Aerosols include dust, other particles, and very small droplets; they have natural sources, human sources, or both. In recent years, researchers have become more and more conscious that atmospheric aerosols have a variety of effects on climate, but many uncertainties remain about what, where, and how strong the effects are. Aerosols stay in the atmosphere only a few days; they undergo chemical transformations, especially in clouds, or get washed back to earth in rain or snow. Therefore, their effects tend to be local, since they disperse before they can travel far in the air.
Although some aerosols, including soot (carbon), absorb heat, scientists have generally believed that the net effect of aerosols is to cool the earth. They do this in two ways: directly, by reflecting sunlight back to space before it can reach the earth's surface, and indirectly, by making the clouds in which they lodge more dense and rainy. Models of global climate change that include aerosols show considerably less future warming, especially in the northern midlatitudes where the aerosols are generated, than those that do not.
Sulfate aerosols, which have both natural and human sources, are thought to cool the earth by reflecting incoming sunlight back into space before it reaches the lower atmosphere. As increased fossil fuel burning puts more sulfate aerosols into the atmosphere, the reasoning goes, this pollution is slowing global warming. Carbon particles, on the other hand, absorb solar radiation, theoretically increasing global warming. So the new finding compounds the question of why the earth has warmed only about half as much as predicted by most climate models.
Hobbs and his colleagues Dean Hegg (University of Washington) and T. Novakov (Lawrence Berkeley National Laboratory) have submitted two papers about their research to the Journal of Geophysical Research. The team was one of the research groups in last year's Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). Using the University of Washington's Convair 131 research aircraft, they took measurements of the size and optical properties of particles in the polluted air extending over the Atlantic Ocean east of a 200-mile urban corridor centered on Washington, D.C.
The carbon they found in the atmosphere was partly elemental carbon (soot) from the burning of forests and the inefficient use of fossil fuels. They also found organic carbon, some from industrial emissions and some from natural sources such as ocean gases.
This new evidence that atmospheric particles have less of a cooling effect than thought conforms with some of Hobbs' earlier research. Some researchers had theorized that smoke from the burning of Amazon forests in Brazil had a large cooling effect worldwide because particles from the smoke would reflect solar radiation back into space. But Hobbs found by making airborne measurements of this smoke that it was strongly absorbing solar radiation as well as reflecting it. "Consequently, we found it had quite a bit less cooling effect than previously estimated," says Hobbs. "That offset the radiation scattering properties that most people thought were dominant."
The next stage of TARFOX is to use the sampling data to calculate the magnitude of the effect of atmospheric particles. Says Hobbs, "You have to know what your particles are and what their optical properties are before you can calculate the relative magnitudes of their scattering, or cooling effect, compared with their absorbing, or warming effect, to find out the net effect." He guesses that the results will still indicate a cooling effect, but signficantly less than previous estimates.
"The findings came as a surprise," say Hobbs. "We had no prior theory." To find out whether these results are specific to the research area or more universal, "Measurements are needed in other urban airsheds."
To find out why large earthquakes do not always follow seismic quiescence, Wyss researched earthquakes in Japan, where much has been published in support of the quiescence hypothesis. There, Wyss found that seismically quiet periods precede significant earthquakes only when sufficient underground stress has accumulated.
Wyss studied three areas around Tokyo that have been in seismic quiescence for three years. Each of the areas has experienced large earthquakes in the past. Within these areas, he mapped and tested asperities, or hard spots, which can supply the source of energy for the earthquake. He assigned each asperity a "b-value" that measured accumulation of underground stress. B-values can help seismologists rule out other reasons for a decrease in the rate of small earthquakes, such as ground water movement. Low b-values indicate a great potential for producing an earthquake; high b-values mean that it is unlikely the ground contains enough stress or energy to produce a quake.
All three of the areas of seismic quiescence near Tokyo contained asperities with high b-values. Therefore, Wyss predicts that there is less than a 20% chance of major earthquakes occurring in those areas any time soon, despite the seismic quiescence.
Wyss thinks his research tends to confirm the theory of seismic quiescence as an indicator of earthquakes to come, despite the "false alarms" such as the Tokyo cases. "False alarms just mean the hypothesis doesn't work in these cases," he said. "Scientists who predict the weather experience false alarms quite frequently." He advocates using multiple methods of earthquake prediction because earthquakes follow a variety of patterns.
Alley's findings are based on ice-core records. Combined with borehole temperatures and stable-isotope records, these are a good proxy for year-round temperatures; taken in conjunction with melt incidents, the ice cores indicate summer temperatures.
Because of its wobbling orbit, the earth today is closer to the sun during the Northern Hemisphere winter and farther away during the Northern Hemisphere summer than it was a few thousand years ago. Scientists have believed that this change probably means that summers in our hemisphere have been cooling and winters have been warming. Thus, a new explanation needs to be found for Greenland's winter cooling.
Alley suggests that ocean heat transport may be the key. "If, over the last few thousand years, the ocean's transfer of heat to the area around Greenland has declined, this would be a simple explanation for the overall cooling," he says. "This is not unlike what happens with the ocean circulation when rapid cooling events have occurred in the past, except it happens more slowly."
During rapid events, such as the Younger Dryas (a cold period of a few hundred years that started between 11,000 and 10,000 years ago), ocean heat transfer slows suddenly and Greenland gets cold. A large part of North America becomes cold, dry, and windy. The area around the Great Lakes dries out, as does the area around the Sahara Desert. Europe becomes colder. Similar changes have happened more slowly in many of these places over the last few thousand years. These changes may be a slow-motion climate event, according to Alley. "The place to check out this possibility is in the oceanographic record."
Alley reported his findings at the May meeting of the American Geophysical Union.
Ground-level ozone is produced from chemical reactions in the atmosphere fueled by air pollutants such as hydrocarbons and nitrogen oxides. The federal Clean Air Act empowers the EPA to establish a National Ambient Air Quality Standard (NAAQS) for ozone to protect human health. The EPA established the new NAAQS in response to new medical data indicating adverse health effects at lower ozone concentrations.
Analysis of ozone levels measured at rural locations in the eastern half of the United States indicates that nearly half the sites will not meet the new ozone standard. Chameides pointed out that since nonattainment of the former standard was mostly limited to urban areas, most people perceive air pollution as an urban problem. "If EPA's new standard better reflects the health effects of ozone pollution," he said, "it suggests that you could probably go just about anywhere in the eastern United States during the summer and encounter unhealthy air."
The new standard of 0.08 parts per million is measured over an eight-hour period, while the former standard of 0.12 parts per million was measured over one hour. Sites are considered in nonattainment if the fourth highest eight-hour averaged ozone concentration measured over three years exceeds 0.08 parts per million. Because summer ozone levels fluctuate very little in rural areas, with a longer-lived daytime maximum and less nocturnal decrease, the country is more likely to reach nonattainment level than the city, where ozone levels drop to near zero at night from a high afternoon peak.
Scientists have known that the new standard would increase the number of nonattainment counties in the United States by a factor of about three. However, Chameides and his collaborators believed that the impact on rural areas had not been adequately considered. To address this issue, they first analyzed 1995 ozone data from 85 rural monitoring sites that were part of the Southern Oxidant Study's Spatial Ozone Network (SON) and the EPA's Clean Air Status and Trends Network (CASTNet). Nearly half of these sites reported ozone readings that would place them in nonattainment with the new standard. Under the previous standards, only 6 of the 85 rural sites had unallowable ozone levels. After publishing these results in Science, Chameides and his colleagues then extended their study to include an analysis of three years' data from SON, CASTNet, and other rural monitoring sites with essentially the same results.
Requiring rural areas to meet the new standards will force a major change in ozone control strategies, since few rural areas control the emissions that cause their pollution problems. "Pollution in rural areas is caused by a complicated combination of distant urban emissions and local and distant rural emissions, so you need to come up with a regional strategy to address rural air pollution," Chameides explained. "The way the EPA guidelines are currently written, local and state agencies must develop a strategy to comply with NAAQS by controlling pollutant emissions within their area. These agencies are not going to be able to solve the problem by controlling local pollutant emissions."
Communities and states would have to take a more regional approach, requiring cooperation of political entities that have not always worked together in the past. "Not only will this change the politics of how we address air pollution in the United States," noted Chameides, "but it will also probably affect the economics of air pollution control."
That scenario now seems unlikely, according to David Wedin of the University of Toronto and David Tilman of the University of Minnesota. In an article in Science, they reported the results of an experiment in which grasses were grown under high nitrogen conditions. The study was funded by NSF's Long-Term Ecological Research Program.
The researchers spent 12 years studying the effects of experimentally added nitrogen in 162 plots in three Minnesota grasslands. "We added nitrogen at rates equivalent to what's deposited from the atmosphere in Minnesota and the Ohio Valley, right up through the amounts of highly agricultural and industrial areas of Europe," said Tilman. "Two of our nine treatments went beyond these rates to try to predict the longer-term effects of nitrogen deposition."
Tilman and Wedin found that while low rates of nitrogen deposition encouraged plant growth and high carbon storage in fields dominated by native "warm-season" prairie grasses, the results were very different in fields dominated by nonnative "cool-season" grasses. These field lost most of the added nitrogen and showed no net storage of carbon. Further, at medium and high rates of nitrogen addition, the native prairie species became extinct, the diversity of vegetation dropped sharply, and the ability of the prairie grasslands to store carbon disappeared.
The researchers found that more than half of the plant species were lost across the nitrogen addition gradient, with the greatest losses occurring at low levels of nitrogen addition--the 1- to 5-gram range, which is comparable to current atmospheric deposition rates in eastern North America and northern Europe. Most of the lost nitrogen leaked into groundwater as nitrate, a pollutant and human health threat throughout the Midwest.
"This could mean earlier spring planting dates for some crops in the future," said David Wolfe, Cornell associate professor in the Department of Fruit and Vegetable Science. "It may also affect the mixture of species in natural plant communities, because only certain plants benefit in this way." The researchers' work was described in a recent paper in Plant, Cell and Environment. Steve Boese, instructor at the College of Charleston, and Jeff Melkonian, Cornell postdoctoral researcher, coauthored the paper with Wolfe.
"Our results are another example of how the increase in carbon dioxide and other greenhouse gases will shake up the plant world," Wolfe said. "Our maps of global vegetation zones will inevitably be altered by these sorts of direct effects on plants, whether or not we also have major changes in climate."
The researchers have focused much of their attention thus far on two crops, beans and cucumbers, that are among a class of plants that tend to wilt when temperatures dip below about 8 degrees Celsius (45 degrees Fahrenheit). They knew from prior experiments that elevated carbon dioxide levels often reduce the rate of water loss from leaves, and they suspected this effect would reduce the amount of chilling damage in these species.
This hypothesis was confirmed by their study. Plants grown and chilled at elevated CO2 levels showed less severe wilting and suffered less permanent leaf damage than plants grown and chilled at current atmospheric CO2 concentrations.
"If carbon dioxide in the atmosphere doubles [from its level before the Industrial Revolution] within the next century as we are expecting," Wolfe explained, "these species may be able to withstand temperatures a few degrees cooler than they do now."
The research is the first to fully document that CO2 can have such an easing effect on chilling damage. Most of the work has been conducted in controlled-environment chambers. The researchers plan to follow up with field experiments and test other plant species.
Wolfe points out that despite this good news, the rapid rise in atmospheric carbon dioxide is still a concern because the gas may change our climate in unpredictable ways.
Said Wolfe: "I still think that fossil fuel emissions, the primary culprit in the carbon dioxide rise, are not good for the planet. Many of the other gases that are produced, such as sulfur dioxide, ozone, and nitrous oxides, can have direct negative effects on plants--and humans, for that matter."
Using a 35,000-file data base of tornadoes in the United States since 1950, meteorology/oceanography professor James O'Brien and undergraduate researcher Mark Bove found that cooler waters off Peru in the wintertime--a phenomenon known as either El Viejo or La Niña, denoting the reverse of the better-known El Niño warming--caused an increase in the number of tornadoes from March through May in Georgia, Alabama, Tennessee, Kentucky, Ohio, Indiana, and Michigan.
O'Brien announced the tornado-belt discovery in June at a NASA meeting on climate variability in the southeastern United States.
O'Brien and Bove calculated that the probability of having four or more springtime tornadoes in the seven-state block increases 300-500% when El Viejo is active in the Pacific the previous winter.
When the waters off Peru cool at least 1 degree Fahrenheit below the average winter sea surface temperature, El Viejo sets in motion a jet stream pattern that brings moist air from the Gulf of Mexico to the Ohio Valley. When the moist air clashes with cold air from Canada, tornadoes can form, O'Brien said.
"I believe our discovery will save a lot of lives by making people in these states aware of the possibility of more tornadoes," O'Brien said. "If we had known in the winter of 1995/96 what we know now, the people in the Ohio-Tennessee Valley could have better prepared for the tornadoes the following spring."
Bove, who did his research for an undergraduate senior thesis under O'Brien's direction, said better tornadic forecasting will allow states more time to designate evacuation routes, prepare shelters, and notify the Federal Emergency Management Agency that funding will be needed.
The researchers also uncovered good news for Texas, Louisiana, Oklahoma, Kansas, and Arkansas. They found that when El Niño warms the Pacific waters off South America, as it will this coming winter, these states--commonly known as "tornado alley"--should see a significant reduction in tornadic activity next spring.
"Most reports of El Niño are about disasters," O'Brien said. But "in the case of US. tornadoes, El Niño is a positive factor."