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.
Radiation balance
Atmospheric gases only absorb some wavelengths of energy but
are transparent to others. The absorption patterns of water vapor (blue peaks)
and carbon dioxide (pink peaks) overlap in some wavelengths. Carbon dioxide is
not as strong a greenhouse gas as water vapor, but it absorbs energy in
wavelengths (12-15 micrometers) that water vapor does not, partially closing
the “window” through which heat radiated by the surface would normally escape
to space. (Illustration NASA, Robert Rohde)
Almost all of the energy which affects Earth's weather is
received as radiant energy from the Sun. The planet and its atmosphere absorb and reflect some of the
energy, while long-wave energy is radiated back into
space. The balance between absorbed and radiated energy determines the average
temperature. Because the atmosphere absorbs some of the re-radiated long-wave
energy, the planet
is warmer than it would be in the absence of the atmosphere:
see greenhouse effect.
The radiation balance is altered by such factors as the
intensity of solar energy, reflectivity of clouds or gases,
absorption by various greenhouse gases or surfaces, emission of heat by
various materials. Any such alteration is a radiative forcing, and causes a new
balance to be reached. This happens continuously as sunlight hits the surface,
clouds and aerosols form, the concentrations of atmospheric gases vary, and
seasons alter the ground cover.
IPCC usage
"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/m2)."
In simple terms, radiative forcing is "...the rate of
energy change per unit area of the globe as measured at the top of the
atmosphere." In the context of climate
change, the term "forcing" is restricted to changes in the
radiation balance of the surface-troposphere system imposed by external
factors, with no changes in stratospheric dynamics, no surface and tropospheric
feedbacks in operation (i.e., no secondary effects induced because of
changes in tropospheric motions or its thermodynamic state), and no dynamically
induced changes in the amount and distribution of atmospheric water (vapour,
liquid, and solid forms).
Climate sensitivity
Radiative forcing can be used to estimate a subsequent change
in equilibrium surface temperature (ΔTs) arising from that
radiative forcing via the equation:
where λ is the climate sensitivity, usually with units in
K/(W/m2), and ΔF is the radiative forcing. A typical value of
λ is 0.8 K/(W/m2), which gives a warming of 3K for doubling of
CO2.
Example calculations
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 (πr2) is equal to 1/4 of the surface area of the
Earth (4πr2), the solar input per unit area is one quarter the
change in solar intensity. This must be multiplied by the fraction of incident
sunlight that is absorbed, F=(1-R), where R is the reflectivity, or albedo, of the
Earth. The albedo of the Earth is approximately 0.3, so F 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.
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.
where C is the CO2 concentration in parts
per million by volume and C0 is the reference concentration.
The relationship between carbon dioxide and radiative forcing is logarithmic,
and thus increased concentrations have a progressively smaller warming effect.
A different formula applies for some other greenhouse gases
such as methane
and N2O
(square-root dependence) or CFCs (linear), with coefficients that can be found e.g.
in the IPCC reports.
Related measures
Radiative forcing is intended as a useful way to compare
different causes of perturbations in a climate system. Other possible tools can
be constructed for the same purpose: for example Shine et al. say
"...recent experiments indicate that for changes in absorbing aerosols and
ozone, the predictive ability of radiative forcing is much worse... we propose
an alternative, the 'adjusted troposphere and stratosphere forcing'. We present
GCM calculations showing that it is a
significantly more reliable predictor of this GCM's surface temperature change
than radiative forcing. It is a candidate to supplement radiative forcing as a
metric for comparing different mechanisms...". In this quote, GCM stands
for "global circulation model", and the
word "predictive" does not refer to the ability of GCMs to forecast
climate change. Instead, it refers to the ability of the alternative tool
proposed by the authors to help explain the system response.
Changes in radiative forcing
The table below (derived from atmospheric radiative transfer
models) shows changes in radiative forcing between 1979 and 2013. The table includes
the contribution to radiative forcing from carbon
dioxide (CO
2), methane (CH
4), nitrous oxide (N
2O); chlorofluorocarbons (CFCs) ; and fifteen other minor, long-lived, halogenated gases. The table includes the contribution to radiative forcing of long-lived greenhouse gases. It does not include other forcings, such as aerosols and changes in solar activity.
2), methane (CH
4), nitrous oxide (N
2O); chlorofluorocarbons (CFCs) ; and fifteen other minor, long-lived, halogenated gases. The table includes the contribution to radiative forcing of long-lived greenhouse gases. It does not include other forcings, such as aerosols and changes in solar activity.
Radiative forcing, relative to 1750, due to carbon dioxide
alone since 1979. The percent change from January 1, 1990 is shown on the right
axis.
The table shows that CO
2 dominates the total forcing, with methane and the CFCs becoming relatively smaller contributors to the total forcing over time. The five major greenhouse gases account for about 96% of the direct radiative forcing by long-lived greenhouse gas increases since 1750. The remaining 4% is contributed by the 15 minor halogenated gases.
2 dominates the total forcing, with methane and the CFCs becoming relatively smaller contributors to the total forcing over time. The five major greenhouse gases account for about 96% of the direct radiative forcing by long-lived greenhouse gas increases since 1750. The remaining 4% is contributed by the 15 minor halogenated gases.
The table also includes an "Annual Greenhouse Gas
Index" (AGGI), which is defined as the ratio of the total direct radiative
forcing due to long-lived greenhouse gases for any year for which adequate
global measurements exist to that which was present in 1990. 1990 was chosen
because it is the baseline year for the Kyoto
Protocol. This index is a measure of the inter-annual changes in conditions
that affect carbon dioxide emission and uptake, methane and nitrous oxide
sources and sinks, the decline in the atmospheric abundance of ozone-depleting
chemicals related to the Montreal
Protocol. and the increase in their substitutes (HCFCs and HFCs). Most of
this increase is related to CO
2. For 2013, the AGGI was 1.34 (representing an increase in total direct radiative forcing of 34% since 1990). The increase in CO
2 forcing alone since 1990 was about 46%. The decline in the CFCs has tempered the increase in net radiative forcing considerably.
2. For 2013, the AGGI was 1.34 (representing an increase in total direct radiative forcing of 34% since 1990). The increase in CO
2 forcing alone since 1990 was about 46%. The decline in the CFCs has tempered the increase in net radiative forcing considerably.
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