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15.2. Measurements in Greenland ice show a rapid rise in atmospheric sulfate emitted by industrial-era smokestacks, followed by a downturn in the 1970s after passage of the Clean Air Act.

countries from the former Soviet Union), and China. Plumes of aerosols are located downwind (to the east) of these sources. Because these particles reflect some of the incoming solar radiation, their net effect on climate is thought to be a regional-scale cooling. The amount of cooling is not well known; it depends on intricate details linked to the size, shape, and color of the mixture of different kinds of particles, as well as their height in the atmosphere. Within days or weeks, these aerosols are scrubbed from the atmosphere by rain, but this may not happen until they have drifted hundreds or thousands of miles downwind.

Ice cores record part of the history of human emissions of aerosols. The sulfate content in an ice core from Greenland (fig. 15.2) reveals an interval from 1650 to almost 1900 when sulfate concentrations generally stayed within a low range, except for brief spikes that mark large volcanic eruptions in the Northern Hemisphere and the tropics. Then, around 1900, the background concentration began a steady rise marking the main part of the industrial era, especially in nearby North America. The downturn in the sulfate values around 1980 reflects the initial effects of the Clean Air Act in reducing SO2 emissions in the United States.

Sulfate emissions during preindustrial times were probably negligible, at least compared to the industrial era. Neither Bronze Age nor early Iron Age metallurgy required extremely hot fires, and smokestack heights were low compared to those during the 1700s and afterwards. Both of these factors would have kept any SO2 emissions close to the ground and within the source areas, rather than sending them higher in the atmosphere where they could be dispersed over large regions. These emissions are not likely to have cooled preindustrial climate significantly.

Sulfate particles are just one of a range of industrial-era aerosols we emit, and the impact of other aerosols on climate is even more uncertain. Dark particles of "black carbon" emitted from modern-day deforestation and other burning are thought to warm the lower atmosphere by absorbing solar radiation, but the size of the warming effect on global temperature is not known. Given that the rate of forest clearance and burning prior to the industrial era was only 5 to 10 percent as large as the modern-day rate, the slow clearance of forests prior to 1700 seems unlikely to have had any measurable effect on global climate because of the aerosols produced.

By some estimates, industrial-era aerosols could have cooled climate enough to cancel perhaps 20 percent of the industrial-era greenhouse-gas warming that would otherwise have occurred. This cooling effect is the second major part of the explanation invoked for the relatively small increase in global temperature during the last century or two despite the sizeable increase in greenhouse gases. Another factor that may yet figure in this story is the effect of industrial-era forest clearance in altering the reflectivity of Earth's surface and its response to solar radiation. In any case, the delayed response of the climate system and the cooling effects from sulfate aerosols are the most likely answer for the apparent discrepancy between the recent warming trends and the increases in greenhouse gases.

Paradoxically, any abrupt, all-out action to roll back both the greenhouse and smokestack emissions of the industrial era would have the effect of intensifying the greenhouse warming, rather than reducing it. If we stopped putting any SO2 in the atmosphere today, the sulfate aerosols now there would rain out within a few weeks, removing their cooling effect and thereby causing some amount of warming almost immediately. But if we stopped putting CO2 in the atmosphere today, it would take more than a century for half of the industrial-era CO2 excess now in the atmosphere to be taken up in the deep ocean. With the industrial cooling effect quickly removed but the warming effect still in place, climate would warm. In addition, for at least a few decades, the unrealized greenhouse warming that is still in the pipeline in response to past greenhouse-gas increases would continue to kick in, pushing temperatures even higher. It's strange, but true: by cleaning up all our industrial emissions, we would amplify global warming, at least for a few decades. Later, as CO2 levels gradually decreased and the climate system slowly responded, climate would begin to cool.

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