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WIREs Clim Change
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The physical concept of climate forcing

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Since the beginning of the debate on global climate change, scientists, economists, and policy makers alike have been using ‘climate forcing’ as a convenient measure for evaluating climate change. Researchers who run complex computer models conceived the theoretical concept of climate forcing in the late 1960s (Charney Report, 1979). This overview describes the development and basics of the physical framework, as radiative energy imbalance in the atmosphere, inflicted by a perturbation in the climate system. Such disturbances and forced changes can alter processes in the climate system, which enhance or dampen the initial effects and thus introduce positive or negative feedback loops. With increased understanding of the nature of the climate system, this basic concept has become more complex and hence more difficult to interpret. The identification of additional anthropogenic disturbances, the interdependence of individual forcings, and difficulties to account for spatial and temporal variabilities of disturbances are only few issues that complicate the overall picture. Although numerous scientific studies exist that evaluate climate forcings by allocating watts per square meter values to individual forcings (Intergovernmental Panel on Climate Change (IPCC) reports, 2010), the actual number of publications that interpret the physical meaning of the climate‐forcing concept remains surprisingly small. Here, this overview focuses on explaining to an interdisciplinary audience the physical interpretation of the concept, including its limitations. It also examines new developments, such as polluter‐based emission scenarios, energy budget approaches, and climate impacts other than temperature change. WIREs Clim Change 2010 1 786–802 DOI: 10.1002/wcc.75

This article is categorized under:

  • Paleoclimates and Current Trends > Climate Forcing
  • Climate Models and Modeling > Knowledge Generation with Models
Figure 1.

Earth's shortwave and longwave radiation fluxes as measured from space. The two images show radiative fluxes in watts per square meters as measured from the NASA Clouds and Earth's Radiant Energy System CERES Instrument in March 2000. The shortwave flux measured by CERES is the portion of the radiative energy received from the Sun that is reflected back to space by the Earth's surface, clouds, and atmosphere. The incoming solar energy at the top of the atmosphere is measured by satellite instruments (not shown) and depends only on the solar output, the position of the Sun to the Earth, and the time and date. The absorbed solar energy of the Earth system is calculated as incoming minus reflected solar flux. CERES also measures outgoing longwave fluxes or thermal radiative energy emitted from the surface, clouds, and atmosphere as shown in the bottom image. The spatial patterns of these fluxes differ significantly albeit their global, long‐term means have to balance in a stable climate. Outgoing thermal radiation is hemispherically symmetric, whereas continents strongly modify reflected solar radiation. (Reprinted with permission from NASA CERES Instrument Team (http://visibleearth.nasa.gov).)

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Figure 2.

The framework of climate forcing in the earth's climate system. This figure conceptualizes the impacts of internal and external perturbations on the climate system. Ellipses inside and outside the gray shaded area represent perturbations that modify (linked with black arrows) atmospheric composition, forcings, and effects. Feedback pathways, which couple physical and chemical processes (rectangles) of the climate system, are shown in blue arrows. Red arrows illustrate the pathways of human impacts symbolized by brown diamonds. In this diagram, the coupling of perturbations and climate response can either be through radiative forcing or nonradiative effects. Note in this illustration climate forcing is synonymous for radiative forcing, whereas other authors combine nonradiative and radiative effects for the climate‐forcing concept. See text for further explanation of the diagram.

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Figure 3.

Energy fluxes and atmospheric temperature profile. Illustrated (Figure 3) are the upward and downward solar (yellow arrows) and terrestrial (red arrows) radiative fluxes, the energy transport from the surface to the atmosphere via evaporation of water and convection of air, and the release of the evaporative energy in the atmosphere via cloud formation (condensation). The blue line represents a typical temperature profile from the surface to the tropopause and above as it evolves from these energetic flux adjustments. The positive radiative energy budget at the surface has to be balanced globally by evaporative and convective cooling with a small contribution from heat uptake of the oceans and land and melting of ice and snow. Convection describes lifting of warm air, which expands and cools. Cloud formation occurs as condensation of water from the cloud base. Outgoing terrestrial radiation from the surface warms the atmosphere due to clouds, water vapor, and natural and anthropogenic GHGs that absorb the radiative energy. The air and clouds emit thermal radiation back to surface and to space. Temperatures increase again in the stratosphere as indicated with the blue curve mainly due to absorption of UV‐light by ozone.

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Figure 4.

Major radiative forcings of present day (2005) relative to the start of the industrial era (about 1750) after IPCC‐AR4.9 The terms or type of perturbations, the corresponding radiative forcing as histograms and listings including error bars, the spatial scale of the forcing agents, and the levels of confidence in the estimates given by the IPCC‐AR4 are listed from left to right. Note that some perturbations, such as aerosols, exert nonradiative effects such as precipitation changes or changes of weather pattern, which may cause additional radiative forcings that are not accounted for in this figure. See text for further information.

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Figure 5.

Schematic of atmospheric temperature adjustments to radiative forcings. The conceptual framework of climate forcings and temperature profiles before (white) and after the perturbation was applied for different versions of atmosphere–ocean/land adjustments (colored lines) is shown in Figure 5. The gray dashed line illustrates the tropopause level where the various forcings are symbolized as arrows. For further information see text and also Ref 31.

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