Saturday, June 16, 2007

Earth's Energy Balance: Heat In.....Heat Out

Imagine how complex the Earth's climate is. Look at the photograph of the Earth from outer space. See the clouds, the oceans, the continents, the ice in the polar regions. Look at the swirling of the clouds, consider the constant motion, the interchange of heat and water vapor from the oceans to the atmosphere. See the illustration below which summarizes these activities that are working every day, night and day, 365 days per year. Where does mankind and our activities fit in? What role do we play? Can we have an effect or are we just here for the ride?

Figure — Details of Earth's energy balance (source: Kiehl and Trenberth, 1997). Numbers are in watts per square meter of Earth's surface, and some may be uncertain by as much as 20%. The greenhouse effect is associated with the absorption and reradiation of energy by atmospheric greenhouse gases and particles, resulting in a downward flux of infrared radiation from the atmosphere to the surface (back radiation) and therefore in a higher surface temperature. Note that the total rate at which energy leaves Earth (107 W/m2 of reflected sunlight plus 235 W/m2 of infrared [long-wave] radiation) is equal to the 342 W/m2 of incident sunlight. Thus Earth is in approximate energy balance in this analysis.


Earth's Annual Global Mean Energy Budget
Kiehl, J. T. and Trenberth, K. E., 1997
Bull. Amer. Meteor. Soc., 78, 197-208.
The purpose of this paper is to put forward a new estimate, in the context of previous assessments, of the annual global mean energy budget. A description is provided of the source of each component to this budget. The top-of-atmosphere shortwave and longwave flux of energy is constrained by satellite observations. Partitioning of the radiative energy throughout the atmosphere is achieved through the use of detailed radiation models for both the longwave and shortwave spectral regions. Spectral features of shortwave and longwave fluxes at both the top and surface of Earth's system are presented. The longwave radiative forcing of the climate system for both clear (125 W·m-2) and cloudy (155 W·m-2) conditions are discussed. We find that for the clear sky case the contribution due to water vapor to the total longwave radiative forcing is 75 W·m-2, while for carbon dioxide it is 32 W·m-2. Clouds alter these values, and the effects of clouds on both the longwave and shortwave budget are addressed. In particular, the shielding effect by clouds on absorption and emission by water vapor is as large as the direct cloud forcing. Because the net surface heat budget must balance, the radiative fluxes constrain the sum of the sensible and latent heat fluxes which can also be estimated independently.

Figure 1. Longwave Emission
PostScript (40 Kb), GIF (5 Kb)
Figure 2. Greenhouse Effect
PostScript (28 Kb), GIF (5 Kb)
Figure 3. Longwave Could Forcing
PostScript (27 Kb), GIF (5 Kb)
Figure 4. Downward Shortwave
PostScript (57 Kb), GIF (5 Kb)
Figure 5. Net Shortwave
PostScript (74 Kb), GIF (5 Kb)
Figure 6. Shortwave Cloud Forcing
PostScript (50 Kb), GIF (5 Kb)
Figure 7. Earth's energy budget (see above)
PostScript (76 Kb), GIF (13 Kb)

No comments: