Cloud Forcing and the Earth's Radiation Budget: New Ideas and New Observations

When the Earth Radiation Budget Experiment (ERBE) produced the rst measurements of cloudradiative forcing (Ramanathan, et al., 1989), the climate community interpreted the results from a context in which the atmosphere was a single column, strongly coupled to the Earth's surface. In this context, all forcings are created equal. When a modeler runs a climate experiment, he creates a perturbation in the energy ow through the system, allows the atmosphere to couple to the surface, and then watches as the surface responds to the coupled disequilibrium. Indeed, the perturbation to the global radiation balance at the tropopause after the atmospheric energy ow equilibrates with the net ow from the surface is the climate forcing, even through the surface has still not completely adjusted to the perturbation. Recently, the modeling community has begun the task of interpreting the way the climate system works by considering systems with more degrees of freedom (Hansen, et al., 1996). In this context, a perturbation can excite several response modes, only some of which tie to the long-term response of the surface temperature. Of course, the response modes are the natural organizations of the temperature, humidity, and pressure elds that underlie the radiation budget. Thus, it appears useful to begin to interpret both theoretical calculations and observations from the standpoint of internally organized objects whose life extends over a much longer time period than the typical GCM time step. Storm systems are the obvious example, although we can incorporate such long-lived phenomena as ocean currents and ecosystem in this Earth system science view.