Climate Forcing
Mark Flanner of University of Michigan and his colleagues have reportedly been measuring the impact of climate forcing using satellite data collected over the last 30 years. “On average, the Northern Hemisphere now absorbs about 100 PetaWatts [10^17 Watts] more solar energy because of changes in snow and ice cover,” according to Flanner. [FIRE-EARTH cannot confirm the accuracy of Flanner’s calculations. Moderator]
“The left image shows how much energy the Northern Hemisphere’s snow and ice—called the cryosphere—reflected on average between 1979 and 2008. Dark blue indicates more reflected energy, and thus more cooling. The Greenland ice sheet reflects more energy than any other single location in the Northern Hemisphere. The second-largest contributor to cooling is the cap of sea ice over the Arctic Ocean. The right image shows how the energy being reflected from the cryosphere has changed between 1979 and 2008. ” Source: NASA-EO
Of the 340 watts per square meter of solar energy that falls on the Earth, 29% is reflected back into space, primarily by clouds, but also by other bright surfaces and the atmosphere itself. About 23% of incoming energy is absorbed in the atmosphere by atmospheric gases, dust, and other particles. The remaining 48% is absorbed at the surface. (NASA illustration by Robert Simmon. Astronaut photograph ISS013-E-8948.)
The surface absorbs about 48% of incoming sunlight. Three processes remove an equivalent amount of energy from the Earth’s surface: evaporation (25%), convection (5%), and thermal infrared radiation, or heat (net 17%). (NASA illustration by Robert Simmon. Photograph ©2006 Cyron.)
The atmosphere radiates the equivalent of 59% of incoming sunlight back to space as thermal infrared energy, or heat. Where does the atmosphere get its energy? The atmosphere directly absorbs about 23% of incoming sunlight, and the remaining energy is transferred from the Earth’s surface by evaporation (25%), convection (5%), and thermal infrared radiation (a net of 5-6%). The remaining thermal infrared energy from the surface (12%) passes through the atmosphere and escapes to space. (NASA illustration by Robert Simmon. Astronaut photograph ISS017-E-13859.)
On average, 340 watts per square meter of solar energy arrives at the top of the atmosphere. Earth returns an equal amount of energy back to space by reflecting some incoming light and by radiating heat (thermal infrared energy). Most solar energy is absorbed at the surface, while most heat is radiated back to space by the atmosphere. Earth’s average surface temperature is maintained by two large, opposing energy fluxes between the atmosphere and the ground (right)—the greenhouse effect. NASA illustration by Robert Simmon, adapted from Trenberth et al. 2009, using CERES flux estimates provided by Norman Loeb.)
Climate Forcings and Global Warming
(Source: Earth’s Energy Budget)
Any changes to the Earth’s climate system that affect how much energy enters or leaves the system alters Earth’s radiative equilibrium and can force temperatures to rise or fall. These destabilizing influences are called climate forcings. Natural climate forcings include changes in the Sun’s brightness, Milankovitch cycles (small variations in the shape of Earth’s orbit and its axis of rotation that occur over thousands of years), and large volcanic eruptions that inject light-reflecting particles as high as the stratosphere. Manmade forcings include particle pollution (aerosols), which absorb and reflect incoming sunlight; deforestation, which changes how the surface reflects and absorbs sunlight; and the rising concentration of atmospheric carbon dioxide and other greenhouse gases, which decrease heat radiated to space. A forcing can trigger feedbacks that intensify or weaken the original forcing. The loss of ice at the poles, which makes them less reflective, is an example of a feedback.
Things that change the balance between incoming and outgoing energy in the climate system are called forcings. Natural forcings include volcanic eruptions. Manmade forcings include air pollution and greenhouse gases. A climate forcing, such as greenhouse gas increases, may trigger feedbacks like the loss of sunlight-reflecting ice. (Photographs ©2008 antonio, ©2008 haglundc, and courtesy Mike Embree/National Science Foundation.)
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