SLS Seminar: Justin Bandoro (MIT)

"Detectability of nonlinear historical forcings and their fingerprints on the climate system"

Evidence of a human influence on the climate system has grown stronger with each successive IPCC scientific assessment report. The formal identification of an anthropogenic climate change “fingerprint” has been achieved in observations of not only atmospheric temperatures, but many components of the earth system, including oceans, the hydrological cycle, and the cryosphere. Most detection and attribution (D&A) studies have been primarily concerned with identifying a single unique anthropogenic fingerprint that encompasses all man-made forcings, including greenhouse gases (GHG) and ozone depleting substances (ODS). But separating the detectability of individual forcings is important for understanding what is driving observed changes in the climate system. A traditional method for D&A studies estimates the climate system’s spatial response patterns (“fingerprints”), and then searches for temporal changes in the pattern similarity statistic between the identified fingerprints and observations. This method implicitly assumes that the forcing evolution is quasi-linear, which has been an adequate approximation for historical GHG forcing. But the historical evolution of stratospheric ozone forcing is highly nonlinear, because ODS emissions were curtailed by the implementation of the Montreal Protocol. This presents a nonlinear case for D&A, and I will present a new method developed to account for nonlinearities of historical forcings for spatial fingerprinting. I will first provide a proof of concept for this method, performing D&A of stratospheric ozone changes using the Whole Atmosphere Community Climate Model. We find markedly enhanced signal-to-noise ratios with the nonlinear method compared to the traditional linear metric, which permits confident detection of an ODS signal that is separate from the GHG signal. In the second part of the talk, I will apply the method to seasonal atmospheric geopotential height changes, to determine the detectability of both ODS and GHG fingerprints using the CMIP5 archive. Both GHG and ODS signals are found to be confidently detectable in the months of December-May.

About the Series

The Atmosphere, Ocean and Climate Sack Lunch Seminar Series is an informal seminar series within PAOC that focuses on more specialized topics than the PAOC Colloquium. Seminar topics include all research concerning the science of atmosphere, ocean and climate. The seminars usually take place on Wednesdays from 12-1pm in 54-915. The presentations are either given by an invited speaker or by a member of PAOC and can focus on new research or discussion of a paper of particular interest.