Soot particles may be twice as bad as the greenhouse gas carbon dioxide in contributing to global warming, suggests a new study.
Grains of soot deposited in snow have also caused about one-quarter of the observed rise in global surface temperature since 1880, suggests the model by James Hansen and Larissa Nazarenko. The pair examined how soot particles affect the atmosphere when they darken snow and ice.
Note that when soot causes ice to melt that makes previously white surface areas become darker and hence to absorb more sunlight and radiate more heat. Hence, the melting of ice and packed snow raises temperatures and causes more ice to melt.
Hansen, director of NASA's Goddard Institute for Space Studies, and Nazarenko, a staff associate there, found soot is twice as potent as carbon dioxide in changing global surface air temperatures in the Arctic and the Northern Hemisphere.
The NASA Goddard Space Flight Center press release page has nice graphs and animations that make clicking here worth the trip.
Hansen and Larissa Nazarenko, both of the Goddard Institute and Columbia University's Earth Institute, found soot's effect on snow albedo (solar energy reflected back to space), which has been neglected in previous studies, may be contributing to trends toward early springs in the Northern Hemisphere, thinning Arctic sea ice, melting glaciers and permafrost. Soot also is believed to play a role in changes in the atmosphere above the oceans and land.
"Black carbon reduces the amount of energy reflected by snow back into space, thus heating the snow surface more than if there were no black carbon," Hansen said.
Soot's increased absorption of solar energy is especially effective in warming the world's climate. "This forcing is unusually effective, causing twice as much global warming as a carbon-dioxide forcing of the same magnitude," Hansen noted.
Hansen cautioned, although the role of soot in altering global climate is substantial, it does not alter the fact greenhouse gases are the primary cause of climate warming during the past century. Such gases are expected to be the largest climate forcing for the rest of this century.
The researchers found that observed warming in the Northern Hemisphere was large in the winter and spring at middle and high latitudes. These observations were consistent with the researchers' climate model simulations, which showed some of the largest warming effects occurred when there was heavy snow cover and sufficient sunlight.
Hansen and Nazarenko used a leading worldwide-climate computer model to simulate effects of greenhouse gases and other factors on world climate. The model incorporated data from NASA spacecraft that monitor the Earth's surface, vegetation, oceans and atmospheric qualities. The calculated global warming from soot in snow and ice, by itself in an 1880-2000 simulation, accounted for 25 percent of observed global warming. NASA's Terra and Aqua satellites are observing snow cover and reflectivity at multiple wavelengths, which allows quantitative monitoring of changing snow cover and effects of soot on snow.
While this may not be immediately obvious this report seems like good news. Why? Because it is a lot cheaper to reduce soot emissions than to reduce carbon dioxide emissions. If a substantial source of warming can be cancelled out cheaply then that buys time (assuming it really is necessary to intervene in the first place) to develop technologies that will allow carbon dioxide emissions to be reduced at much lower cost.
Soot is not the only warming pollutant which would be cheaper to reduce than carbon dioxide. Methane also has warming effects and reduction of methane emissions could be done cheaply. A reduction in methane emissions would have the added benefit of reducing ozone at ground level.
Reduction in soot emissions would also yield substantial health benefits. A recent paper published in the medical journal Circulation C. Arden Pope of Brigham Young University and colleagues have found that particulate pollutants increase the incidence of cardiopulmonary diseases and ischemic heat attack.
Tiny particles of pollutants emitted by automobiles, power plants and factories significantly increase the risk of dying from cardiovascular disease in the United States, according to a study led by Brigham Young University epidemiologist Arden Pope.
The research was published in "Circulation: Journal of the American Heart Association" on Dec. 15. Statistical links between air pollution and increased mortality were reported by Pope and others in the mid-1990s. In March of 2002 he and colleagues reported associations between air pollution and lung cancer, as well as the broad category of cardiopulmonary disease, which includes both heart and lung ailments.
The new study narrows the latter finding by identifying a strong link between particulate air pollution and ischemic heart disease (the type that causes heart attacks), and also a link between pollution and the combined category of irregular heart rhythms, heart failure and cardiac arrest. It also suggests general biological pathways through which pollution might cause these diseases that lead to death – increased inflammation and nervous system aberrations that change heart rhythm.
"Not only do we show a statistical link between particulate air pollution and these types of heart disease," Pope said, "but we see specific patterns that are consistent with mechanistic pathways that may help explain how air pollution causes those diseases. The study discusses recent advancements in cardiovascular medicine that have explored the role of inflammation in the development and progression of atherosclerosis. The study results are consistent with recent findings that air pollution may provoke low-grade pulmonary inflammation, accelerate the progression of atherosclerosis, and alter cardiac function. These results add biological plausibility that air pollution really is a risk factor for heart disease."
The EPA has declared that the annual average level of PM2.5 particles in the air should not exceed 15 micrograms per cubic meter. Pope's study showed that each 10 micrograms-per-cubic-meter increase in fine particulate air pollution is accompanied by an 18-percent increase in risk of death from ischemic heart disease and a 13-percent increase in risk of death from altered heart rhythm, heart failure or cardiac arrest.
Further analysis also showed higher risks associated with air pollution for former and current smokers – 26 percent and 94 percent, respectively. Pope notes that "smoking is clearly a much larger risk factor, but air pollution increases the risk of cardiovascular death in non-smokers and seems to add additional risk to smokers."
Pope emphasized that the study's findings are "good news." Since the early 1980s, the annual U.S. average of PM2.5 has dropped from 21 to 14 micrograms per cubic meter.
If we consider both the uncertainties in the current climate models (witness the sudden discovery of the greater importance of soot outlined above) and the advantages both in costs and in health benefits for the reduction in warming pollutants other than carbon dioxide it seems foolish to rush into a major reduction in carbon dioxide emissions. There are easier pickings that provide more benefits. A reduction in particulates would reduce heart disease and cancer and make us healthier in other ways. A reduction in methane emissions would reduce ozone pollution and therefore also reduce harm to the lungs and other organs.
Now, almost 12 years later, Dr. Hansen says that too much emphasis has been placed on the effects of fossil fuels combustion. Instead, Hansen says that warming over the past century was mostly driven by gases such as methane and chlorofluorocarbons.
In a report in Proceedings of the National Academy of Sciences, he says, "We suggest that a strategy to slow global warming focus on reducing air pollution, especially tropospheric (ground level) ozone, methane and black carbon particles." The report notes that the growth rate of non-carbon greenhouse gases has "declined in the past decade."
Dr. Hansen states that global warming can be prevented "without any economically wrenching actions." He says that "human health and ecological costs of these pollutants are counted in billions of dollars in the United States and impacts are reaching devastating levels in the developing world. A strategy focused on reducing these pollutants, which are not essential to energy production, should unite the interests of developed and developing nations."
Update: See a previous work by Hansen and colleagues from May 2003. Note that soot emissions are high in areas that are economically less developed.
Black carbon or soot is generated from traffic, industrial pollution, outdoor fires and household burning of coal and biomass fuels. Soot is a product of incomplete combustion, especially of diesel fuels, biofuels, coal and outdoor biomass burning. Emissions are large in areas where cooking and heating are done with wood, field residue, cow dung and coal, at a low temperature that does not allow for complete combustion. The resulting soot particles absorb sunlight, just as dark pavement becomes hotter than light pavement.
Update II: On a related note Richard Muller discusses the limitations of current methods to estimate historical temperatures.
The disagreement is not political; most of it arises from valid issues involving physics and mathematics. First the physics. An accurate thermometer wasn’t invented until 1724 (by Fahrenheit), and good worldwide records didn’t exist prior to the 1900s. For earlier eras, we depend on indirect estimates called proxies. These include the widths of tree rings, the ratio of oxygen isotopes in glacial ice, variations in species of microscopic animals trapped in sediment (different kinds thrive at different temperatures), and even historical records of harbor closures from ice. Of course, these proxies also respond to other elements of weather, such as rainfall, cloud cover, and storm patterns. Moreover, most proxies are sensitive to local conditions, and extrapolating to global climate can be hazardous. Chose the wrong proxies and you’ll get the wrong answer.
When you hear some claim that a particular year is the hottest year on record for x many hundreds or thousands of years take it with a grain of salt. As Muller's article shows, there are unresolved questions about how to accurately estimate historical temperatures in previous centuries on planet Earth.
|Share |||Randall Parker, 2003 December 23 03:53 PM Climate Trends|