Commissioned by the World Climate Research Program, an international team of 25 scientists, with participation from the DOE’s Lawrence Livermore National Laboratory, worked over 4 years to assess all evidence related to climate sensitivity. Combining temperature records since the industrial revolution, paleoclimate records of prehistoric temperatures, satellite observations since 1980, and detailed models that examine the physics of climate feedbacks such as clouds, the team found that the lines of evidence corroborated each other such that climate sensitivity is likely (66% chance) between 2.6 and 3.9 °C in baseline calculations with >66% chance of being between 2.3 and 4.5 °C under other plausible assumptions.
The severity of climate change is closely related to how much the Earth warms in response to greenhouse gas increases. For 40 years, the uncertainty range of climate sensitivity has stubbornly been fixed at between 1.5 and 4.5 °C. By convincingly narrowing the uncertainty range in this key quantity, the study makes possible a more confident view of what will happen as greenhouse gasses increase.
Climate sensitivity, the global surface temperature response to CO2 doubling, has been uncertain for decades. Evidence relevant to climate sensitivity comes from areas of science as diverse as the changes in temperature from past ice ages to the simulations of clouds by large-eddy simulation models. The team developed a novel Bayesian methodology which permitted the first-ever rigorous combination of evidence from the 3 major lines of evidence: (1) the amount of warming since the late 1800s, (2) the changes in temperature found in paleoclimate records, and (3) the study of climate feedbacks using modern satellite observations and detailed cloud models such as large-eddy simulations. The three lines of evidence were found to largely agree such that climate sensitivity is likely in the middle or upper portions of the canonical 1.5˚C and 4.5˚C range. The fact that the three lines of evidence agree – and that these lines of evidence are independent – means there is increased confidence in the resulting narrower range of climate sensitivity.