Abstract
Equilibrium climate sensitivity (ECS) to doubling of atmospheric CO2 concentration is a key index for understanding the Earth’s climate history and prediction of future climate changes. Tropical low cloud feedback, the predominant factor for uncertainty in modeled ECS, diverges both in sign and magnitude among climate models. Despite its importance, the uncertainty in ECS and low cloud feedback remains a challenge. Recently, researches based on observations and climate models have demonstrated a possibility that the tropical low cloud feedback in a perturbed climate can be constrained by the observed relationship between cloud, sea surface temperature and atmospheric dynamic and thermodynamic structures. The observational constraint on the tropical low cloud feedback suggests a higher ECS range than raw range obtained from climate model simulations. In addition, newly devised modeling frameworks that address both spreads among different model structures and parameter settings have contributed to evaluate possible ranges of the uncertainty in ECS and low cloud feedback. Further observational and modeling approaches and their combinations may help to advance toward dispelling the clouds of uncertainty.
Highlights
Physical predictions of the Earth’s climate variability and changes are ongoing challenges for the geoscience community
Historical observations including satellite and implementation of satellite simulator enable to evaluate the reproducibility of the modeled clouds and its variations on seasonal, interannual, and decadal timescales
The accumulated observations suggest that the large-scale controls of low cloud cover (LCC) can be applied to the long-term cloud change under global warming
Summary
Physical predictions of the Earth’s climate variability and changes are ongoing challenges for the geoscience community. Enhanced cold air advection near the surface is concurrent with stronger convective mixing in the BL and increased LCC (Klein et al 1995; Mansbach and Norris 2007; Myers and Norris 2015) These observations and modeling-based relationships between the LCC and the large-scale conditions can be applied to longer-term cloud changes including cloud feedback in a warming climate (see section). Recent studies applied the observed relationship between the LCC variation and the large-scale atmospheric and SST conditions (Table 1) for reducing the uncertainty in the low cloud feedback in a changing climate (e.g., Dessler 2013; Zhai et al 2015; Qu et al 2015b; Myers and Norris 2016; Brient and Schneider 2016). Further works are needed to evaluate the uncertainty range of cloud feedback and constrain the ECS uncertainty among multiple PPEs developed by different climate models
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