With all the attention surrounding carbon dioxide these days, it is easy to forget that there are a number of other important natural and human-driven factors (“forcings” in climate circles) that influence Earth’s climate.

Among the most important of these are sulfate aerosols, microscopic particles smaller than a millionth of a meter suspended in the air. Sulfate aerosols are produced primarily from sulphur dioxide (SO₂) emitted during the combustion of fossil fuels. Along with ozone precursors, they are primary causes of acid rain and of lung irritation and ground-level haze or smog in polluted areas.

Sulfate aerosols also have a strong cooling effect on Earth, both through their ability to scatter incoming light and because of their propensity to increase cloud formation and reflectivity.

Among the most significant changes in climate change modeling between the 2001 IPCC Third Assessment Report (TAR) and the Fourth Assessment Report (AR4) in 2007 was a revision of the expected trajectory of human-induced sulfate aerosol emissions. In the earlier report, scientists assumed that aerosols would increase in rough proportional to economic growth. The authors of the 2007 report realized that emissions of aerosols, which have direct and immediate negative health effects to those in the area surrounding their emission, will likely be targeted for reductions as countries like China and India become wealthier. This emissions reduction would mirror a similar process that occurred in Europe and the United States.

Sulfate aerosols are the most significant substance in a category of aerosols tending to help cool the climate. Aerosols decrease radiative forcing in two ways: through direct aerosol effects as a result of an increased scattering and absorption of incoming solar radiation, and through indirect effects resulting from their ability to serve as cloud condensation nuclei.

An increased number of cloud condensation nuclei have a number of different effects: they increase the reflectivity of clouds by making them denser and giving them higher liquid water content, they increase the height of clouds, and they increase cloud lifetime.

Figure One, below, shows the major climate forcings over the past 120 years. The major positive forcings include CO₂ at 1.66 watts per meter squared (W m^-2), methane (CH₄) at 0.46 W m^-2, nitrous oxide (N₂O) at 0.16 W m^-2, and various halocarbons (CFCs, HCFCs, etc.) at 0.34 W m^-2. Aerosol direct effects account for -0.5 ± 0.4 W m^-2 negative forcing, with SO₂ comprising -0.4 W m^-2. Indirect effects are around -0.7 W m^-2, with a large uncertainty range of -1.8 to -0.3 W m^-2.

Aerosols are the primary reason why Earth is still at around 380 parts per million CO₂-equivilent (CO₂e), rather than the 460 ppm CO₂e projected if all the positive forcings were added together. Conveniently enough, aerosols pretty much cancel out the warming from all the non-CO₂ greenhouse gases.

Figure One. Radiative forcing of major climate factors over the past 123 years. Figure from Hansen et al 2005.

There are a number of different projections for sulfate aerosol emissions over the next century based on assumptions regarding the rate of economic growth, population growth, and technological development. Figure Two, below, shows an aggregation of all models of anthropogenic sulfate emissions used in the most recent IPCC report. Specific scenarios vary widely, but the median value across all models results by the year 2100 in sulfate aerosol emissions of 35 million metric tons, roughly one half of current emissions.

Figure Two. Projections of future aerosol emissions for SRES (Special Report on Emissions Scenarios) and post-SRES scenarios. Figure from the third working group of the latest IPCC report.

A reduction of anthropogenic SO₂ of around 50 percent worldwide over the next century, as projected in the most recent IPCC report, would result in a significant warming effect on the global climate. Sulfates are extremely short-lived particles, and emission reductions would have immediate effects on radiative forcing. A 50 percent reduction in sulfate aerosol emissions would reduce by half their current radiative forcing of -0.83 W m^-2. This change in forcings would increase global temperatures by roughly 0.36 degrees C (.64 F) relative to a scenario where aerosol emissions remain constant.

Figure three below shows the practical implications of a reduction in aerosols in the next century. If current greenhouse gas concentrations remain constant at current levels, scientists project about 1.34 degrees C (2.41 F) warming relative to pre-industrial temperatures by the end of the century (the world has already warmed 0.74 degrees C (1.33 F) in the past century, and 0.60 degrees C (1.08F) additional warming is in the pipeline as a result of Earth’s thermal inertia). A reduction of anthropogenic atmospheric sulfate aerosols by 50 percent means that 1.34 degrees C (2.41 F) warming suddenly becomes 1.70 degrees C (3.06 F).

Constant 2005 GHG Concentrations
Constant SO2	1.34 degrees C (2.41 F)
Reduced SO2	1.70 degrees C (3.06 F)
Figure Three. Based on a simple calculation of radiative forcings of the current atmospheric concentration of greenhouse gases at equilibrium, assuming a climate sensitivity of roughly 0.87 degrees K. Also assuming that anthropogenic SO2 represent only 72 percent of total atmospheric SO2 flux and that the indirect aerosol effects of SO2 account for around 62 percent of total indirect aerosol forcing, or -0.43 W m-2.

Expected changes in sulfate aerosol emissions over the next century in light of growing concerns over the health impacts of air quality in rapidly developing countries therefore may result in significant global temperature increases. An expected reduction of SO₂ emissions by 50 percent by 2100 will result in an equilibrium temperature increase of 0.36 degrees C contrasted with a world where SO₂ emissions remained constant. This additional warming means that current atmospheric GHG concentrations commit Earth to around 1.7 degrees C temperature increase relative to pre-industrial surface mean temperature.

The data suggest an urgency for the world to take immediate and effective action to begin reducing emissions if a mean temperature increase of more than 2.0 degrees C is to be avoided. It suggests also that meeting such a goal will be considerably harder than has been assumed in the past based on a business-as-usual approach to sulfate aerosol emissions.