Richard Melson

April 2006

Earth Energy Budget

In the early 21st century, it's become clear that air pollution can significantly reduce the amount of sunlight reaching Earth, lower temperatures, and mask the warming effects of greenhouse gases. Climate researcher James Hansen estimates that "global dimming" is cooling our planet by more than a degree Celsius (1.8F) and fears that as we cut back on pollution, global warming may escalate to a point of no return. Regrettably, in terms of possibly taking corrective action, our current understanding of global dimming has been a long time in the coming, considering the first hints of the phenomenon date back to 18th-century observations of volcanic eruptions.

In this slide show, follow a series of historic events and scientific milestones that built the case for global dimming. Click on the image at left to begin.—Susan K. Lewis

Thanks to Spencer Weart of the American Institute of Physics,

whose book The Discovery of Global Warming made this feature possible.

A hypertext version of the book can be found at

Earth Radiation Budget

Overview of Earth Radiation Budget

Most input of the Earth energy is received from the Sun. The solar energy is short-wave radiation. Although the Earth also receives electromagnetic energy from the other bodies in space, it's negligible, compared with solar energy [Table 1]. The incident solar energy (shortwave) may be reflected and absorbed by the Earth's surface or the atmosphere. And Earth's surface and atmosphere also emit the radiation (longwave).

The Earth Radiation Budget is the balance between incoming energy from the sun and the outgoing longwave (thermal) and reflected shortwave energy from the Earth.

Solar Energy

The radiant solar Energy is from nuclear energy and the temperature of the Sun is 6000K. The spectrum of the solar radiation received at the top of the atmosphere is well approximated by the spectrum of a blackbody having a surface temperature of about 6000K. Thus Sun may be considered as a blackbody.

The solar energy reaching the Earth is traditionally quantified as the solar constant which is the annual average solar irradiance received outside the Earth's atmosphere or surface normal to the incident radiation at the Earth's mean distance from the Sun

(about 1370 W/m2).

The actual solar irradiance from the Sun.

The actual solar irradiance varies by 3.4% from the solar constant during the year due to the eccentricity of Earth's orbit about the Sun.

Surface radiation budget

The Surface Radiation Budget is divided into downward shortwave radiation, reflected shortwave radiation, downward longwave radiation, upward longwave radiation, net radiation.

They are dominated by clouds.

The downward shortwave radiation may be reflected to the space absorbed in the atmosphere and absorbed at the ground.

Esun = AEsun + Eatm + (1-Asfc)Esfc

where Esun is the incident solar radiation, A is albedo of TOA (Top of atmophere), Eatm is the energy of absorbed in atmosphere, Esfc is downward irradiance at surface and Asfc is surface albedo.

This equation implicity includes scattering and multiple reflectors between the surface and clouds. Usually the incident solar radiation and TOA albedo is measured by satellite measurement.(ERBE) The reflected solar radiation is the product of surface albedo and the downward solar radiation, the surface albedo should be determined. One method to estimate surface albedo is the minimum albedo technique. Because few locations are likely to be cloud-covered for an entire month, the minimum albedo is likely to be represent the clear-sky albedo. It can be calculated from narrow band AVHRR observations. The downward longwave radiation is mostly from the atmosphere. It depends on the temperature and moisture of the atmosphere. The water vapor and other gases, aerosols absorb some solar energy and emit some longwave radiation energy computation of downward longwave radiation from the atmosphere is difficult, even when the distributions of water vapor, carbon dioxide, cloudiness, and temperature are measured. Some satellite measurements like TOVS estimates downward longwave radiation. Little longwave radiation is reflected by the surface: natural surface emission is dominant. It is also difficlut to measure and define the surface temperature especially vegetation surface. To combine the above four components makes the calculation of net radiation at the surface. This is not accurate because the errors in each accumulate. So it is developed the research to use some satellite measurements-NOAA, GOES etc.

Earth Radiation Budget Experiment

April 15, 2006