Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 1/1/2006
Publication Date: 6/6/2006
Citation: Ramanathan, V., Inamdar, A.K. 2006. The radiative forcing due to clouds and water vapor. Book Chapter. pp. 119-151
Interpretive Summary: This chapter analyzes measurements of the earth’s radiation budget at the top-of-atmosphere, based on the Earth Radiation Budget Experiment (ERBE) instrument on board the NOAA-9 (National Oceanic and Atmospheric Agency) satellite, to summarize our understanding of the effect of clouds on earth, earth’s greenhouse effect and its relation with atmospheric water vapor. The ERBE instrument scans the entire earth from pole-to-pole and can provide a twin view of the earth: one without clouds and one with clouds. Results show that clouds have a net cooling effect on the earth. This cooling is several times the heating expected from a doubling of carbon dioxide concentrations and shows that earth would be much warmer without clouds. Atmospheric greenhouse effect is defined here as the difference between the energy emitted by the earth’s surface and that at the top of atmosphere, or in other words, the radiative energy trapped by the atmosphere. Measurements of atmospheric greenhouse effect have also been made from top-of-atmosphere radiation budget measurements over the globe including oceans and land. Results show that atmospheric water vapor acts as a major greenhouse gas trapping radiation emitted from the earth’s surface and preventing more of it from escaping to space. A warmer earth will hold more water vapor in the atmosphere and this additional water vapor will, in turn, cause increased warming of the earth. This amplifying effect of water vapor is known as the "water vapor positive feedback", and has been deduced from seasonal and inter-annual differences in atmospheric greenhouse effect measurements from ERBE. However, whether this holds true for long term climate change is still open to debate, owing to lack of long term records of accurate water vapor measurements in the atmosphere.
Technical Abstract: This chapter utilizes results from the spaceborne Earth Radiation Budget Experiment (ERBE), launched in 1984 aboard the NOAA-9 (National Oceanic and Atmospheric Agency) satellite, to summarize our understanding of the radiative forcing due to water vapor and clouds. The effect of clouds on the radiation balance of the earth is referred to as the cloud-radiative forcing. The atmospheric greenhouse effect (Ga) is defined as the difference between the surface and the top of atmosphere emission, or in other words, the effect of the atmosphere in reducing the longwave cooling to space. The five-year global mean energy budgets derived from the ERBE instrument, suggest that clouds reduce the absorbed solar radiation by 48 W m-2 (Cs) while enhancing the greenhouse effect by 30 W m-2 (Cl), resulting in a net cooling of the global surface-atmosphere system by 18 W m-2 on average. This value is several times the 4 W m-2 heating expected from doubling of CO2 and thus suggests that the Earth would probably be substantially warmer without clouds. Five-year average regional distributions of Cs and Cl reveal the major climatic regimes and organized cloud systems of the earth such as the deep convective-cirrus, extra-tropical storm tracks and marine strato-cumulus coastal systems. Global and regional averages of the atmospheric greenhouse effect have been derived from ERBE, which include the effect of oceans and land. Maps of the regional distributions of the normalized atmospheric greenhouse effect (with respect to surface temperature, (Ts) and precipitable water (w) display a remarkable consistency and qualitatively portray the role of large-scale dynamic water vapor transport on atmospheric greenhouse effect. Examination of the annual cycles reveals that Ga, w, and Ts are highly correlated consistent with other studies, and the water vapor feedback sensitivity parameter, dGa/dTs, tracks the response of the equatorial convection and large-scale circulation of water vapor and its greenhouse effect. The main features deducted from this study were: (1) increase in equatorial convection in response to warming, moistens the mid to upper troposphere between 100 N and 100 S and contributes to the so-called supergreenhouse effect; (2) the Hadley cell response to the equatorial convection is to dry the sub-tropics and extra tropics, which compensates for some of the equatorial enhanced greenhouse effect; (3) on tropical and global scales, however, there is a net moistening effect, and the feedback is positive and is similar to that of the fixed relative humidity models. The analyses presented here confirm that water vapor has a positive feedback for global-scale changes on seasonal to inter-annual timescales, and these results can provide a starting point for validating general circulation models of climate.