USGCRP-sponsored research continues to advance understanding of the causes, magnitude, and consequences of changes in land cover and in terrestrial and marine ecosystems. The FY96 edition of Our Changing Planet included a summary of recent key findings. Highlights of more recent findings follow:

• New understanding has been gained of the contemporary global carbon cycle through the measurement and analysis of changes in the atmospheric concentration of oxygen and the ratio of 13C° to 12C° in atmospheric carbon dioxide. These measurements and analyses support the hypothesis that terrestrial ecosystems of the mid-latitudes of the Northern Hemisphere have functioned as a significant carbon sink (up to about a third of the fossil fuel emissions) in the first half of the 1990s. Without this sink, the causes of which are not yet understood, the rate of carbon dioxide accumulation in the atmosphere would have been even faster.

• Large-scale ecosystem modeling efforts are making important progress. The models being developed are prognostic and can be used to simulate a range of ecological responses to changes in climate and the chemical composition of the atmosphere. Among the responses that can start to be considered are time-dependent changes in the distribution of terrestrial plant communities across the globe as climate changes.

• Long-term ecosystem-level experiments are now providing evidence that biodiversity plays a critical role in the carbon and nutrient cycles of terrestrial ecosystems. In addition, regular measurements are now being taken that will lead to improved estimates of the effects of interannual climate variability on carbon exchange between land ecosystems and the atmosphere. This information can be used to develop and test process-based ecosystem models that are important components of the larger Earth system models. These models are critical research tools in global change science and assessment, and will benefit greatly from improved representation of ecosystem processes.

• New land-cover data for South America, Southeast Asia, and the conterminous United States have been released on the World Wide Web http://amazon.unh.edu/pathfinder/ and http://edcwww.cr.usgs.gov/landdaac/. The data were developed from Landsat products. The new information for South America and Southeast Asia will facilitate better estimates of rates of deforestation and of the flux of carbon to the atmosphere associated with forest clearing. The land-cover data for the conterminous United States, which have a 1-km resolution, are an important information base for resource managers working on regional-scale planning.

• Recent modeling studies have shown that unusual physical conditions along the break in the Georges Bank's shelf during the late 1950s and 1960s can be traced to changes in the strength of the cold Labrador Current. The dynamics of planktonic animal populations on the Bank are thought to be strongly influenced by biological and physical events in the surrounding deep basins in the Gulf of Maine, the offshore slope water, and upstream waters on the Scotian Shelf. Changes in planktonic populations in turn influence the marine resource populations of these ecosystems. Scientists are evaluating the impact of such decadal-scale climate-related events on the seasonal cycle of heating and cooling that controls water column structure, local circulation, and plankton dynamics.

• Recent studies in the tropical Pacific Ocean indicate that iron, which is relatively abundant in waters near land, may be the limiting nutrient in determining primary production of marine life in the blue waters of the central ocean basins. In a series of field experiments involving controlled additions of iron salts to surface waters, scientists documented dramatic plankton blooms and concomitant draw down of other excess nutrients. These results are encouraging studies of factors controlling primary production, carbon cycling, and ocean-climate impacts elsewhere in the world ocean.