Radiative Forcing Components

Transcription

1 Radiative Forcing Components

2 Content Definition of Radiative Forcing Radiation Balance Climate sensitivity Solar forcing Forcing due to atmospheric gas

3 Definition of Radiative Forcing In climate science, radiative forcing or climate forcing, is defined as the difference of insolation (sunlight) absorbed by the Earth and energy radiated back to space. Typically, radiative forcing is quantified at the tropopause in units of watts per square meter of the Earth's surface. A positive forcing (more incoming energy) warms the system, while negative forcing (more outgoing energy) cools it. Causes of radiative forcing include changes in insolation and the concentrations of radiatively active gases, commonly known as greenhouse gases and aerosols.

4 Definition of Radiative Forcing The IPCC AR4 report, defines radiative forcing as: "Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. In this report radiative forcing values are for changes relative to preindustrial conditions defined at 1750 and are expressed in Watts per square meter (W/m 2 )." Intergovernmental Panel on Climate Change (IPCC)

5 Radiation Balance Earth's energy budget or Earth's radiation balance describes the net flow of energy into Earth in the form of shortwave radiation and the outgoing infrared longwave radiation into space. Thus, the energy fluxes are important to understand climate change, defined by changes in Earth's energy balance.

6 edu.larc.nasa.gov/energy_budget/

7 Climate Sensitivity Climate sensitivity is the equilibrium temperature change in response to changes of the radiative forcing. Therefore climate sensitivity depends on the initial climate state, but potentially can be accurately inferred from precise palaeoclimate data. Slow climate feedbacks, especially changes of ice sheet size and atmospheric CO 2, amplify the total Earth system sensitivity by an amount that depends on the time scale considered.

8 Climate Sensitivity Although climate sensitivity is usually used in the context of radiative forcing by carbon dioxide (CO 2 ), it is thought of as a general property of the climate system: the change in surface air temperature (ΔTs) following a unit change in radiative forcing (RF), and thus is expressed in units of C/(W/m 2 ). For this to be useful, the measure must be independent of the nature of the forcing (e.g. from greenhouse gases or solar variation); to first order this is indeed found to be so.

9 Climate Sensitivity For coupled atmosphere ocean global climate models, the climate sensitivity is an emergent property: it is not a model parameter, but rather a result of a combination of model physics and parameters. By contrast, simpler energy balance models may have climate sensitivity as an explicit parameter. The terms represented in the equation relate radiative forcing (RF) to linear changes in global surface temperature change (ΔTs) via the climate sensitivity λ. It is also possible to estimate climate sensitivity from observations; however, this is difficult due to uncertainties in the forcing and temperature histories.

10 Solar Forcing Radiative forcing (measured in Watts per square meter) can be estimated in different ways for different components. For the case of a change in solar irradiance (i.e., "solar forcing"), the radiative forcing is simply the change in the average amount of solar energy absorbed per square meter of the Earth's area. Since the cross sectional area of the Earth exposed to the Sun (πr 2 ) is equal to 1/4 of the surface area of the Earth (4πr 2 ), the solar input per unit area is one quarter the change in solar intensity.

11 Solar Forcing This must be multiplied by the fraction of incident sunlight that is absorbed,, where is the reflectivity, or albedo, of the Earth. The albedo of the Earth is approximately 0.3, so is approximately equal to 0.7. Thus, the solar forcing is the change in the solar intensity divided by 4 and multiplied by 0.7. Likewise, a change in albedo will produce a solar forcing equal to the change in albedo divided by 4 multiplied by the solar constant (1,362 W/m 2 ).

12 Solar Irradiance Spectrum

13 Forcing Due to Atmospheric Gas For a greenhouse gas, such as carbon dioxide, radiative transfer codes that examine each spectral line for atmospheric conditions can be used to calculate the change ΔF as a function of changing concentration. These calculations can often be simplified into an algebraic formulation that is specific to that gas. A different formula applies for some other greenhouse gases such as methane and N 2 O (square root dependence) or CFCs (linear), with coefficients that can be found e.g. in the IPCC reports.

14 Radiative forcing for doubling CO 2

15 Radiative forcing for eight times increase of CH 4

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18 Changes in radiative forcing of long lived greenhouse gases between 1979 and

19 The Radiative Forcing bar chart: AR5 version Climate Change 2013: IPCC AR5 WG1 SPM, Fig. SPM 05

20 Radiative Forcing Components