules found in the deep sea, among other methods. After using this to gauge the surface air temperatures, the researchers look at the impact of dust and greenhouse gases. During glacial periods – intervals of time during an ice age – radiation effects had a fairly small impact on anomalies in the surface air temperature. But warmer periods such as the one we live in today showed an increased climate sensitivity that has not been accounted for in previous models. Under this scenario, the study finds a likely temperature increase of 4.78 to 7.36 degrees Celsius by 2100, assuming the most pessimistic greenhouse emission scenario will come true.
The authors make sure to point out uncertainties in their model. The reconstruction of historic temperatures and radiation levels is difficult and can range widely between different studies, affecting the estimate of climate sensitivity. This uncertainty is best resolved by looking at the collective data gathered across multiple studies – and the overlap with other models indeed suggests that reductions in greenhouse emissions are essential to avoid potentially large temperature rises.
Last year, world leaders signed up to the Paris Agreement postulating a long-term goal to keep the rise in global average temperatures below 2 degrees Celsius to avoid the most extreme consequences of climate change. Only last week, news broke that global sea ice levels are near an all-time low for this time of year since records were kept, contributing to fears of rising sea levels across the globe. Analysing climate sensitivity over the last 784,000 years suggests that avoiding such effects of global warming may be even harder than previously assumed, and therefore require more political efforts.
This article was written by Hans-Joachim Sonntag and edited by Teodora Aldea.