Historical time-variation in climate sensitivity related to the nature of the forcing - ICAS external seminar

Institute for Climate and Atmospheric Science. Speaker Jonathan Gregory, University of Reading.

Abstract: The CMIP5 historical model-ensemble mean shows significant time-variation on decadal timescales in climate sensitivity (which is inversely proportional to dR/dT, the slope from regression of global-mean TOA radiative response R against global-mean surface air temperature T in 30-year periods). Climate sensitivity is relatively high around 1930 and since 1980, and relatively low around 1900 and 1960. By comparison with historicalGHG and historicalNat, it appears that the low climate sensitivity is due to volcanic forcing, which causes briefly dominant large excursions in R and T, and the high climate sensitivity is the value applicable to greenhouse-gas (GHG) forcing. The variation occurs because in decades around 1930 there were no large volcanic eruptions, so the GHG forcing was unopposed, and in recent years volcanic forcing was overwhelmed by GHG forcing due to the rapid increase in greenhouse-gas concentrations while anthropogenic aerosol forcing has been fairly steady. These decadal variations in climate sensitivity cannot be discerned in any single historical AOGCM integration, because they are obscured by unforced variability, but decadal variations in climate feedback of similar magnitude are found in the AGCM amip-piForcing experiments with observed SSTs, despite the real-world history being only a single realisation. Moreover amip-piForcing more closely resembles the historicalNat experiment than the historical experiment, especially in that amip-piForcing and historicalNat both indicate exceptionally low climate sensitivity since the 1980s. The time-variations of climate sensitivity are related to the geographical patterns of SST change, which cause changes in R that are not directly related to variations in T.

Bio: I am employed at two institutions:

NCAS-Climate, located in the Department of Meteorology of the University of Reading (80% of my time), since 2003. I am a senior scientist in the core team and a professor in the department.

Met Office Hadley Centre in Exeter (20%) of my time, since November 1990. I am a Met Office Science Fellow in climate change at the Hadley Centre.

I served as a lead author of the projections and ocean observations chapters and the technical summary of the IPCC WG1 Fourth Assessment Report (2007), and of the sea-level chapter and the technical summary of the IPCC WG1 Fifth Assessment Report (2013). The European Research Council funded my project "Seachange", on sea-level change due to ocean density and circulation change, with an Advanced Grant during 2010-2016, and they have offered an Advanced Grant for my project "Couplet", on transient climate change in the coupled atmosphere--ocean system. I was awarded the FitzRoy prize of the Royal Meteorological Society in 2016 for my work on sea-level and climate sensitivity, and elected a Fellow of the Royal Society and a Fellow of the American Geophysical Union in 2017. My research interests in both places relate to mechanisms of global and large-scale change in climate and sea level on multidecadal and longer timescales. I work with colleagues in both Reading and Exeter on these subjects, as well as with collaborators in other universities and climate centres; I hope to promote more such contacts between the academic community and the Met Office in climate change research.

The seminar will be followed by cake and coffee in the School foyer.