Radiative Forcing of Climate Change Expanding the Concept and Addressing Uncertainties (2005) / Chapter Skim
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1 Introduction
1 Introduction
Pages 11-27
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From page 11... ...
It is in that context that the U.S. Climate Change Science Program asked the National Academies to examine the current state of knowledge regarding the climate forcings associated with gases, aerosols, land use, and solar variability and to identify relevant research needs (see Appendix B for full statement of task)
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For the purposes of this report, the Sun, volcanic emissions, and human-caused emissions of greenhouse gases and changes to the land surface are considered external to the climate system.
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Examples include changes in solar energy output, volcanic emissions, deliberate land modification, or anthropogenic emissions of greenhouse gases, aerosols, and their precursors. A climate feedback is an internal climate process that amplifies or dampens the climate response to an initial forcing.
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In this case, greenhouse gas concentrations can be treated as a forcing, and changes in global mean temperature can be considered a response. Over the past 1000 years, CO2 concentrations appear to have varied in response to surface temperature changes prior to large-scale fossil fuel burning during the nineteenth and twentieth centuries, while during the latter period they can be considered primarily as a forcing of surface temperature changes (Gerber et al., 2003)
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Climate forcing: An energy imbalance imposed on the climate system either exter nally or by human activities. · Direct radiative forcing: A climate forcing that directly affects the radiative budget of the Earth's climate system; for example, added carbon dioxide (CO2)
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At the same time, the Earth and its atmosphere emit energy to space, resulting in an approximate balance between energy received and energy lost. The so-called greenhouse gases, such as water vapor, CO2, methane (CH4)
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, although with a stratosphere in radiative equilibrium this is equivalent to TOA radiative forcing. Stratospheric adjustment is most important for forcings that affect the stratospheric thermal structure, such as well-mixed greenhouse gases.
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The level of water in the bucket is analogous to Earth's global heat content (which is determined to a first order by mean temperature and water vapor pressure)
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This means that in addition to calculating radiative forcing at the tropopause, one must also quantify radiative forcing at the surface and its atmospheric distribution. THEORETICAL DEVELOPMENT OF THE RADIATIVE FORCING CONCEPT The concept of radiative forcing is based on the hypothesis that the change in global annual mean surface temperature is proportional to the imposed global annual mean forcing, independent of the nature of the applied forcing.
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From page 20... ...
At the surface, solar heating far exceeds longwave cooling, and this radiative heating is balanced by convective transport of latent and sensible heat from the surface to the atmosphere. In addition, the sum of surface radiative heating and tropospheric radiative cooling is zero, thus maintaining radiation energy balance for the whole surface-troposphere column, and it is this radiation balance that is perturbed by the addition of greenhouse gases and aerosols.
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From page 21... ...
, who used a three-dimensional GCM to calculate the global mean surface temperature change due to a doubling of CO2 and a 2 percent change in solar insolation. They found that surface temperature estimated by the GCM can be scaled with the initial forcing as in Equation 1-3.
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After the IPCC Second Assessment Report (IPCC, 1996) , the term radiative forcing came to imply climate forcing (i.e., a term that forces climate changes)
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Currently this is accomplished either through statistical approaches or by forcing a regional model with boundary conditions from a global climate model. THE RADIATIVE FORCING CONCEPT AND CLIMATE POLICY The concept of radiative forcing has provided a clear mechanism for conceptualizing the Earth's climate as a closed system with a detectable metric of change: global mean surface temperature.
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, limiting concentrations (e.g., greenhouse gas stabilization scenarios) , and limiting temperature changes and impacts (e.g., environmental scenarios)
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From page 25... ...
The system may be further coupled to an optimization scheme to determine optimal investment rates in reductions of greenhouse gas emissions. In these optimization schemes the damage function is dependent on the global averaged surface temperature, while the cost function depends on the level of greenhouse gas emissions abatement (Nordhaus and Boyer, 2000)
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GWPs compare the integrated radiative impact of a one time-unit of emissions of greenhouse gas X to the integrated radiative forcing impact of a one timeunit of CO2 emissions (IPCC, 2001)
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From page 27... ...
In addition, the current concept is not useful for evaluating how the rate of technical transformation, which depends on economic and policy drivers, affects the trade-off between two greenhouse gases. At present, integrated assessment models are used to consider the combined scientific and economic factors that contribute to the global warming impacts of different forcings (e.g., Manne and Richels, 2001)
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