December 23, 2003
Soot Plays Bigger Role In Global Warming Than Previously Believed

James Hansen and Larissa Nazarenko say soot is responsible for a substantial portion of global warming.

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.

Soot is a potent warming pollutant.

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.

James Hansen has been making a similar argument at least since the year 2000.

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

kaitlyn said at November 30, 2004 7:15 PM:

I was wondering if an experts on global warming can email me for my science fair project. Thanks!

Diane said at October 7, 2005 7:51 AM:

I am currently writing a paper on soot emissions. I am supposed to come up with different types of policies that could help reduce soot emissions in the industry sectors and the average citizen. What do you suggest?

William Hoffman said at September 28, 2006 9:27 PM:

Soot on ice can have no effect on global temperatures. If it absorbs light and accelerates ice melting, the effect is to create water at 0 from ice at 0. This is a latent heat effect, not a sensible heat effect.

Second, taking you at your word that Hansen is of this persuasion, others of his general view suggest that soot is responsible for a widespread cloud cover increase that reflects sunlight, delaying the "real" Global Warming.

Excuses abound, rationalizations dominate the rhetoric and the fact remains that there is no Global Warming. Get a DVD telling more in detail about the poor scientific basis for such theories at

Invent Horsepower said at October 5, 2006 12:38 PM:

In response to WH:

First with that rational we never need to worry about warming temperatures as long as there is some ice on the North or South Poles since we just need to give them time to interact with each other.

In cold climates sublimation would not be insignificant in ice destruction. From observation in the Midwest I have observed how dirt aids this process when air temperature are below freezing. Yes ice melts at 0 C but it might have been much colder than that a few days before it began to melt.

The global dimming theory I am familiar with credits the ice in the jet exhaust for reflecting solar energy away from the earth not black soot. By the way I don't think there would be a cloud in the sky in Iowa today, if it weren't for jet trails. As an example, today these clouds cover about a fourth of the sky and perhaps they are thick enough to reflex 20% of the sun. Using these numbers these jet trails(man made clouds) reflex about 5% of the solar rays on this particular day.

Anyway this brings me back to another point about polar ice. You talk about the polar ice fields as just big heat sinks. But they also act as big reflectors. So as more and more land or sea are exposed less and less solar radiation is reflected back to space.

So no I don't think we should scoff at the notion of global warning. But it is more than just a function of green house gases which these rather new theories on soot and global dimming suggest. But, lets not depend on global dimming to cool the earth since it effects other things too. For one it also reduces the amount of solar radiation the crops get which could effect yields.

The President of the United States says we are addicted to oil. In my opinion that is bad for a variety of reasons. If someone is concerned about climate change and carbon dioxide formation why should I argue with them if I didn't agree. After all the solution to reducing carbon dioxide and reducing the dependence on oil is the same. Consume less energy!

Lloyd said at December 28, 2006 7:09 PM:

Global warming as usually reported is totally distorted. The problem is there is too much talk and not enough data quoted. The warming proponents in particular are prone to talk-talk-talk but show no data. What we need to do is to share information more and to understand what it is saying, especially on global warming, where potentially costly decisions are being proposed nearly every day by pundits to reduce CO2 unnecessarily etc. Mainly their comments are aimed at coal use, but not any other fossil fuel use, all of which produce CO2 which at best all manmade CO2 can cause only a miniscule change in warming or cooling and mighty small in effects compared to El Nino effects on global warming. Certainly Mt. Washington data shows so-called global warming is not happening summers in North America, and not in any way that is harmful winters. Winter warming in evidence (look, it's still very cold) saves energy, the summer cooling in evidence saves energy as well. Following is my analysis of the data sets:

1. See page 4450 of the Mt. Washington AMS temperature report, it shows the diverging winter warming/spring warming trend and summer/fall cooling trend, straight line averages for these past 69 years (report is essentially up to present time). This combination does not indicate global warming, but rather that another ice age is just starting (more below). The peaking temperature data of ice cores (attached) also is a key indicator.

Per Mt. Washington report, winter and spring temperatures (still very cold) are increasing, likely due to more cloud cover thus a greater greenhouse effect up north in winter, thus slightly more snow in northern climes must be falling somewhere (on average); this will be provable over time. And declining summer and fall temperatures (that's the big tip off) results from overall more northern snow reflecting sunlight causing increased summer and fall cooling. This the traditional view by scientists, and I doubt if it has changed. I think most climate scientists haven't seen Mt. Washington data, apparently it's been kept under wraps. Keep in mind the summer temperature dip is still very slight (page 4450), the new ice age is just barely beginning. And while this dip is not statistically significant, it's very significant it has been dropping for 70 odd years, not increasing.

2. Additional indicators that climate temperatures have peaked and we are starting the big fall in summer temperatures (predicted by ice core data attached) is that satellite measured temperatures are basically flat, and TSI (sun energy reaching earth, satellite measured) is only slowly increasing now, and possibly peaked. The fact that overall winter/spring temperatures are higher causing the average peak to increase slightly by 0.24C these past 69 years is meaningless in the larger scheme of things. Higher winter temperatures mainly indicate a larger greenhouse effect in winter (it is still very cold) due to more cloud cover and thus more greenhouse effect and also more snowfall up north. Hence we have likely peaked in temperature as the ice core data suggests, and we are in the early stages of another ice age. We may see this new ice age start to accelerate in our lifetimes.

Unlike the Al Gore movie, which strings along film clips of natural disasters that happen all the time, an irresponsible way to prove something that isn't in fact apparently happening, this Mt. Washington U.S. weather station hard data is something we can rely on that is totally not influenced by human habitation or innuendo, pundits repeating more pundits, etc. Fortunately it is factual (along with ice core data and satellite measurements), and measuring what we need to know to predict future climate trends, we just need to pay attention to the data to see it.

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