A team of 25 scientists, including ATMS and GEOL professor Cristian Proistosescu, has made progress on a 40-year old question: How much will the Earth warm if human activity doubles the concentration of atmospheric Carbon Dioxide? This number, called Equilibrium Climate Sensitivity, traces its history to the first comprehensive assessment of the effects of carbon dioxide on climate, prepared in 1969 by the National Academies of Sciences. This assessment, known as the Charney report, summarized its finding in terms of this Equilibrium climate sensitivity, and found a likely range of 1.5 to 4.5 degrees Celsius per doubling of CO2. This broad uncertainty has proven quite recalcitrant, as the likely range remained essentially unchanged through five assessment reports of the Intergovernmental Panel on Climate Change. The team, organized under the auspices of the World Climate Research Program (WCRP), has finally made progress by carefully bringing together three independent lines of evidence: basic physical understanding of atmospheric processes, observations of changes in Earths' temperature and heat content since the pre-industrial era, and the geologic record of climate change over the past few million years. By combining these different estimates in a novel statistical framework the team was able to constrain Equilibrium Climate Sensitivity to a likely range of 2.6 to 3.9 degrees Celsius for a doubling of CO2. The report has been published in Reviews of Geophysics: https://doi.org/10.1029/2019RG000678
This range rules out scenarios of less severe global warming, indicating a need for drastic action to reduce carbon emissions. At the same time, the results indicate that scenarios of extremely severe climate change, as suggested by several recent numerical climate models, are unlikely. The results can be summarized as: "We won't get any free passes on climate change, but we are not yet doomed, provided we act now"
New work reduces the uncertainty in our projections of how much the Earth will warm if Carbon Dioxide concentrations double.