THE NATIONAL ACADEMIES PRESS
Radiative Forcing of Climate Change:
Expanding the Concept and Addressing Uncertainties
Changes in climate are driven by natural and human-induced perturbations of the Earth s energy balance. These climate drivers or “forcings” include variations in greenhouse gases, aerosols, land use, and the amount of energy Earth receives from the Sun. Although climate throughout Earth’s history has varied from “snowball” conditions with global ice cover to “hothouse” conditions when glaciers all but disappeared, the climate over the past 10,000 years has been remarkably stable and favorable to human civilization. Increasing evidence points to a large human impact on global climate over the past century. The report reviews current knowledge of climate forcings and recommends critical research needed to improve understanding. Whereas emphasis to date has been on how these climate forcings affect global mean temperature, the report finds that regional variation and climate impacts other than temperature deserve increased attention.
What is radiative forcing? The influence of a factor that can cause climate change, such as a greenhouse gas, is often evaluated in terms of its radiative forcing. Radiative forcing is a measure of how the energy balance of the Earth-atmosphere system is influenced when factors that affect climate are altered. The word radiative arises because these factors change the balance between incoming solar radiation and outgoing infrared radiation within the Earth’s atmosphere. This radiative balance controls the Earth’s surface temperature. The term forcing is used to indicate that Earth’s radiative balance is being pushed away from its normal state.
Radiative forcing is usually quantified as the ‘rate of energy change per unit area of the globe as measured at the top of the atmosphere’, and is expressed in units of ‘Watts per square metre’ (see Figure 2). When radiative forcing from a factor or group of factors is evaluated as positive, the energy of the Earth-atmosphere system will ultimately increase, leading to a warming of the system. In contrast, for a negative radiative forcing, the energy will ultimately decrease, leading to a cooling of the system. Important challenges for climate scientists are to identify all the factors that affect climate and the mechanisms by which they exert a forcing, to quantify the radiative forcing of each factor and to evaluate the total radiative forcing from the group of factors.
For those who are somewhat mathematically inclined, visit the article noted below for a description of the CO2 greenhouse effect:
The CO2 problem in 6 easy steps
We often get requests to provide an easy-to-understand explanation for why increasing CO2 is a significant problem without relying on climate models and we are generally happy to oblige. The explanation has a number of separate steps which tend to sometimes get confused and so we will try to break it down carefully.
Step 1: There is a natural greenhouse effect.
Step 2: Trace gases contribute to the natural greenhouse effect.
Step 3: The trace greenhouse gases have increased markedly due to human emissions
Step 4: Radiative forcing is a useful diagnostic and can easily be calculated
Step 5: Climate sensitivity is around 3ºC for a doubling of CO2
Step 6: Radiative forcing x climate sensitivity is a significant number
The article concludes with the observation that with current forcings, there would be a 1.2 Celsius degree rise above the pre-industrial base “at equilibrium” and that with an experienced 0.7 degree rise, there is still 0.5 degrees “in the pipe”. It is probably retained by the oceans and yet to be released to the atmosphere.
RF Fig. 1 IPCC WG1 CH02 Radiative Forcing
1750 – 2005
IPCC AR4 WG1 Figure 2.21. Components of RF for emissions of principal gases, aerosols and aerosol precursors and other changes. Values represent RF in 2005 due to emissions and changes since 1750. (S) and (T) next to gas species represent stratospheric and tropospheric changes, respectively. The uncertainties are given in the footnotes to Table 2.13. Quantitative values are displayed in Table 2.13.
RF Fig. 2 IPCC WG1 CH02 Radiative Forcing
(IPCC AR4 WG1 Figure 2.22)
Integrated RF of year 2000 emissions over two time horizons (20 and 100 years). The figure gives an indication of the future climate impact of current emissions. The values for aerosols and aerosol precursors are essentially equal for the two time horizons. It should be noted that the RFs of short-lived gases and aerosol depend critically on both when and where they are emitted; the values given in the figure apply only to total global annual emissions. For organic carbon and BC, both fossil fuel (FF) and biomass burning emissions are included. The uncertainty estimates are based on the uncertainties in emission sources, lifetime and radiative efficiency estimates.