Balancing Earth’s Radiant Energy Budget
Climate is defined as the average state of the atmosphere, hydrosphere and land over a given time period. Thus, measurements of radiant energy within the Earth’s atmosphere are at the heart of the climate change discussion. How climate changes is directly related to how our planet balances the amount of incoming sunlight with outgoing radiant energy. For scientists, to measure all incoming and outgoing energy is to have a bottom line ledger sheet on the sum total of all the physical motions and interactions of our world’s climate system. Over the course of a year and over the entire globe, is the Earth’s total energy budget in balance? If not, the Earth is either heating up or cooling down. So if scientists are to understand climate and accurately predict future climate change, then they must determine what drives the changes within the Earth’s radiation balance.

These global scale composite images show where more or less shortwave radiant energy is reflected back into space (top), and where more or less longwave radiant energy is emitted to space (bottom). These measurements were acquired by CERES, flying aboard NASA’s Terra satellite, on May 25, 2001.

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 balance—the 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 ERBE’s 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 Earth’s 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 Earth’s Radiant Energy System (CERES) sensor aboard the joint NASA/NASDA Tropical Rainfall Measuring Mission (TRMM) satellite. Twin CERES instruments were also launched aboard NASA’s Terra satellite in December 1999, and the pair will again fly aboard NASA’s 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 ERBE’s same measurement objectives with better than twice the former sensor’s 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 Earth’s atmosphere that will help us more accurately account for the effects of aerosols and clouds on climate.

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The Earth Radiation Budget Satellite (ERBS) was deployed from the Space Shuttle Challenger in October 1984 by NASA Astronaut Sally Ride. ERBS was the first of three spacecraft to carry copies of the Earth Radiation Budget Experiment (ERBE) sensor, which have provided scientists with measurements of the Earth’s radiant energy budget for almost two decades. (Image courtesy NASA GSFC)

Remote Sensing
Introduction
Balancing Earth’s Radiant Energy Budget
Dust in the Wind
Abstract Art or Arbiters of Energy?
Serendipity and Stratospheric Ozone
The Chemistry of Earth’s Atmosphere
Where Storm Clouds Gather
Conclusion

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Aerosols and Climate Change
Clouds and Radiation
Why isn’t Earth Hot as an Oven?

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Precipitation
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