Global dimming is the reduction in the amount of sunshine reaching Earth's surface. Both global dimming and global brightening, the opposite effect, have been observed. Dimming is caused by an increased blockage in the atmosphere of light from the sun. Clouds and aerosols—small particles emitted by burning fuels—can both contribute to dimming. Because dimming reduces the amount of solar energy reaching Earth, it tends to cool the planet, masking or offsetting global warming.
Global dimming has been confirmed for the period of 1960 to 1990, during which time the amount of sunlight reaching Earth's surface decreased about 4%. However, the dimming trend has been reversed from about 1990 to at least 2007 over most of the world. There has been much confusion in the scientific literature over the measurement of global dimming, as well as some sensational media coverage of the subject. Dimming has been hailed both as an imminent threat to human life and as salvation from global warming (and therefore a reason not to curtail the burning of fossil fuels). However, both claims are inaccurate. Although global dimming has reversed, as of 2007 the brightening had not proceeded far enough in all areas to undo the earlier decades of dimming.
Historical Background and Scientific Foundations
The possibility that particles suspended in Earth's atmosphere might cause cooler weather was recognized in the eighteenth century by American statesman and experimenter Benjamin Franklin (1706–1790), who suggested that a large volcanic eruption in Iceland might explain a spell of cold weather observed in Europe. In the 1960s, scientists first became aware of the extent to which tiny aerosol particles suspended in the air as haze could travel hundreds of miles or more from their sources. In the 1970s and 1980s, analysis of ancient ice samples retrieved from the depths of the Greenland ice cap showed traces left on global climate by volcanic eruptions that placed millions of tons of ash and sulfate particles in the atmosphere.
In the middle and late 1970s, early efforts at computer modeling of climate included aerosol particles. Researchers concluded that the tendency of aerosols would be to increase the planet's albedo (reflectivity), both by scattering light back into space and by increasing cloud formation. A cloud droplet or ice crystal is more likely to form around a tiny particle of air pollution than in clean air, other conditions being equal, so aerosols make cloud formation more likely. Aerosols also make denser, more opaque clouds, because smaller, more numerous cloud droplets tend to form when there are many aerosol particles present. However, it was not realized that a decrease in the amount of sunlight reaching the planet's surface— global dimming—would be the result. In 1987, satellite photographs showed that cloudiness was enhanced over major ocean shipping lanes because aerosols from the diesel fuel burned by ships were seeding cloud formation.
In the mid 1980s, Israeli meteorologist Gerry Stanhill reported that the amount of sunlight reaching the ground in Israel had fallenby22% from the1950s to the1980s. (It was Stanhill who coined the term “global dimming.”) At about the same time, German meteorologist Beate Liepert noted dimming in the Bavarian Alps. Both scientists began independently to search the scientific literature for measurements of the amount of sunlight reaching Earth's surface over time. They discovered that researchers had independently reported dimmings of 9% in Antarctica, 10% in parts of the United States, 16% in parts of the United Kingdom, and up to 30% in parts of Russia.
These direct measurements of diminished sunlight were confirmed by data on pan evaporation rates. The pan evaporation rate is the amount of water that evaporates over a given period of time from an exposed pan. This rate had decreased in step with the proposed global dimming. Stanhill concluded that the amount of sunlight reaching Earth had, in fact, decreased by 2 to 4% from about 1960 to 1990. Yet this seemed to contradict global warming, which had already been well established by millions of measurements: if aerosols were increasing cloud cover and so darkening Earth, how could the planet be getting warmer?
The role of clouds in global warming and cooling is one of the largest sources of uncertainty in global climate modeling: clouds have a paradoxical double effect on the heating of the planet, reflecting heat radiation back at Earth's surface like a blanket and reflecting solar radiation back out into space like a mirror. The blanket effect tends to warm Earth, the mirror effect to cool it. Thus, confirming enhanced cloud formation in some areas by aerosol pollution did not necessarily confirm the occurrence of a cooling effect, whether regional or global.
To complicate the picture, aerosols can affect climate directly, as well as indirectly (through increased cloud formation). In the 1990s, Project INDOEX (Indian Ocean Experiment) collected data showing that aerosol pollution from India was causing a dark layer of haze some 2 miles (3.2 km) thick over the northern Indian Ocean, and that this brown cloud was reducing the amount of sunlight reaching the ocean by over 10%. The toxic clouds of pollution were also spawning increased cloud cover, which was cooling the ocean by reflecting more solar energy into space.
It is now known that the net effect of sulfate aerosols is cooling. However, some other aerosols cause warming. In particular, the dark, carbon-rich particles called soot or black-carbon particles are excellent absorbers of sunlight. They heat the air around them, and when they sift down to the surface, darkening Arctic snow, they make melting more likely. Melting Arctic snow cover exposes dark land, which accelerates global warming.
Impacts and Issues
Wide agreement that global dimming was indeed a global issue was not reached until about 2004, and even since then a large number of scientists have maintained that the phenomenon is not truly global but regional, being especially confined to urban areas, where there are more aerosols in the air. This dispute is partly about words, because if many regional dimming effects occur, then they will produce a dimming of the world as a whole. In any case, most scientists agree that global dimming has reversed itself since about 1990.
In 2005, the journal Nature reported that although the amount of solar radiation reaching Earth's surface decreased by 4 to 6% from 1960 to 1990, and that the dimming trend had reversed since that time, brightening had not gone far enough yet to undo all the dimming of earlier decades. Measurements by NASA's Aqua satellite showed global brightening to a degree that agreed with the warming of the oceans, which had been measured and had also been correctly predicted by climate-change computer models.
WORDS TO KNOW
AEROSOL: Particles of liquid or solid dispersed as a suspension in gas.
ALBEDO: A numerical expression describing the ability of an object or planet to reflect light.
FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.
GLOBAL BRIGHTENING: Any increase in the amount of sunlight reaching Earth's surface; in particular, any decrease in or reversal of global dimming, the blockage of sunlight by aerosols (fine particles) in Earth's atmosphere.
GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.
PAN EVAPORATION RATE: Rate at which water evaporates from an open pan exposed to the sky. Pan evaporation rate depends on wind, sunlight, humidity, and temperature. Measurements of pan evaporation rates have declined globally over the last century or so: global dimming has been proposed as a possible contributor to this effect.
SAHEL: The transition zone in Africa between the Sahara Desert to the north and tropical forests to the south. This dry land belt stretches across Africa and is under stress from land use and climate variability.
SOLAR ENERGY: Any form of electromagnetic radiation that is emitted by the sun.
The main cause of global dimming is sulfate aerosols. Sulfate aerosols have a net cooling effect on Earth, as opposed to black carbon and soot particles, which have a net warming effect. As sulfate aerosol pollution has decreased in the 1990s and beyond, therefore, brightening has replaced dimming. To complicate the picture, the eruption of the volcano Mount Pinatubo in 1991 ejected about 20 million tons of sulfur dioxide into the atmosphere, increasing both darkening and cooling. As the global climate recovered from the Pinatubo eruption through the 1990s and as sulfate aerosols from human sources decreased due to anti-pollution laws, global dimming—whether viewed as a collection of regional phenomena or a single global phenomenon—has decreased.
The effects of global dimming are disputed. A few studies have attributed the occurrence of some droughts to dimming: less solar radiation reaching the surface means less evaporation, which may have decreased rainfall and even caused famine in the Sahel region of northern Africa. However, this connection is far from proven. There is broader scientific agreement that dimming may have partly masked the extent of global warming, and that global brightening is now revealing the full warming effect that human activities have already had. This unmasking may explain why global warming effects lagged model predictions for some years but have exceeded them since the mid-1990s. Other scientists caution that the feedbacks between aerosols, evaporation, humidity, albedo, and cooling are too complex for such conclusions to be drawn, given present knowledge. What is certain is that the world is rapidly warming, regardless of changes in the global dimming effect.
In the early 2000s, some urged that because of global dimming's masking effect on global warming, it would be self-destructive to burn less fossil fuel. In 2005, the Reuters news service ran a headline that read, “Fossil Fuel Curbs May Speed Global Warming.” However, concentrations of greenhouse gases are steadily increasing, and aerosol emissions could not increase enough to overcome the warming effects of these gases. Furthermore, aerosols are a direct health hazard, causing many hundreds of thousands of deaths from cancer and respiratory disease around the world each year. One kind of pollution cannot be undone simply by increasing another kind.
Primary Source Connection
This source describes the contents of a new NASA study on the relationship between global dimming and global warming. Global dimming is a process by which less direct sunlight reaches Earth's surface due to aerosol particles, dust, and pollution in the atmosphere that reflect sunlight back into space. Global dimming was observed throughout the second half of the twentieth century. The new NASA study noted that global dimming and global warming could occur simultaneously, with global dimming lessening the effect of global warming.
Stephen E. Cole is on the Earth Science News Team at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
GLOBAL ‘SUNSCREEN’ HAS LIKELY THINNED, REPORT NASA SCIENTISTS
A new NASA study has found that an important counterbalance to the warming of our planet by greenhouse gases—sunlight blocked by dust, pollution and other aerosol particles—appears to have lost ground.
The thinning of Earth's “sunscreen” of aerosols since the early 1990s could have given an extra push to the rise in global surface temperatures. The finding, published in the March 16 issue of Science, may lead to an improved understanding of recent climate change. In a related study published last week, scientists found that the opposing forces of global warming and the cooling from aerosol-induced “global dimming” can occur at the same time.
“When more sunlight can get through the atmosphere and warm Earth's surface, you're going to have an effect on climate and temperature,” said lead author Michael Mishchenko of NASA's Goddard Institute for Space Studies (GISS), New York. “Knowing what aerosols are doing globally gives us an important missing piece of the big picture of the forces at work on climate.”
The study uses the longest uninterrupted satellite record of aerosols in the lower atmosphere, a unique set of global estimates funded by NASA. Scientists at GISS created the Global Aerosol Climatology Project by extracting a clear aerosol signal from satellite measurements originally designed to observe clouds and weather systems that date back to 1978. The resulting data show large, short-lived spikes in global aerosols caused by major volcanic eruptions in 1982 and 1991, but a gradual decline since about 1990. By 2005, global aerosols had dropped as much as 20 percent from the relatively stable level between 1986 and 1991.
The NASA study also sheds light on the puzzling observations by other scientists that the amount of sunlight reaching Earth's surface, which had been steadily declining in recent decades, suddenly started to rebound around 1990. This switch from a “global dimming” trend to a “brightening” trend happened just as global aerosol levels started to decline, Mishchenko said.
While the Science paper does not prove that aerosols are behind the recent dimming and brightening trends— changes in cloud cover have not been ruled out—another new research result supports that conclusion. In a paper published March 8 in the American Geophysical Union's Geophysical Research Letters, a research team led by Anastasia Romanou of Columbia University's Department of Applied Physics and Mathematics, New York, also showed that the apparently opposing forces of global warming and global dimming can occur at the same time.
The GISS research team conducted the most comprehensive experiment to date using computer simulations of Earth's 20th-century climate to investigate the dimming trend. The combined results from nine state-of-the-art climate models, including three from GISS, showed that due to increasing greenhouse gases and aerosols, the planet warmed at the same time that direct solar radiation reaching the surface decreased. The dimming in the simulations closely matched actual measurements of sunlight declines recorded from the 1960s to 1990.
Further simulations using one of the Goddard climate models revealed that aerosols blocking sunlight or trapping some of the sun's heat high in the atmosphere were the major driver in 20th-century global dimming. “Much of the dimming trend over the Northern Hemisphere stems from these direct aerosol effects,” Romanou said. “Aerosols have other effects that contribute to dimming, such as making clouds more reflective and longer-lasting. These effects were found to be almost as important as the direct effects.”
The combined effect of global dimming and warming may account for why one of the major impacts of a warmer climate—the spinning up of the water cycle of evaporation, more cloud formation and more rainfall—has not yet been observed. “Less sunlight reaching the surface counteracts the effect of warmer air temperatures, so evaporation does not change very much,” said Gavin Schmidt of GISS, a co-author of the paper. “Increased aerosols probably slowed the expected change in the hydrological cycle.”
Whether the recent decline in global aerosols will continue is an open question. A major complicating factor is that aerosols are not uniformly distributed across the world and come from many different sources, some natural and some produced by humans. While global estimates of total aerosols are improving and being extended with new observations by NASA's latest generation of Earth-observing satellites, finding out whether the recent rise and fall of aerosols is due to human activity or natural changes will have to await the planned launch of NASA's Glory Mission in 2008.
“One of Glory's two instruments, the Aerosol Polarimetry Sensor, will have the unique ability to measure globally the properties of natural and human-made aerosols to unprecedented levels of accuracy,” said Mishchenko, who is project scientist on the mission.
Stephen E. Cole
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