New satellite tools provide best insight yet into gravity of world pollution and deforestation

The first observations from a new Earth-orbiting monitor were released on 6 June, providing the most complete view assembled to date of the world’s air pollution. MOPITT (Measurements of Pollution in the Troposphere) tracks the air pollutant carbon monoxide from aboard NASA’s Terra spacecraft as it circles the Earth from pole to pole 16 times daily, while scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, are blending the new data with output from a computer model of Earth’s atmosphere to develop the world’s first global maps of long-term lower-atmosphere pollution.

MOPITT demonstrates a new capability to make global observations of carbon monoxide, both a toxin and a representative tracer of other types of pollution. By tracking carbon monoxide plumes, scientists are able to follow other pollutants, such as nitrogen oxides, that are produced by the same combustion processes but cannot be directly detected from space.

The most dramatic set of MOPITT global observations, gathered between March to December 2000 are immense clouds of carbon monoxide from forest and grassland fires in Africa and South America, which have travelled rapidly across the Southern Hemisphere as far as Australia during the dry season, with researchers surprised to find a strong source of carbon monoxide in Southeast Asia during April and May 2000. The new maps show air pollution plumes from this region travelling over the Pacific Ocean to North America, often at fairly high concentrations. While fires are the major contributor, researchers suspect that at times industrial sources may also contribute to these events.

Although MOPITT cannot distinguish between individual industrial sources in the same city, it can map different sources that cover a few hundred square miles. The results are accurate enough to differentiate air pollution from a large metropolitan area, for example, from a major fire in a national forest.

NCAR scientist Jean-Francois Lamarque helped create MOPITT’s fully global maps of carbon monoxide by blending information from the satellite measurements with output from an atmospheric chemistry model developed at NCAR. “Most of the information contained in the maps comes from the data, not the model,” Lamarque explained, “but the model fills in the blanks in a very smart way.” The blending technique, also enables scientists to work backwards from the observations to pinpoint pollution sources, a major goal of the experiment.

“MOPITT information will help us improve our understanding of the linkages between air pollution and global environmental change, and it will likely play a pivotal role in the development of international environmental policy,” commented atmospheric chemist Daniel Jacob of Harvard University, who used MOPITT data to study air pollution from Asia.

Meanwhile, researchers from the Department of Geography and the Global Land Cover Facility at the University of Maryland in the United States say that estimates of deforestation that are significantly more accurate than those used by the United Nations, which recently showed that the rate of deforestation had slowed globally (see related story).

These forest measures were produced by analysing data from NASA’s new Earth Observing System Terra’s MODIS (Moderate Resolution Imaging Spectroradiometer) instrument and Maryland’s continuous fields method for tree cover mapping. Among the first assessments of forest cover produced using the method is one showing the percentage tree cover over the United States at a spatial resolution of 250m.

“It is essential that we know accurately how much forest cover there is on our planet to help us conserve what is left and to make the best use of forest resources,” said John Townshend, a professor in the university’s department of geography. “The UN Food and Agriculture Organization (FAO) has recently made an enormous effort to derive statistics for forest cover for the world based on reports from the countries themselves. But these figures usually are simply not comparable from country to country, and it is abundantly clear that for many countries the FAO estimates are highly inaccurate.”

Many scientists and environmentalists have criticised the methods the FAO used in assessing the extent of the world’s forestation and its change over time. Maryland researchers say the FAO analysis has many inherent scientific and political problems because of its use of a number of different definitions of what constitutes a forest. These definitions were based on both intended land use and land cover estimates. One result of this classification approach was that in some cases areas from which all the trees had been harvested were still counted by the FAO as forest because that was the land’s ‘intended use’. In addition, the latest FAO report and its two earlier ones aren’t comparable, the researchers say, because all three use different methodologies.

“Our work and that of many other scientists show that the way forward to more useful estimates of forest cover lies with the use of remote imaging data,” said Ruth DeFries, an associate professor in the department of geography and the university’s Earth System Science Interdisciplinary Center. “Remote sensing offers a globally repeatable and verifiable methodology which doesn’t have the problems of bias that are inherent when forest estimates are provided by various agencies or institutions within each country.”