A colorful look at climate sensitivity

Abstract

Abstract. The radiative response to warming and to changing concentrations of CO2 is studied in spectral space. If, at a particular wavenumber, the emission temperature of the constituent controlling the emission to space does not change its emission temperature, as is the case when water vapor adopts a fixed relative humidity in the troposphere or for CO2 emissions in the stratosphere, spectral emissions become independent of surface temperature, giving rise to the idea of spectral masking. This way of thinking allows one to derive simple, physically informative, and surprisingly accurate expressions for the clear-sky radiative forcing, the radiative response to warming, and hence climate sensitivity. Extending these concepts to include the effects of clouds leads to the expectation that (i) clouds dampen the clear-sky response to forcing; (ii) diminutive clouds near the surface, which are often thought to be unimportant, may be effective at enhancing the clear-sky sensitivity over deep moist tropical boundary layers; (iii) even small changes in high clouds over deep moist regions in the tropics make these regions radiatively more responsive to warming than previously believed; and (iv) spectral masking by clouds may contribute substantially to polar amplification. The analysis demonstrates that the net effect of clouds on warming is ambiguous, if not moderating, justifying the assertion that the clear-sky (fixed relative humidity) climate sensitivity – which, after accounting for surface albedo feedbacks, we estimate to be about 3 K – provides a reasonable prior for Bayesian updates accounting for how clouds are distributed, how they might change, and deviations associated with changes in relative humidity with temperature. These effects are best assessed by quantifying the distribution of clouds and water vapor and how they change in temperature rather than geographic space.

Overview: Quasi-Lagrangian observations of Arctic air mass transformations – Introduction and initial results of the HALO–(AC)3 aircraft campaign

“…Nevertheless, the current ability to model them is still limited and, therefore, future model-based projections of Arctic climate changes are highly uncertain (Smith et al, 2019;Cohen et al, 2020;Block et al, 2020;Linke et al, 2023). In particular, the model representations of the effects and development of clouds (Pithan et al, 2014;Kretzschmar et al, 2020;Stevens and Kluft, 2023), and of the interactions of the atmosphere with sea ice, snow on sea ice, and ocean physics as well as biogeochemical feedback processes are challenging (Rinke et al, 2019;Huang et al, 2019;Pefanis et al, 2020). In addition, the role of aerosol particles in Arctic amplification has not been sufficiently investigated (Schmale et al, 2021;Dada et al, 2022;Gong et al, 2023).…”