Abstract
A commonly-used model of the global radiative budget assumes that the radiative response to forcing, R, is proportional to global surface air temperature T, R=lambda T. Previous studies have highlighted two unresolved issues with this model: first, the feedback parameter lambda depends on the forcing agent; second, lambda varies with time. Here, we investigate the factors controlling R in two atmosphere–slab ocean climate models subjected to a wide range of abrupt climate forcings. It is found that R scales not only with T, but also with the large-scale tropospheric stability S (defined here as the estimated inversion strength area-averaged over ocean regions equatorward of 50^circ {}). Positive S promotes negative R, mainly through shortwave cloud and lapse-rate changes. A refined model of the global energy balance is proposed that accounts for both temperature and stability effects. This refined model quantitatively explains (1) the dependence of climate feedbacks on forcing agent (or equivalently, differences in forcing efficacy), and (2) the time evolution of feedbacks in coupled climate model experiments. Furthermore, a similar relationship between R and S is found in observations compared with models, lending confidence that the refined energy balance model is applicable to the real world.
Highlights
The response of the climate system to external forcing is often interpreted using the global top-of-atmosphere energy balance framework N =F+R=F+ T (1)(Gregory et al 2002), which states that the net radiative imbalance N equals the sum of the effective radiative forcing F (Sherwood et al 2015) and the radiative response R, which is assumed to scale with the global surface air temperature anomaly T
The slab models are subjected to a variety of forcing agents, including greenhouse gases ( CO2, CH4 ), solar irradiance ( S0 ), tropospheric sulphate aerosol ( SO4 ), black carbon aerosol (BC), volcanic aerosol (VOLC), ocean heat uptake (OHU), and idealized, uniform surface forcings (UNIF)
The range of R/T that we find in equilibrium climate change for a range of forcing agents indicates that the assumption of proportionality R ∝ T is not accurate
Summary
(Gregory et al 2002), which states that the net radiative imbalance N equals the sum of the effective radiative forcing F (Sherwood et al 2015) and the radiative response R, which is assumed to scale with the global surface air temperature anomaly T. Can vary in time; large variations in occurred during the historical period (Gregory and Andrews 2016; Zhou et al 2016; Andrews et al 2018), and in most coupled climate models, climate feedbacks evolve towards more positive values over time under CO2 forcing (e.g., Murphy 1995; Senior and Mitchell 2000; Winton et al 2010; Andrews et al 2012; Armour et al 2013; Andrews et al 2015; Proistosescu and Huybers 2017; Ceppi and Gregory 2017) These issues suggest that the radiative response may depend on variables other than just global surface temperature. The depth of the slab is set to 50 m everywhere in HadSM3, whereas it varies spatially in CAM4-SOM, being determined from a reference coupled atmosphere-ocean simulation
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