Radiative Forcing

Images
figure 8
Changes in radiative forcing due to increases in greenhouse gas concentrations between 1765 and 1990. Houghton, J.T., G.J. Jenkins, J.J. Ephraums, eds, 1990: 1990 Intergovernment Panel on Climate Change, Cambridge University Press.Figure 2.2, page 55.
figure 9
Comparison of Anthropogenic Greenhouse Forcing with Natural Variations of Radiative Forcing.
figure 10
Reconstructed solar irradiance from 1874-1988. Houghton, J.T., G.J. Jenkins, J.J. Ephraums, eds, 1990: 1990 Intergovernment Panel on Climate Change, Cambridge University Press.
figure 11
Global warming climate runs with CO₂ and sulfate aerosols (A consortium for the Application of Climate Impact Assessments).
accompaning graph
Comparing the Global Heating and Cooling from Fossil Fuel Combustion(Stephen E. Schwartz, Brookhaven National Laboratory)

Their radiative forcing effects are expressed in Wm^-2decade^-1 for five different periods, ending with the decade of the 1980s. Note that forcing due to carbon dioxide has increased but not directly in proportion to its increased concentration, as previously discussed. The impacts of others, such as the CFCs and HCFCs, have increased more dramatically even though their abundance is very low, because their impact per molecule is high and their lifetimes are large.
Note that the magnitude of the increase in radiative forcing for the most recent decade on the previous graph is about 0.55 Wm^-2 (figure 8). The total cumulative radiative forcing since the Industrial Revolution due to anthropogenic greenhouse gases is about 2.77 Wm^-2.
We should ask how this 0.55 Wm^-2decade^-1 compares with natural fluctuations in radiative forcing (figure 9). For instance, the luminosity of the sun fluctuates, which causes variations in the amount of energy the earth receives from the sun. It would be useful to compare measurements of this variation with present and projected future contributions due to enhanced greenhouse effects.
Other natural variations are due to orbital changes of the earth around the sun, changes in the radius of the sun, and effects of volcanoes.
Variability of orbital parameters of the earth's revolution about the sun is reduced by internal feedback mechanisms, but its effect is estimated to be about 0.035 Wm^-2decade^-1, which is much less than the 0.55 Wm^-2decade^-1 due to anthropogenic greenhouse gases. Variability of total output of the sun over short periods (e.g., a few years) is about 1.4 Wm^-2decade^-1 (see figure 10), but this variability tends to be cyclical over longer periods. Averaged over the last 100 years, variability of total output of the sun has been about 0.1 Wm^-2decade^-1, again far less than 0.55 Wm^-2decade^-1. The rate of heat loss due to heat flow jfrom the center of the earth is 0.06 W/m².
Changes in radius of the sun and changes in sunspot activity also lead to natural variability of about 0.1 Wm^-2decade^-1. A major volcano releases large amounts of sulfate and other particles that may persist in the stratosphere for 1 to 3 years and lead to global cooling due to reflection of incoming solar radiation. This can change radiative forcing by 0.2 to 0.4 Wm^-2decade^-1 for the period of time that the dust lingers in the stratosphere. Because such major eruptions occur only once per decade or so, their average effect is far less than 0.55 Wm^-2decade^-1.
What about the effect of changes in landuse? A previous table showed different albedos (reflectivities) for different types of surfaces. Humans have deforested Europe and the US and are now deforesting tropical areas and changing them to agricultural land. Humans also have built cities, eliminated natural prairies, and installed irrigation systems enabling crops to be grown in near-desert regions. These anthropogenic changes have increased the albedo of the planet by approximately 0.006 over the last 1,000 years, giving a change in radiative forcing of about 0.01 Wm^-2decade^-1.
From these comparisons, it should be apparent that human emission of greenhouse gases have increased the radiative forcing of the planet far in excess of natural variations.
It has been discovered recently that sulfur dioxide released by burning of coal further complicates the calculations of human impact on planetary surface temperatures. Sulfur dioxide (SO₂) emitted when coal is burned is transformed into sulfate particles in the atmosphere which reflect solar radiation directly back to outer space. When the sulfate particles dissolve in cloud droplets, they tend to "brighten" the cloud and further increase the solar radiation reflected back to outer space from the top of the cloud. Therefore sulfate particles make both direct and indirect contributions to cooling the lower atmosphere. As we learned in the discussion of the sulfur cycle, sulfates have a relatively short lifetime in the atmosphere (1-3 weeks). This means that the cooling effect of sulfate particles is not global but rather occurs near and downwind of major industrial areas. The magnitude of this effect on a regional climate is thought to be about -0.3 to -0.9 Wm^-2 for direct forcing and as much as -0.8 Wm^-2 for indirect forcing. The impact of this negative forcing is a cooling effect of 0.8 to 1.6⁰C in the global mean temperature. An important difference between CO₂ warming and aerosol cooling is that the longer lifetime of CO₂ means continuing emissions even at a non-increasing rate, lead to increased radiative forcing. For aerosols, with limited lifetimes, constant emission rates simply replace the aerosol particles being washed out of the atmosphere. So the aerosol radiative forcing for emissions is constant.
Recent estimates of the effects of sulfate aerosol particles are shown in the next two global maps (figure 11) that give warming due to carbon dioxide with and without considering the sulfate cooling.
This issue will be further discussed in lectures on climate modeling.
One goal of the US Clear Air Act and its amendments (and comparable actions in Europe) over the last 25 years is the reduction of sulfur emissions from combustion of coal. These efforts have achieved some success, as is shown by the accompanying graph , which shows that the ratio of sulfur to carbon dioxide in emissions has decreased. The dilemma presented by this situation is that our continuing efforts to reduce sulfur emissions will unmask increasing amounts of global warming due to rising CO₂.
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