Radiative Forcing and Climate Change

A perturbation to the atmospheric concentration of an important greenhouse gas, or the distribution of aerosols, induces a radiative forcing that can affect climate. Radiative forcing of the surface-troposphere system is defined as the change in net radiative flux at the tropopause due to a change in either solar or infrared radiation (IPCC, 1996a). Generally, this net flux is calculated after allowing for stratospheric temperatures to re-adjust to radiative equilibrium. A positive radiative forcing tends on average to warm the surface; a negative radiative forcing tends to cool the surface. This definition is based on earlier climate modeling studies, which indicated an approximately linear relationship between the global mean radiative forcing at the tropopause and the resulting global mean surface temperature change at equilibrium. However, recent studies of greenhouse gases (e.g., Hansen et al., 1997) indicate that the climatic response can be sensitive to the altitude, latitude, and nature of the forcing.

The resulting change in radiative forcing can then drive changes in the climate. A positive radiative forcing tends on average to warm the Earth's surface; a negative radiative forcing tends to cool the surface. Changes in radiative forcing can occur either as a result of natural phenomena or due to human activities. Natural causes for significant changes in radiative forcing include those due to changes in solar luminosity or due to concentrations of sulfate aerosols following a major volcanic eruption. Human related causes include the changes in atmospheric concentrations of greenhouse gases and in aerosol loading discussed earlier.

2.310.25 Climate Forcings

J -CFCs vegetal ion forced and other volcanic

0.3+0.15 stratospheric tropospheric cloud surface 0.4±0.2 ozone aerosols changes alterations aerosols

- well-mixed tropo spheric greenhouse ozone -0.2+0,1

-greenhouse gases -

(indirect via 03) ! other anthropogenic forcings 1 | natural forcings -» 1

Figure 1.4 Estimated change in globally and annually averaged anthropogenic radiative forcing (Wm~2) resulting from changes in concentrations of greenhouse gases and aerosols from the pre-industrial time to 1998 and to natural changes in solar output from 1850 to 1998. The error bars show an estimate of the uncertainty range. Based on

1.4.1 Explaining the past record

As shown in Figure 1.4, analyses of the direct radiative forcing due to the changes in greenhouse gas concentrations since the beginning of the Industrial Era (roughly about 1800) give an increase of about 2.3 Wm~2 (Hansen et al., 1998). To put this into perspective, a doubling of CO2 from pre-industrial levels would correspond to about 4 Wm~2; climate models studies indicate this would give 1.5 to 4.5°C increase in global temperature. Approximately 0.5 Wm~2 of the increase has occurred within the last decade. By far the largest effect on radiative forcing has been the increasing concentration of carbon dioxide, accounting for about 64 percent of the total change in forcing. Methane has produced the second largest change in radiative forcing of the greenhouse gases.

Changes in the amounts of sulfate, nitrate, and carbonaceous aerosols induced by natural and human activities have all contributed to changes in radiative forcing over the last century. The direct effect on climate from sulfate aerosols occurs primarily through the scattering of solar radiation. This scattering produces a negative radiative forcing, and has resulted in a cooling tendency on the Earth's surface that counteracts some of the warming effect from the greenhouse gases. In the global average, increases in amounts of carbonaceous aerosols, which absorb solar and infrared radiation, have likely counteracted some of the effect of the sulfate aerosols. Aerosols can also produce an indirect radiative forcing by acting as condensation nuclei for cloud formation.

There is large uncertainty in determining the extent of radiative forcing that has resulted from this indirect effect, as indicated in Figure 1.4.

Changes in tropospheric and stratospheric ozone also affect climate, but the increase in tropospheric ozone over the last century and the decrease in stratospheric ozone over recent decades have had a relatively small combined effect on radiative forcing compared to CO2. The radiative forcing from the changes in amount of stratospheric ozone, which has primarily occurred over the last few decades, mostly as a result of human-related emissions of halogenated compounds containing chlorine and bromine, is generally well understood. However, the changes in concentration of tropospheric ozone over the last century, and the resulting radiative forcing, are much less well understood.

Change in the solar energy output reaching the Earth is also an important external forcing on the climate system. The Sun's output of energy is known to vary by small amounts over the 11-year cycle associated with sunspots, and there are indications that the solar output may vary by larger amounts over longer time periods. Slow variations in the Earth's orbit, over time scales of multiple decades to thousands of years, have varied the solar radiation reaching the Earth, and have affected the past climate. As shown in Figure 1.4, solar variations over the last century are thought to have had a small but important effect on the climate, but are not important in explaining the large increase in temperatures over the last few decades.

Evaluation of the radiative forcing from all of the different sources since pre-industrial times indicates that globally-averaged radiative forcing on climate has increased. Because of the hemispheric and other inhomogeneous variations in concentrations of aerosols, the overall change in radiative forcing is much greater or much smaller at specific locations over the globe.

Any changes induced in climate as a result of human activities, or from natural forcings like variations in the solar flux, will be superimposed on a background of natural climatic variations that occur on a broad range of temporal and spatial scales. Analyses to detect the possible influence of human activities have had to take such natural variations into consideration. As mentioned earlier, however, recent studies suggest that the warmer global temperatures over the last decade appear to be outside the range of natural variability found in the climate record for the last four hundred to one thousand years.

1.4.2 Projecting the future changes

In order to study the potential implications on climate from further changes in human-related emissions and atmospheric composition, a range of scenarios for future emissions of greenhouse gases and aerosol precursors has been produced by the IPCC Special Report on Emission Scenarios (SRES), for use in modeling studies to assess potential changes in climate over the next century for the current IPCC international assessment of climate change. None of these scenarios should be considered as a prediction of the future, but they do illustrate the effects of various assumptions about economics, demography, and policy on future emissions. In this study, four SRES "marker" scenarios, labeled A1, A2, B1, and B2, are investigated as examples of the possible effect of greenhouse gases on climate. Each scenario is based on a narrative storyline, describing alternative future developments in economics, technical, environmental and social dimensions. Details of these storylines and the SRES process can be found elsewhere (Nakicenovic et al., 1998; Wigley, 1999). These scenarios are generally thought to represent the possible range for a business-as-usual situation where there have been no significant efforts to reduce emissions to slow down or prevent climate changes.

Figure 1.5 shows the anthropogenic emissions for four of the most important gases to concerns about climate change, CO2, N2O, CH4, and SO2. Carbon dioxide emissions span a wide range, from nearly five times the 1990 value by 2100 to emissions that rise and then fall to near their 1990 value. N2O and CH4 emission scenarios reflect these variations and have similar trends. However, global sulfur dioxide emissions in 2100 have declined to below their 1990 levels in all scenarios, because rising affluence increases the demand for emissions reductions. Note that sulfur emissions, particularly to mid-century, differ fairly substantially between the scenarios. Also, the new scenarios for sulfur emissions are much smaller than earlier analyses (e.g., IPCC, 1996a), largely as a result of increased recognition worldwide of the importance of reducing sulfate aerosol effects on human health, on agriculture, and on the biosphere.

In this study the global climate change consequences of SRES scenarios were calculated with the reduced form version of our Integrated Science Assessment Model (ISAM) (Jain et al., 1996; Kheshgi et al., 1999). The model consists of several gas cycle sub-models converting emissions of major greenhouse gases to concentrations, an energy balance climate model for the atmosphere and ocean, and a sea level rise model. In this study, updated radiative forcing analyses (Jain et al., 2000) for various greenhouse gases have been used. Based on results from the carbon cycle submodel within ISAM, Figure 1.6 shows the derived changes in concentrations of carbon dioxide for the four SRES scenarios. Over the next century, CO2 concentrations continue to increase in each scenario, reaching concentrations from 548 ppm to 806 ppm. Even though emissions decline in some of the scenarios, the long atmospheric lifetime of CO2 results in continued increases in concentration over the century.

The upper panel of Figure 1.7 shows the derived globally averaged radiative

Figure 1.5 Anthropogenic emissions in the SRES marker scenarios for CO2, CH4, N2O, and SO2. Note that the SRES emission values are standardized such that emissions from 1990 to 2000 are identical in all scenarios.

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