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4 Rethinking the Global Radiative Forcing Concept
Pages 83-99

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From page 83...
... THE CURRENT CONCEPT Global-annual mean adjusted radiative forcing at the top of the atmosphere is, in general, a reliable metric relating the effects of various climate perturbations to global mean surface temperature change as computed in general circulation models (GCMs)
From page 84...
... or Incorporates fast calculate indirect and fixed surface temperatures atmospheric feedbacks in semidirect effects of (Shine et al., 2003) and the simulation of climate aerosols adjusted atmospheric forcing and response Not as computationally temperatures (in both Insensitive to the altitude expedient as the standard troposphere and at which forcing is radiative forcing stratosphere)
From page 85...
... forcing components nonradiative forcing Allows characterization Further work is needed to of teleconnected response quantify links of regional to a regionally isolated nonradiative forcing to forcing regional and global Nonradiative forcing climate response alters radiative forcing Some types of nonradiative and thus provides a more forcing are not easily complete characterization quantified in watts per of radiative forcing square meter, thus it is not clear how to compare them to radiative forcing Ocean heat content Can be used to calculate Observations may have the net radiative insufficient frequency and imbalance of the Earth spatial coverage to Offers a valuable accurately determine the constraint on the radiative imbalance at the performance of climate necessary resolution models NOTE: TAR = IPCC Third Assessment Report (IPCC, 2001)
From page 86...
... It is inadequate to describe fully the radiative effects of several anthropogenic influences including absorbing aerosols, which lead to a positive radiative forcing of the troposphere with little net radiative effect at the top of the atmosphere; effects of aerosols on cloud properties (including cloud fraction, cloud microphysical parameters, and precipitation efficiency) , which may modify the hydrological cycle without significant radiative impacts; perturbations of ozone in the upper troposphere and lower stratosphere, which challenge the manner in which the stratospheric temperature adjustment is done; and surface modification due to deforestation, urbanization, and agricultural practices and surface biogeochemical effects.
From page 87...
... Yet another limitation of the concept is that the assumption of a constant, linear relationship between changes in global mean surface temperature and global mean TOA radiative forcing does not always hold. This linear relationship breaks down for absorbing aerosols, which may have small TOA forcing, but disproportionately larger surface forcing due to absorption of solar radiation (Lohmann and Feichter, 2001; Ramanathan et al., 2001a)
From page 89...
... GLOBAL MEAN RADIATIVE FORCING AT THE SURFACE The TOA radiative forcing might not be directly related to surface temperature if a forcing agent changes the vertical distribution of heating in the atmosphere. Well-known examples of such cases are the direct radiative forcing of black carbon (BC)
From page 90...
... One way to address this limitation of the traditional radiative forcing concept is to calculate the global mean radiative forcing at the surface along with that at the top of the atmosphere. Considering the surface radiative forcing may enable quantification of the effects of aerosols on the surface
From page 91...
... Like the other metrics discussed above, global mean radiative forcing at the surface would not allow characterization of the regional structure of forcing. Reporting surface radiative forcing along with that at the TOA is im
From page 92...
... The traditional global mean radiative forcing provides no information about this regional structure, so many researchers have begun to present estimates of radiative forcing on a regional scale as derived from models or observational campaigns. A large number of modeling studies have been carried out to characterize the spatial variability in aerosol forcing due to direct, indirect, and semidirect effects (IPCC, 2001)
From page 93...
... REGIONAL NONRADIATIVE FORCING Some forcings affect the climate system in nonradiative ways, in particular by modifying the hydrological cycle or vegetation dynamics. These nonradiative forcings generally have radiative impacts, but describing them only in terms of this radiative impact does not convey fully their influence on climate variables of societal relevance.
From page 94...
... 94 RADIATIVE FORCING OF CLIMATE CHANGE Sulfate 90N 0.4 60N 0.36 0.32 30N 0.28 0.24 0 0.2 0.16 30S 0.12 0.08 60S 0.04 0 90S 180 150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180 Mineral Dust 90N 0.6 60N 0.54 0.48 30N 0.42 0.36 0 0.30 0.24 30S 0.18 0.12 60S 0.06 0 90S 180 150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180 Sea Salt 90N 0.1 60N 0.09 0.08 30N 0.07 0.06 0 0.05 0.04 30S 0.03 0.02 60S 0.01 0 90S 180 150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180 Organic and Black Carbon Aerosols 90N 0.2 60N 0.18 0.16 30N 0.14 0.12 0 0.1 0.08 30S 0.06 0.04 60S 0.02 0 90S 180 150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180 FIGURE 4-1 Annual mean aerosol optical depth predicted by an aerosol chemical transport model due to sulfate, mineral dust, sea salt, and organic and black carbon aerosols. SOURCE: Collins et al.
From page 95...
... , and in the atmosphere calculated from the aerosol optical depths shown in Figure 4.1. SOURCE: Adapted from Collins et al.
From page 96...
... refers to these kinds of aerosol forcings as thermodynamic forcing because the spatial patterns of diabatic heating are changed. Several nonradiative forcings involve the biological components of the climate system.
From page 97...
... As is the case for regional radiative forcing, further work is needed to quantify links between regional nonradiative forcing and climate response. Another consideration in devising metrics for nonradiative forcings is enabling direct comparison with radiative forcings, computed in units of watts per square meter.
From page 98...
... so as to independently 2 confirm the calculation of radiative imbalance from the changes in ocean heat storage. Another issue is whether the spatial and temporal sampling of the ocean heat content accurately captures the regions and depths at which heat changes are occurring.
From page 99...
... heat content integrated over the region from 20N to 20S (solid line) and over the entire globe (dashed line)


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