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ing the photochemical reactions that both produce and consume OH are of critical importance to an assessment of changes in the oxidizing power of the troposphere. This understanding is currently sketchy, in part, because instrumentation for measuring OH has only recently become available. A recognition of the current importance of tropospheric OH has played a critical role in environmental policy, notably in the replacement of chlorofluorocarbons with products amenable to oxidation by OH in the troposphere.
The PEM-Tropics A flew at the end of the dry season in the southern Pacific. A key discovery from this flight was that the South Pacific is covered by a blanket of biomass-burning pollution transported 10,000 miles or more from agricultural and forest fires in South America and southern Africa. This biomass burning has a profound impact on ozone and aerosols over the South Pacific with likely implications for climate. Scientists expect to take a closer look at pollution flowing from Asia next spring with NASA's Trace-P project. —REBECCA RENNER
Homing in on the missing sink At a packed session on carbon sequestration held during the American Geophysical Union's (AGU) meeting in June, Pieter Tans of the National Oceanic and Atmospheric Administration (NOAA) presented new evidence that appeared to strengthen previous research attributing a strong C0 2 sink to North America. The session also made clear that important differences persist between estimates of the size of what many researchers call the "missing sink." Tans, who is chief scientist at NOAA's Climate Monitoring and Diagnostics Laboratory, has yet to publish his new work, which is based on an analysis of the average concentration differences of C0 2 and CO observed at sampling sites over the North Atlantic and North Pacific basins. Much of Tans's work has focused on assessing atmospheric levels of C0 2 , and he was the first researcher to posit that North America harbors a large sink for absorbing the compound. Research to date has shown that large water bodies, forests, and agriculture absorb C0 2 . "Since the air tends to move from west to east at these tem-
perate latitudes, one would expect to see a small increase in the concentrations of C0 2 as well as CO during the crossing of the North American continent," Tans explained. "That would have to be caused by the burning of fossil fuels which produces both C0 2 and CO. The average emissions per square kilometer are large over that continent. We see an average increase in CO, but a decrease in C02." To verify the new measurements, Tans said he is currently looking at other trace gases, such as SF6 (all of which is anthropogenic in origin) and the isotopic ratios of C0 2 . "The latter has the potential to distinguish between oceanic as opposed to terrestrial and fossil fuel sources of C02," he said. Tans's work, as well as that of Ralph Keeling of the Scripps Institution of Oceanography in California, added to the growing body of evidence from atmospheric measurements that there is a large, as-yet unidentified carbon sink on land in northern temperate latitudes, according to Eric Sundquist, a research geochemist with the U.S. Geological Survey who co-organized the session.
In a dramatic change of tone, the Chinese government has offered "strong support" for international efforts to combat global climate change under the Kyoto Protocol, the international pact to reduce greenhouse gas emissions. China is the world's second largest C02 emitter, but as a developing country, officials there have argued that the protocol will limit China's economic growth. After three years of discussions with U.S. and other country officials, Chinese officials now say that projects designed to abate greenhouse gases can encourage economic growth, as well as help China address its own serious local air pollution problems, a U.S. State Department official said. However, China did not agree to reduce C02 emissions. For the first time, China publicly states plans to "increase significantly" its reliance on renewable energy sources, greatly expand natural gas in its energy supply, increase the use of coal-bed methane and clean coal technologies, and to work with other countries to reach agreement on Kyoto pact provisions that will give developed countries credit for emissions reductions achieved by sponsoring energy-efficient projects in developing countries. China's climate change pledge is part of a joint US-China statement on cooperative environmental protection and sustainable development efforts, signed in Beijing in mid-May.
Diesel takes a hit In a landmark action, southern California air regulators in June adopted the first in a series of rules designed to ban dieselpowered transit buses, garbage Continued on Page 333A
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On the basis of nearly a dec ade of observations of the ratio of atmospheric oxygen to nitrogen (02/N2), Keeling, an associate professor of geochemistry, re ported that the most important source of variability in atmo spheric C02, at least in the short term, is the land biota. Recent research by an interna tional group led by David Schimel of the Max Planck Institute for Biogeochemistry in Germany hints that this variability could be much greater than previously be lieved. During his AGU presentation, Schimel discussed the results of an in fluential paper pub lished earlier this year (Science 2000, 287(11)2004-2006). Schimel's group used models with historical data from 1895 to 1993 to show that the terres trial carbon sink varied by more than 100% from year to year.
Tans argued that Schimel's analysis of C0 2 uptake by North America, as presented at the AGU meeting, did not go far enough. "It is a very good guess that, if one extends their type of analysis to all of the temperate and highlatitude ecosystems of the north ern hemisphere, only an insignif icant sink of C0 2 will be found in all of those ecosystems,"
C02 levels recorded by tall tow
Schimel's work, ers like these operated by Ν0ΑΑ which points to a have the potential to bridge the yawning gap between atmos much weaker sink pheric measurements and read than the ongoing work by Tans, impli ings taken on the ground. cates land use as a major source of variability, and Tans said. This is insignificant in Sundquist says there is "growing comparison with the magnitude consensus that the large uniden that would satisfy the constraints tified terrestrial carbon sink in formed by the large-scale obser the northern hemisphere must be vations of atmospheric C0 2 , isorelated somehow to human land topic ratios, and 0 2 /N 2 . use." "Therefore, it appears that we Schimel's models indicate that have a serious discrepancy here "land use recovery processes could between the ecological and landbe driving a sink that's twice as use approach ("from the bottom large as the current" physiological up") and the atmospheric pat ecosystem sink in the United terns ("top down"). There must be States, associated with the uptake serious errors or omissions in ei of carbon dioxide, he said. ther one of those approaches or in both," Tans said. "We've systematically taken some of the most productive "We've arrived at the point lands with the highest carbonwhere both approaches have storing potential and put them been evaluated and seem to be into agriculture, and mis radically on their own terms quite robust," changes [how much carbon they Schimel agreed. "Therefore, I can take up]." Schimel explained. think we have a very important 3 3 2 A • AUGUST 1, 2000 /ENVIRONMENTAL SCIENCE & TECHNOLOGY / N E W S
scientific puzzle—why does the atmosphere appear to be seeing so different of a signal than the signal that we retrieved from the national forest inventories, which represent many orders of magni tude more data points than sam ples of the atmosphere (though each one is much more local)?" To answer that question, Schimel is involved with an interna tional group investi gating the potential of measurements on tall towers. These towers "have the enormous ad vantage of record ing a dense time series, in contrast to aircraft, so that they can see all of the timescales of vari ability," Schimel said. Tans agreed that such towers "would certainly be a step towards bridging the spatial scales from local ecosystems to very large regions." NOAA initiated such tall tower studies in 1992, he said, not ing that two have been operated for a number of years as a part of the Ameriflux network. The tall towers, in combina tion with the new vegetation sen sors flying on the NASA TERRA satellites, (Environ. Sci. Technol. 1999 33 (13), 271A). researchers could begin to look at interannual variability, seasonal variability, and vegetation activity, associat ing it with fluxes that are ob served at the towers, Schimel said. "I think that it's only at this regional scale that we can get closer and understand whether it's the biology, the atmosphere, or the measuring techniques that are causing this persistent differ ence between the different ap proaches," he concluded. —KELLYN BETTS
