Concluding Remarks

Current chemical transport models of the atmosphere, which typically include 50-100 chemical compounds and 150-250 chemical reactions, reproduce with reasonable success the global behavior of the chemical system in the atmosphere. Differences between the results provided by these models remain substantial and will have to be addressed in the future. These models are used to explain the dramatic changes that have occurred in the chemical composition of the atmosphere over the last century and to predict changes in the future oil the basis of plausible emission scenarios.

• Increases in the atmospheric concentration of long-lived greenhouse gases since the preindustrial era have led to a climate forcing of about 2.5 W m~2 (IPCC, 1996). The inter-annual variability in this trend is not well understood and involves complex interactive processes between the atmosphere, the ocean, and the continental biosphere. Coupled earth system models with a detailed representation of global biogeochemical cycles will help address these issues.

• The aerosol load of the atmosphere has also increased as a result of human activities, specifically biomass burning and fossil fuel combustion. Sulfate aerosols have produced a direct cooling effect that can reach locally more than 2 W m~2 over industrialized areas, but is much smaller on the global scale. The indirect radiative effects of aerosols (through changes in the optical properties and lifetimes of the clouds) remain rather uncertain, but could be larger than the direct effects (IPCC, 1996). Changes in upper level clouds (i.e., cirrus) could lead to a warming of the earth's surface. Progress in this area requires a better understanding of aerosol mi-crophysics and chemistry. The role of nonsulfate aerosols (and specifically multicomponent aerosols) will also have to be included in comprehensive model calculations.

• The oxidizing power of the atmosphere has likely decreased significantly, especially in the Northern Hemisphere, as a result of human activities. As a result, the lifetime of methane may have increased by 10-15% since the preindustrial era. At the same time, the abundance of tropospheric ozone has increased perhaps by as much as a factor of 2-3 in the Northern Hemisphere. Enhanced biomass burning fluxes of NOv, CO, and hydrocarbons from tropical ecosystems are likely to be important. Future changes in tropospheric ozone are predicted to be largest in the tropics (India, China). These projected increases in tropical emissions are likely to have a disproportionate impact on global atmospheric chemistry because of the vigorous upward transport that characterizes the region. The global budget of ozone, however, remains, rather uncertain due to the lack of systematic observations, especially in the tropics and in the Southern Hemisphere. The impact of future commercial aircraft operations on upper tropospheric ozone in the Northern Hemisphere will probably become significant during the 21st century. • In the stratosphere, the link between long-term buildup of anthropogenic chlorine and ozone decline is now firmly established (WMO, 1999). Recently, the decline in mid-latitude ozone has slowed, but late winter/early spring ozone values in the Arctic were often unsually low during the 1990s. The Antarctic ozone hole, which is observed in September/October, continues unabated. Increasing concentrations of carbon dioxide together with the observed stratospheric ozone losses have caused a cooling of the lower stratosphere and a negative radiative forcing of the climate system. All of these highlight the existing link between ozone and climate issues that society has been facing.

Biogeochemistry is inherently a broad subject and clearly requires interdisciplinary approaches. Today, as the community regards the earth as a complex nonlinear system, studies of atmospheric chemistry and biogeochemistry cannot be dissociated from studies of the physical climate system. Interactions between the ocean, the continental biosphere, and the atmosphere are therefore central themes for the science of the 21st century. The challenges for the new Max Planck Institute for Biogeochemistry in Jena are particularly exciting.

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