Climate change projections are driven by changes in planetary heat balance, known as climate forcings. While current forcings due to anthropogenic greenhouse gases have been determined to within about 10%, forcings associated with anthropogenic aerosols have proven far less tractable. As a result, uncertainty concerning total, industrial-era climate forcing is enormous. Climate forecasters, by and large, have ignored this uncertainty. For example, the 100-year climate projections by the Intergovernmental Panel for Climate Change assume perfect knowledge of current climate forcings and consider uncertainties only with regard to future forcings. It is a seldom-acknowledged fact, therefore, that these projections rest on the plausible, but untested, assumption that forcings to date can be deduced from the temperature record.

To test this assumption requires, first and foremost, a dramatic improvement in knowledge of aerosol forcings. Here we evaluate the feasibility of this goal by outlining a strategy for empirical quantification of aerosol direct (or clear-sky) forcing. This is the primary objective of the CALIPSO (lidar-in-space) satellite mission, scheduled to join a suite of formation-flying satellite sensors known as the A-Train in early 2005. The envisioned strategy involves carefully coordinated satellite and in-situ observations for global, 4-dimensional mapping of aerosol amount and aerosol type and for studying aerosol covariations with clouds, surface albedo, and relative humidity. In-situ observations are required to provide data on key quantities that cannot be measured from space, such as single scatter albedo and the fraction of extinction due to condensed water. The combined data set will be used to constrain a model-based calculation of direct aerosol forcing. This strategy builds on the approach to the direct aerosol forcing problem that is being developed for the MODIS data set. Finally, the 4-dimensional aerosol data acquired via this strategy, when combined with planetary albedo data from CERES, may permit an empirical quantification of aerosol indirect forcing.