Increase in Radiative Forcing due to Humans

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 12) that give warming due to carbon dioxide with and without considering the sulfate cooling.

This issue will be further discussed in units 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 Figure 13 , 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.

PREVIOUS: Natural Variations of Radiative Forcing

Back