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Particles of Doubt

It's April 2012, and the earth's average temperature just hit a new high.
But you're shoveling snow.

Sulfate particles could be a key reason why much of the eastern United States hasn't kept pace with the global warm-up of this century. Our world's climate may become even more skewed if some nations continue to increase industrial emissions of sulfur while others cut back. To better understand how sulfates affect our climate, NCAR scientists are looking at sulfate behavior on every scale--microscopic to global--and using their knowledge to improve computer-based projections of climate that help guide policy.

Graphic courtesy NCAR Climate and Global Dynamics Division
This depiction of present-day sulfate data used in the NCAR climate system model shows the annual average sulfate content in milligrams for a column above each square meter of the earth's surface. The greatest concentrations appear in eastern North America and eastern Asia.


Benjamin Franklin surmises that volcanic aerosols might affect climate.


Tambora erupts in the East Indies, emitting millions of tons of sun-blocking aerosols. The world plunges into a multiyear cool spell that includes New England's legendary "year without a summer" in 1816.


Coal burning is the prime culprit behind a bout of sulfate-laden smog in London on December 5-8 that kills more than 4,000 people. Visibility in some places is less than one foot. Four years earlier, a similar episode near a steel mill and a sulfuric-acid plant in Donora, Pennsylvania, sickens 6,000 and kills 20.


The U.S. Congress passes the Clean Air Act, largely out of concern over smog. The effects of air chemistry on climate are virtually ignored.

Late 1980s:

Robert Charlson (University of Washington) and colleagues explore the atmospheric impact of dimethyl sulfide, one of the most common forms of airborne sulfur. Their 1987 paper in Nature invigorates research on sulfates and climate.


The Philippines' Mt. Pinatubo erupts, cooling global climate for several years. Its sulfates mix with human-produced chemicals to help deplete stratospheric ozone. The volcano's climatic effects eclipse those of a human disaster: smoke from massive oil fires set during the Persian Gulf war.


As evidence grows that sulfates have a stronger cooling effect on global climate than previously thought, the Intergovernmental Panel on Climate Change (IPCC) produces new global-warming predictions that account for these effects. The IPCC projects a warming over the next century of between 1.0 and 4.5 degrees Celsius (1.8-8.1 degrees Fahrenheit).


The next IPCC report will include new estimates of sulfur's climatic effects drawn from work at NCAR and other research centers.

As particles go, sulfates aren't the most elegant. In liquid, a pure sulfate particle resembles a perfect sphere. In the lower atmosphere, according to NCAR senior scientist Jeffrey Kiehl, "sulfates are usually messy." Water, soot, minerals, sea salt, and other airborne materials cling to them.

Take a bunch of those messy compounds, scatter them around the earth's atmosphere in dense clumps, and you have a sense of what climate researchers like Kiehl are up against. Humans are now adding twice as much sulfur as nature does to the atmosphere. Sulfates not only reflect sunlight, they appear to change the atmosphere's cloud makeup to promote even more reflection--but these effects are spread unevenly around the globe (see graphic on next page). For all these reasons, sulfates are the premier wild card in the computer simulations used by scientists to recreate past climates. Kiehl and his colleagues are using NCAR's latest global climate model, along with new models focusing on aerosol chemistry, to see how sulfates have changed this century's climate and how they might alter the next century's as well.

After being emitted by coal burning or other industry, sulfur reacts in clouds with ozone or hydrogen peroxide to form sulfates. A key element in acid rain, these aerosols (airborne particles) are concentrated above the world's industrial centers--eastern North America, Europe, and eastern Asia. Overall, it's believed that up to half of this century's potential global warming from increased levels of carbon dioxide and other greenhouse gases has been negated by the effects of sulfates.

The main reason for this is well understood. Sulfates excel at scattering sunlight in all directions, including back out to space. "The typical size of sulfate aerosols [around one micron, or one millionth of a meter] happens to match the peak scattering wavelength of sunlight," explains Kiehl.

Along with their direct scattering effect, sulfates have a secondary effect that is much trickier for scientists to model. Sulfates act as nuclei for water vapor to condense around, which means that adding sulfates can boost the total number of droplets in a cloud. That, in turn, increases the cloud's ability to reflect sunlight. In some recent climate models, the cooling from these indirect effects has rivaled--or even eclipsed--the warming from greenhouse gases. However, most global models can only predict the mass of sulfates or liquid water in a cloud, or they project the number of aerosols or droplets using empirical relationships. An actual count usually requires far more computer time than present systems and budgets allow.

With these problems in mind, NCAR in the spring of 1998 began an ambitious experiment to see how well a global climate model can deal with sulfur. At the heart of this project is NCAR's climate system model (CSM). The CSM was launched in the mid-1990s for use by the university community at large, as well as by NCAR researchers. Its strong suit among similar models worldwide is that, unlike the others, it needs no midcourse correction to keep its coupled ocean and atmosphere from drifting into unrealistic territory.

The first step in this project is to see how well the model will reproduce the 20th-century rise in global temperature, as well as its regional patterns. Two separate 100-year simulations were performed in mid-1998, one with sulfates and one without. While many models use carbon dioxide as a proxy for all greenhouse gases (to save on computer time and expense), this project included five other important greenhouse gases, along with the best possible estimates of their global prevalence at each year during the study period (1870-1990). Sulfates were added several times each model year, since they have a stronger seasonal cycle than the greenhouse gases.

Kiehl worked with NCAR colleagues Mary Barth and Phil Rasch to design the sulfate component of the CSM. Barth studies sulfates on scales that range from a single cloud to the entire globe. In a model developed by NCAR's Chin-Hoh Moeng to simulate eddies in the atmosphere, Barth accounts for 100 chemical reactions that involve sulfur and a number of other chemicals emitted by industry and nature. She's looking forward to results from the merging of sulfate aerosols, cloud chemistry, and climate within the CSM: "It'll be interesting to see how they all interact with each other."

While the CSM examines global climate and chemistry as they evolve over decades, another NCAR model looks at seasonal variations. The MOZART model can depict up to two years of global interactions involving 50 chemical compounds and 150 chemical reactions. MOZART's first edition, completed in 1997, was built largely to study the formation and fate of ozone and other photochemical oxidants. A revised version, created in 1998, includes sulfates and other aerosols.

MOZART's two-year range is ideal for comparison with field projects that now gather air-chemistry data across vast regions. "You run the model and then compare it to observations--and if it doesn't agree, you try to understand why," says Didier Hauglustaine, a long-term NCAR visitor from France's Centre National de la Recherche Scientifique. Eventually, MOZART could serve as the nucleus of a multifaceted earth-system model that ties into the CSM. "The future will bring changes in the relationships between the atmosphere, the continental biosphere, and the ocean," says Guy Brasseur, director of NCAR's Atmospheric Chemistry Division. "To predict these changes, we need integrated earth-system models that link physical climate processes with biogeochemical cycles."

For NCAR's sulfate-modeling pioneers, there is still plenty of uncharted territory. It's not known how high sulfur molecules climb in the atmosphere before they're converted into sulfates, or how much higher the sulfates go before they wash out. The location of these heights is critical to understanding the impact of sulfates on clouds. To help fill in the blanks, a large, multiagency field study is planned by the National Science Foundation's Center for Clouds, Chemistry and Climate (C4) for late 1998 and early 1999. Led by Veerabhadran Ramanathan (Scripps Institution of Oceanography) and Paul Crutzen (Max-Planck-Institut, Mainz, Germany), the Indian Ocean Experiment (INDOEX) will study the three-dimensional routes that sulfur emissions take from India to the relatively pristine air of the Southern Hemisphere. "We really have very little information on what that vertical distribution is," says Kiehl.

Sulfates are sprinters in the climate race: they stay in the atmosphere only a few days to weeks before falling or raining out. In contrast, greenhouse gases like carbon dioxide are marathon runners, remaining airborne for years, even centuries. Because they have so little time to roam, sulfates tend to affect the climate mainly over regions where they're emitted. Their short lifespan also means that any big change in sulfur emissions might have a relatively prompt impact on local climate (although weather patterns would likely create too much "noise" for scientists to directly detect such a link). In the United States, where concern over acid rain led to stringent regulation in the 1970s and 1980s, sulfur emissions have gradually stabilized. However, they are rising quickly in regions like China, India, and Southeast Asia.

Tom Wigley, Mary Barth, and Jeffrey Kiehl collaborate on sulfate-related modeling at NCAR.
How will all this affect the climate of the next century? The NCAR modeling team launched the second half of its "before and after" experiment later in 1998. The goal here is to simulate the atmosphere from 1990 to 2100, again including a full array of greenhouse gases and sulfates. The uncertainties this time are as much social as scientific. One group based at NCAR, A Consortium for the Application of Climate Impact Assessments (ACACIA), is looking closely at the interplay between policy and predictions.

"We know that sulfur emissions are very costly to the economy. We've gone through that in the U.S.," says ACACIA head Tom Wigley. He believes that other nations will also implement controls on sulfates "when their economic well-being reaches a level similar to ours." If this is true, then a century-long extrapolation of today's huge growth in sulfates across the developing world might not be accurate. A more measured projection of sulfate levels has been developed for the 21st-century climate project by ACACIA's Steve Smith, along with Wigley and Hugh Pitcher (Battelle Pacific Northwest Laboratory).

By late 1998, the first results of the 21st-century climate project are expected to be in. Even those results will be merely one portrait of sulfates and their many shades of gray. However, bolstered by high-quality data and modeling, it should be one of the most accurate pictures to date. By 2000, results from the INDOEX field project and from MOZART modeling will have brought further realism to our understanding of sulfates' paths and effects in the atmosphere.

"This is a very slow process," notes Kiehl. "I think that's frustrating for policy people, who want the definitive answer on the role of aerosols on climate. It's going to take a much longer time to find this answer than it took for greenhouse gases because of the complexities we're talking about." Still, he adds, "It makes it a very interesting problem for scientists."

On the Web

NCAR climate system model
A Consortium for the Application of Climate Impact Assessments
NCAR Atmospheric Chemistry Division/Strategic Plan 1998-2002

HL Contents The Eleven-Year Switch Particles of Doubt A World of Cycles A More Perfect Science The Art of Counting Raindrops A Turbulent Situation Wired for Weather

Other issues of UCAR Highlights


Edited by Bob Henson, bhenson@ucar.edu

Prepared for the Web by Jacque Marshall
Last revised: Mon Apr 10 13:23:27 MDT 2000