“Simpson's Law” and the Spectral Cancellation of Climate Feedbacks

Abstract

These terms give the radiative response to vertically uniform warming (Planck), deviations from uniform warming (lapse, LR), changes in specific humidity (water vapor, WV), and changes in surface albedo and clouds. Precise definitions for the first three, which typically have the largest magnitude, will be given below.Although this decomposition has become fairly standard (Flato et al., 2013;Sherwood et al., 2020), it also suffers from various drawbacks. Perhaps the most basic drawback is that the conventional Planck feedback, which gives the "reference response" relative to which the other feedbacks are computed, is not a good null hypothesis for the system response (Roe, 2009): It assumes that specific humidity stays fixed with temperature change, even though we now know from theory, models, and observations that fixed relative humidity

An Analytical Model for Spatially Varying Clear-Sky CO2 Forcing

“…2a for various values of l. The value l 5 10.2 cm 21 minimizes the errors in this comparison and yields an excellent fit, and will be used henceforth. Note that this value is close to the l 5 11-11.5-cm 21 range reported in Jeevanjee and Fueglistaler (2020b) and Wilson and Gea-Banacloche (2012) from direct fits to the spectroscopy, and that all values in this range yield a reasonable fit in Fig. 2a.…”

“…These are reference absorption coefficients evaluated at a fixed pressure and temperature, which we take to be p ref 5 100 hPa and T ref 5 250 K. Here n denotes wavenumber (rather than frequency), n 0 5 667.5 cm 21 , k 0 5 50 m 2 kg 21 is a representative mass absorption coefficient at n 0 (discussed further below), and the ''spectroscopic decay'' parameter l 5 10.2 cm 21 sets the rate at which k ref declines exponentially away from band center. The parameters l and k 0 may be obtained by fitting (1) to modeled absorption spectra, but the parameters turn out to depend somewhat on details of the fit (Jeevanjee and Fueglistaler 2020b;Wilson and Gea-Banacloche 2012). Instead, we opt to determine these parameters via optimization as described in section 3.…”

Joint Dependence of Longwave Feedback on Surface Temperature and Relative Humidity

“…Note that the Planck, lapse rate, and water vapor feedbacks are included in λ cs . The moist adiabat is not satisfied in the mid-latitudes, but we note that the lapse rate feedback is small when RH is fixed (R. Cess, 1975;Held & Shell, 2012;Zelinka et al, 2020;Jeevanjee et al, 2021). We exclude the RH-feedback associated with a change in RH with surface warming for simplicity and because its value in the global mean is < 0.1 Wm −2 K −1 (Held & Shell, 2012;Zelinka et al, 2020).…”

“…We first address Question 2 by exploring the state dependence of cs E  as a function of both s E T and RH. We use RH as a state variable because RH-based feedbacks have certain advantages over specific humidity ( v E q) based feedbacks both from a thermodynamic point of view (Held & Shell, 2012) and from a radiative point of view (Jeevanjee et al, 2021), as specific humidity already has a de facto strong temperature dependence through the Clausius-Clapeyron relation. To compute radiative transfer we use PyRADS, a validated lineby-line column model (Koll & Cronin, 2019).…”

“…based feedbacks both from a thermodynamic point of view (Held & Shell, 2012) and from a radiative point of view (Jeevanjee et al, 2021), as specific humidity already has a de facto strong temperature dependence through the Clausius-Clapeyron relation. To compute radiative transfer we use PyRADS, a validated lineby-line column model (Koll & Cronin, 2019).…”