Radiative Forcing of Climate Change Expanding the Concept and Addressing Uncertainties (2005) / Chapter Skim
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Pages 144-158
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From page 144... ...
The relative ease of calculating radiative forcing and the associated temperature response has enabled the use of climate models, simpler versions of those models, and chemical transport models to investigate the many factors that may influence climate. In short, the TOA radiative forcing concept still has considerable value and should be retained as a standard metric in future climate research.
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From page 145... ...
Indeed, the relationship between TOA radiative forcing and surface temperature is not valid if there is significant variation in the vertical distribution of radiative forcing. For example, the direct radiative forcing of black carbon and other absorbing aerosols leads to a reduction in surface heat input while increasing atmospheric heating.
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From page 146... ...
Report global mean radiative forcing at both the surface and the top of the atmosphere in climate change assessments. · Develop parameterizations for using surface forcing in integrated assessment and simple climate models.
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From page 147... ...
No metrics for quantifying nonradiative forcing have been accepted. Unlike traditional radiative forcing, which can be directly related to surface temperature, nonradiative forcings are not easily linked to a single climate variable.
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From page 148... ...
Another consideration in identifying potential metrics for nonradiative forcings is their significant regional variation; any new metrics will have to be able to characterize the regional structure in forcing and climate response. Further work is needed to quantify links between regional nonradiative forcing and climate response, whether the response occurs in the region, in a distant region through teleconnections, or globally.
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From page 149... ...
To improve the representation of the indirect effect in climate models, fundamental research is needed on the physical and chemical composition of aerosols, aerosol activation, cloud microphysics, cloud dynamics, and subgrid-scale variability in relative humidity and vertical velocity. RECOMMENDATIONS: · Conduct integrated and comprehensive field investigations to quantify indirect aerosol radiative forcings -- for example, in closure experiments with redundant observational and modeling studies.
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From page 150... ...
Radiative transfer models relating aerosol columns and optical properties to the corresponding radiative forcing are thought to be relatively mature but must be tested further with field closure studies that provide multiple constraints for the models for a range of environments. Assessments of past and future radiative forcings are compromised by the poor characterization of aerosol sources and sinks.
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From page 151... ...
Knowledge of what triggers abrupt climate changes is still quite limited; more research is needed to determine the possible role of radiative and nonradiative climate forcings, such as human-caused increases in greenhouse gases or land-use changes. Indeed, past abrupt climate
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From page 152... ...
· Determine the probability that future radiative and nonradiative forcings (e.g., reductions in aerosol emissions, continued tropical deforestation) could induce an abrupt climate change.
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From page 153... ...
The shortness of the instrumental record and of accurate monitoring of climate forcings, however, limits the confidence with which climate change since preindustrial times can be attributed to specific forcings. Proxy records obtained from ice cores, sediments, tree rings, and other sources provide a critical tool for extending knowledge of both climate forcings and climate response further back in history.
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From page 154... ...
Such benchmark measurements of radiative and other climate forcings and climate variables are needed immediately. Because the radiative forcings and the climate responses are highly dependent on wavelength, space-based observations with high spectral resolution are needed to isolate the signatures of the relevant radiative processes and components.
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From page 155... ...
A network of surface stations intended to characterize the surface energy budget could help better understand and monitor nonradiative forcings, although care would be needed in determining the siting and density of stations to appropriately account for the impact of landscape heterogeneity. RECOMMENDATIONS: Continue observations of climate forcings and variables without interruption for the future in a manner consistent with established climate monitoring principles (e.g., adequate cross-calibration of successive, overlapping datasets)
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From page 156... ...
In addition, the policy community has focused primarily on global mean radiative forcing and the associated response in surface temperature. Given the increasing realization of the significance of geographically dependent climate forcings, the policy community will need new forcing metrics and guidance on how to apply them.
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From page 157... ...
Many simplified climate models have focused on global mean surface temperature as the primary climate response to forcings, although more recently they have considered regional temperature changes and other societally relevant aspects of climate, such as sea level. It is important that models used for policy analysis incorporate further complexities in the radiative and nonradiative forcing concepts, as identified in this chapter.
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From page 158... ...
RECOMMENDATIONS: Encourage policy analysts and integrated assessment modelers to move beyond simple climate models based entirely on global mean TOA radiative forcing and incorporate new global and regional radiative and nonradiative forcing metrics as they become available. · Devise practical tools to relate new forcing metrics that may be introduced in the future to simple measures of climate change.
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