[PAOCQ] Tim Cronin (MIT)

“Spectrally-resolved thinking about climate feedbacks”

A warmer earth radiates more energy in infrared wavelengths, and likely darkens slightly in solar wavelengths. When climate model projections are analyzed, these rates of change in global-mean radiative fluxes per degree of surface warming are expressed as feedback parameters (in units of W/m^2 per K of warming or W/m^2/K), and are usually integrated across all wavelengths. In this talk, I will discuss how investigating the spectral dimension of climate feedbacks can help deepen our understanding and reveal unquestioned assumptions.

In the first part of the talk, following Cronin & Dutta (2023), I will talk about the most widely used “no-feedback” radiative response to warming in terrestrial infrared wavelengths — often called the Planck feedback. The simplest estimate of this radiative response is given by the Stefan-Boltzmann law as ~3.8 W/m^2/K, yet analysis of climate models yields a Planck feedback averaging ~3.3 W/m^2/K. This difference of 0.5 W/m^2/K is large compared to the uncertainty in the net climate feedback, yet it was not studied carefully prior to our work. Using spectrally-resolved reasoning and radiative transfer models, I will discuss the main reasons for the 0.5 W/m^2/K difference and how it may matter for our thinking about climate change.

In the second part of the talk, I will turn to the shortwave. Water vapor is the dominant absorbing gas here, too, with a dense forest of absorption lines that extend out well into the near-infrared and weakly even into the visible spectrum. As water vapor absorption strengthens in a warmer world with more column water vapor, the planet darkens in solar wavelengths, leading to a shortwave water vapor feedback that is about 15% of the total water vapor feedback, or ~0.3 W/m^2/K. I will argue that we can obtain a back-of-the-envelope estimate of this feedback parameter by considering the spectroscopic properties of water vapor in the shortwave, in an atmosphere that contains other scattering substances and a mostly-absorbing surface.

PAOC Colloquium —

Interdisciplinary seminar series that brings together the whole PAOC (Program in Atmospheres, Oceans, and Climate) community. Seminar topics include all research concerning the physics, chemistry, and biology of the atmospheres, oceans and climate, as well as talks about societal impacts of climatic processes.

Contact: paoc-colloquium-comm@mit.edu