Balancing Earths Radiant Energy Budget
In 1978, NASA launched its Nimbus-7 satellite carrying a new sensor, called the Earth Radiation Budget (ERB) experiment, designed to measure direct solar irradiance, reflected shortwave radiation (visible light), and emitted longwave radiation (heat) every day over the entire Earth. This was the first space-based sensor capable of self-calibrating so that its total solar irradiance measurements were accurate to within ± 0.5 percent. The Nimbus-7 ERB collected nine years of global-scale data upon which scientists began long-term climate studies. In the interest of extending the ERB data set and improving upon its measurement capabilities, NASA launched three more Earth Radiation Budget Experiments (renamed ERBE) in the 1980s.
In addition to total solar irradiance, ERBE measured the reflected solar and emitted thermal radiation from the Earth-atmosphere-ocean system. These observations revealed that over the course of a year the global radiation budget is in balancethe Earth reflects and emits roughly the same amount of energy back into space that it receives from the sun. The data also showed that the average annual, global contribution by clouds is they reflect 17 Wm-2 more shortwave energy (visible light) than they trap as longwave energy (heat). Yet, due to calibration uncertainties, deficiencies in ERBEs sampling method, and the limitations of existing angular dependence models there still exists a significant uncertainty (about ± 5 Wm Wm-2) regarding our understanding of Earths radiation budget. Part of this uncertainty lies in our limited knowledge of the spatial distribution of clouds as well as the optical properties of these clouds over time. Moreover, we cannot be sure how the distribution and optical properties of clouds will change over time. The endeavor to address these issues began with the 1997 launch of the Clouds and the Earths Radiant Energy System (CERES) sensor aboard the joint NASA/NASDA Tropical Rainfall Measuring Mission (TRMM) satellite. Twin CERES instruments were also launched aboard NASAs Terra satellite in December 1999, and the pair will again fly aboard NASAs Aqua satellite launched in early 2002. Many of the sampling and accuracy limitations on ERBE were addressed in the design of CERES so that it allows scientists to meet ERBEs same measurement objectives with better than twice the former sensors accuracy. Ultimately, scientists anticipate that CERES will not only extend the ERBE data set, but it will provide the first long-term global measurements of the radiative fluxes within the Earths atmosphere that will help us more accurately account for the effects of aerosols and clouds on climate.