Clouds damp the radiative impacts of polar sea ice loss

Ramdane Alkama,Giovanni Forzieri,Patrick C Taylor,Lorea Garcia-San Martin,Gregory Duveiller,Herve Douville,Alessandro Cescatti,Didier Swingedouw

doi:10.5194/tc-14-2673-2020

Abstract

Abstract. Clouds play an important role in the climate system: (1) cooling Earth by reflecting incoming sunlight to space and (2) warming Earth by reducing thermal energy loss to space. Cloud radiative effects are especially important in polar regions and have the potential to significantly alter the impact of sea ice decline on the surface radiation budget. Using CERES (Clouds and the Earth's Radiant Energy System) data and 32 CMIP5 (Coupled Model Intercomparison Project) climate models, we quantify the influence of polar clouds on the radiative impact of polar sea ice variability. Our results show that the cloud short-wave cooling effect strongly influences the impact of sea ice variability on the surface radiation budget and does so in a counter-intuitive manner over the polar seas: years with less sea ice and a larger net surface radiative flux show a more negative cloud radiative effect. Our results indicate that 66±2% of this change in the net cloud radiative effect is due to the reduction in surface albedo and that the remaining 34±1 % is due to an increase in cloud cover and optical thickness. The overall cloud radiative damping effect is 56±2 % over the Antarctic and 47±3 % over the Arctic. Thus, present-day cloud properties significantly reduce the net radiative impact of sea ice loss on the Arctic and Antarctic surface radiation budgets. As a result, climate models must accurately represent present-day polar cloud properties in order to capture the surface radiation budget impact of polar sea ice loss and thus the surface albedo feedback.

Highlights

Solar radiation is the primary energy source for the Earth system and provides the energy driving motions in the atmosphere and ocean, the energy behind water phase changes, and the energy stored in fossil fuels
Our results indicate that 66 ± 2% of this change in the net cloud radiative effect is due to the reduction in surface albedo and that the remaining 34±1 % is due to an increase in cloud cover and optical thickness
Considering the SW cloud radiative effect (SWcre) and LW cloud radiative effect (LWcre) components, we find that the SWcre (Fig. 3c, d) shows a similar pattern of correlation as the NETcre (Fig. 3a, b) but with a stronger magnitude, while LWcre generally shows the opposite correlations (Fig. 3e, f)

Summary

Introduction

Solar radiation is the primary energy source for the Earth system and provides the energy driving motions in the atmosphere and ocean, the energy behind water phase changes, and the energy stored in fossil fuels. A fraction (Loeb et al, 2018) of the solar energy arriving to the top of the Earth atmosphere (short-wave radiation; SW) is absorbed at the surface. Earth’s surface and atmosphere emit thermal energy back to space, called outgoing long-wave (LW) radiation, resulting in a loss of energy (Fig. 1). The balance between these energy exchanges determines Earth’s present and future climate. The change in this balance is important over the Arctic, where summer sea ice is retreating at an accelerated rate (Comiso et al, 2008), surface albedo is rapidly declining and surface temperatures are rising at a rate double that of the global average (Cohen et al, 2014; Graversen et al, 2008), impacting sub-polar ecosystems (Cheung et al, 2009; Post et al, 2013) and possibly mid-latitude climate (Cohen et al, 2014, 2019)

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