Radiative Forcing Plot
Radiative Forcing combines the instantaneous heating due to greenhouse gases, aerosols (sulphate and smoke), and external forcing (solar variablility, volcanoes).
Java source code
Curves
(Radiative Forcing Module)
Major components
Red: Total
Black: Carbon Dioxide (CO2)
Green: Other greenhouse gases (see below)
Blue: Aerosols (sulphate and smoke)
Yellow: Solar and volcano
In expert complexity level, these are sub divided as follows
Other greenhouse gases
(see Oghga Module)
Green: Methane (CH4)
Brown: Nitrous Oxide (N2O)
Grey: Tropospheric Ozone (O3T)
Light-pink CFCs
Purple: HFCs
Pink: Stratospheric Ozone
light-green: Stratospheric water
Aerosols
Blue: Black carbon (soot -warming)
Dark Blue: Organic carbon (white smoke -cooling)
Cyan: Sulphate aerosols -direct effect
Light blue: Sulphate aerosols -indirect effect of cloud condensation nuclei
External forcings
Solar Variability
Volcanos (dust)
Units
Watts per square metre
(note this is a unit of power not energy)
Controls and Options
Cyan: Sulphate Aerosol R.F. (total) in 2000
Yellow: Solar variability R.F. in 2000
These both scale the whole curve up or down (see below)
Black (expert level): R.F. for CO2 doubling
A climate model parameter (see below)
BCOCWig Option (expert level): Choose Wigley rather than Joos formula for Carbon aerosol R.F. (Oghga Module)
Discussion
See also
Radfor module -how it works
Radiative Forcing is a convenient concept for combining various influences on the heat balance of the earth. It refers to the change in instantaneous heating at the top of the troposphere, compared to the preindustrial climate. For further explanation of the concept, see IPCCTAR WG1 Chapter six.
Carbon Dioxide
For CO2 the radiative forcing is proportional to the logarithm of its concentration. Consequently doubling the CO2 concentration has the same warming effect, regardless of the baseline.
Increasing the parameter "radiative forcing for CO2 doubling" (expert level only) does not have a big effect on the temperature, since the effect of CO2 on the latter is prescribed by the "climate sensitivity" (temperature rise for CO2 doubling). However it adjusts the relative influence of CO2 compared to other greenhouse gases, aerosols and solar variability. Both these parameters are used together, in fitting the simple model to a range of GCM predictions. See
temperature plot
climate module
Other gases
The other gas curve shows the sum of the radiative forcings from CH4, N2O, O3, CFCs and HFCs. For more detail see:
Other gas plot
F-gas plot
Oghga module -how it works
Currently the radiative forcing from the other gases combined is almost as great as that from CO2. However methane and tropospheric ozone have much shorter atmospheric lifetimes than CO2, and so they become relatively less important in the longer term.
Aerosols
Sulphate aerosols and white smoke reflect sunlight and there have a cooling effect (negative radiative forcing), whilst black smoke (soot) absorbs sunlight and has a warming effect.
Sulphate gases also combine with water to form droplets of sulphuric acid, which act as condensation nuclei for cloud droplets, which then reflect even more sunlight. This is known as the "indirect" effect.
Since these droplets are then removed from the atmosphere as acid rain, their atmospheric lifetime is very short, so these effects are greatest in industrial regions. In developed countries, the cooling effect of aerosols was greatest in 1970s-1980s and is now declining due to attempts to reduce acid rain by using of cleaner fuels or putting "scrubbers" on chimneys.
The magnitude of the sulphate aerosol radiative forcing (especially the indirect effect) is very uncertain, so
an adjustable parameter is provided for you to experiment with this. Try to get a good fit between the calculated and measured temperatures.
See Temperature Plot
The radiative forcing from carbon aerosols is even less certain. In 2000 the radiative forcing (W/m2) was assumed to be:
| Fossil Fuel | Biomass Burning |
White (organic) | -0.1 | -0.4 |
Black (soot) | +0.2 | +0.2 |
In the future, Bern-CC model simply scales these to CO emissions, whilst Wigley-Raper model assumes the fossil component scales with sulphate aerosols and the biomass part with "gross deforestation". By default the former is used, but the "BCOCWig" option (expert level) selects an approximation to the latter.
See Oghga module
Solar Variability and Volcanos
The direct impact of solar variability is rather small, however various mechanisms have been proposed which may amplify this. Here the curve from Hoyt and Schatten has been scaled to produce a forcing of 0.3W/m2 in 2000 compared to 1750 (as proposed in IPCCTAR-WG1-ch6).
As well as the regular eleven year sunspot cycle, there seems to be increased heating during the period 1910-1940, and a slight cooling thereafter. This helps to explain part of the shape of the historical temperature curve, but it cannot explain the particularly rapid warming since 1970.
Since the magnitude of this effect is very uncertain, an adjustable parameter is provided for you to experiment with this.
See Temperature Plot
In addition, the dust produced by major volcanic eruptions has a short term impact on global climate, the most recent example being Mt Pinatubo in 1991. These spikes are also visible in the carbon cycle, due to the feedback from temperature. However this forcing has no impact on the long term trends.
Different spatial distribution of forcing
It is assumed in this plot, that we can compare and sum the radiative forcing caused by various greenhouse gases, aerosols, and solar variability, which helps to illustrate the relative importance of various factors. However it must be emphasised that forcings with different regional distributions, cannot be considered to offset each other. For example, sulphate aerosols have a short lifetime so their cooling effect is felt mainly in industrialised areas of northern continents. Stratospheric ozone depletion is biased towards the polar regions. The effect of greenhouse gases may be greatest during winter nights, whilst the effect of increased solar activity may be greatest during summer days.
Some of these effects are considered by unevenly distributing the radiative forcing between the four surface boxes of the climate model.
See Climate Module
Correspondence with IPCC data
If you select expert complexity level, and then click the button "ipccdata" (top panel) you will see circles superimposed on this plot, which show the data published in the IPCCTAR-WG1-SRES appendix. These should be comparable with the model calculations, when "no-policy SRES scenarios" is also selected from the mitigation menu.
More about fit to IPCC data
Note:
Two sets of circles are given for CO2 R.F.,from Bern-CC and W/R models. To match these data you should move the "radiative forcing for CO2 doubling" parameter to 3.71. For some GCM-fits this parameter is lower, but then is offset by a higher climate sensitivity (see above and temperature plot).
One set of circles corresponds to the sum of BC+OC aerosol forcing using the Wigley formula (see above).