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Global Cooling?

Florida Energy Extension Service and Mike West

As we leave behind of one of the harshest winters for the eastern United States in a decade, with thirteen major winter storms (at last count) and below normal temperatures, some are humorously hoping that global warming will come to the rescue. However, climatologists remind us that the past ten to twelve winters have actually been warmer than normal; apparently we've gotten soft. To be serious, the real effects of full scale anthropogenic global warming could cause a devastating rise in sea levels and widespread drought. But, without naturally occurring greenhouse gases such as carbon dioxide, methane, and water vapor, Earth's average surface temperature would be an icy 4°F, instead of the balmy 62°F we now enjoy. 

The amount of carbon dioxide in the atmosphere has increased steadily since measurements were taken back in 1958. Human caused global warming accounts for about a 1°F rise in temperature from 1890. However, this amount of actual global warming is less than the best climate models predict, considering the amount of carbon dioxide in the atmosphere. Apparently, there are other effects that tend to cool the earth that partially counter-act the warming effect of human caused greenhouse gases. This fact has spawned some interesting research. Atmospheric aerosols, mostly sulfates which also are products of fossil fuel burning and other human activity, reflect sunlight back into space and reduce the amount of energy available to warm the planet. It is ironic that the reflection effect of man-made sulfate aerosol may be partially or completely offsetting the greenhouse effect of man-made carbon dioxide. Sulfate aerosol is formed in two ways: directly via chemical reactions to form sulfur dioxide and hydrated ammonium sulfate salts, and by oxidation of sulfur dioxide by hydrogen peroxide in cloud droplets; these partially evaporate leaving a concentrated submicron aerosol droplet. The aerosol hovers in the lower troposphere (ground level to 25,000 feet) for a few days, reflecting sunlight back into space and increasing the reflectivity of clouds. The sulfate aerosol is composed of oxidized sulfur particles about 0.1 to 1.0 m in diameter. The main source of sulfate particles in the atmosphere is sulfur dioxide from industry (responsible for two-thirds). 

Phytoplankton, volcanoes, swamps and bogs also contribute. About half of the sulfur gases emitted into the atmosphere are removed by precipitation, by plants, or are absorbed by soil and seawater. The precipitation that washes out the aerosol becomes acidic. Aerosols have a short atmospheric residence time. This means that the particles cannot get very far from their source before they are washed out, dictating that cooling effects should be localized and most significant in areas with the highest emissions of sulfur gases. About 90 percent of sulfur gases are emitted in the Northern Hemisphere, where fossil fuels release five times the amount of natural emissions. In the Southern Hemisphere, one-fourth of sulfur gas emissions are caused by human activity. Since mixing of the air over the two hemispheres requires about a year, sulfur aerosol remains in the hemisphere in which it was formed. Both greenhouse gas and sulfate gas emissions have increased steadily since records have been kept. As models predict, the earth has warmed due to the greenhouse effect. But the Northern Hemisphere has warmed more slowly, possibly due to sulfates in the troposphere. Each gram of sulfate represents about 100 square feet of reflective surface area; about 35 billion kilograms of anthropogenic sulfur are converted to aerosol each year. Of the total amount of sunlight reflected by the aerosol, about 15 to 20 percent is sent back to space. About three percent of the sun's direct radiation does not reach the surface because of this sulfate aerosol, and about 0.25 percent is reflected back to space. To evaluate the significance of this number, the amount of solar radiation reaching the earth is about 200 Watts (think of two 100 Watt light bulbs) for each square meter, so the loss is about half a Watt/m² averaged over the entire planet. Since 90% of the aerosol is in the Northern Hemisphere, the loss may be 1 Watt/m² there. These may be small numbers, but they are significant: carbon dioxide greenhouse warming amounts to a heat gain of about 1.5 Watts/m², and methane and nitrous oxide add another 1.0 Watts/m². 

Localized effects are even more fascinating. Researchers at the Max Plank Institute for Chemistry used data about the sources and wind-driven spreading of industrial sulfate gases to predict where and how much sulfate aerosol will form. When they plotted their results on a map, three large masses of haze appeared in the Northern Hemisphere: one centered over the Ohio Valley in the U.S., one over Europe, and a third over the Middle East. This makes it clear that the effects of sulfate aerosol are localized: The Ohio region experiences a loss of almost 1 Watt/m² while the Midwest has a net gain of 1.7 Watt/m² due to the combined effect of man-made greenhouse gases and sulfate aerosol. However, all of these findings are based on crude assumptions. 

Changes in carbon dioxide levels for the last four years may be the most revealing bit of information. During this time, the steady increase in CO₂ weakened, then the level actually declined, leveled, and declined again. More than a few experts are finding this difficult to explain, since the amount of CO₂ released from burning fossil fuels has not declined. Some speculate that the eruption of Mount Pinatubo in the Philipines may be a part of this atmospheric puzzle. Others have observed that the ratio of carbon 13 to carbon 12 in the atmosphere has increased, indicating an increase in CO₂ uptake by land-based plant life (which prefers carbon 12). Charles Keeling of the University of California at San Diego is troubled by the considerable error in the expected measurements of this ratio. Although the decrease in CO₂ has not yet been explained, experts believe it is temporary. Ralph Keeling, also of UCSD, sums up the situation: "That the carbon dioxide growth will stay low is doubtful, but this is relevant at least in the sense that it shows we don't really know what's happening with the respect to the most important man-made greenhouse gas." Finally, a definitive conclusion.

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