Radiative Forcing

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Radiative Forcing
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Radiative forcing definition
A change in the net radiation budget at the top
of the atmosphere, whether due to a change in
the net incoming solar radiation, or a change in
the emission of infrared radiation is called
Radiative Forcing.
Radiative forcing perturbs the balance
between incoming and outgoing radiation, and the
Earths climate adjusts to restore the balance.
Positive radiative forcing acts to warm the
surface, Negative radiative forcing acts to cool
the surface.
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Radiative forcing has the units of the Solar
constant, thus, Watts/m2.

• Examples
• Solar constant declines by 1. This leads to a
  radiative
• forcing of -3.4 W/m2.
• CO2 is doubled with respect to pre-industrial
• concentrations this leads to a radiative
  forcing of
• ?4 W/m2 (not taking into consideration
  feedback effects).
• Increased aerosol content in the atmosphere will
  lead
• to a negative radiative forcing.

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The Radiative forcing of a greenhouse gas is
calculated
For greenhouse gases with trace
concentrations (CFCs, etc.) Radiative forcing
(Current concentration pre-industrial
concentration) ? Radiative efficiency
The radiative efficiency is the radiative
forcing that would be derived from the increase
in the concentration of the gas by 1 ppbv. It
depends on the detailed absorption spectrum of
the gas.
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For non-trace greenhouse gases (CO2, CH4, etc.)
The equation to calculate the current radiative
forcing is non-linear, as the absorption bands of
these gases tend already to be partially
saturated.
Example The radiative forcing of CO2 (Hansen et
al. 1998)
where
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Solar Forcing
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Global Warming Potential
Is an index that measures the capability of an
increase of 1 kg of a given greenhouse gas to
contribute to global warming. This index is
measured relative to a reference greenhouse gas,
generally CO2. It depends not only on the
radiative efficiency of the gas, but also on the
lifetime of the gas in the atmosphere.
The global Warming Potential (GWP) is calculated
over a specific time horizon (TH).
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Change in Radiative forcing between 1860 and 2000