Cloud Radiative Effect

Clouds can act as a greenhouse ingredient to warm the Earth by trapping outgoing longwave (LW) infrared radiative flux at the top of the atmosphere (TOA) . Clouds can also enhance the planetary albedo by reflecting shortwave (SW) solar radiative flux back to space to cool the Earth. The net effect of the two competing processes depends on the height, type, and the optical properties of the clouds. The cloud radiative effect (CRE) on the Earth’s present-day radiation budget can be inferred from satellite data by comparing upwelling radiation in cloudy and non-cloudy regions. The figure at right shows that cloud conditions exert a global and annual SW CRE of approximately -50 W/m2 and a mean LW CRE of approximate 30 W/m2. The net global mean CRE is approximately -20 W/m2 implying a strong net cooling effect of clouds on the current climate. Given the large magnitude of SW and LW CRE, clouds have the potential to cause significant climate feedback. However, the sign of the feedback on climate change cannot be determined from the sign of CRE in the current climate, but depends instead on how sensitive the properties are that govern the LW and SW CRE. Thus, estimates of cloud feedback require process-level understanding and modeling of the nontrivial factors on which clouds depend. Moreover, since clouds modify the general circulation and hydrologic cycle through their interactions with the atmosphere, ocean, and land, comprehensive global climate models are seen as a crucial tool in our quest for an adequate understanding of the interactions between clouds and climate.

 

Figure
from the IPCC AR5 report (Chapter 7, Fig.7.7a-b): distribution of annual-mean top of the atmosphere (a) shortwave, (b) longwave cloud radiative effects averaged over the period 2001-2011 from the CERES EBAF (ed2.6) data set.