• How might the sources and sinks change in the future with changing land management, climate, and chemical inputs?

  • As global use of anthropogenic nitrogen increases, is there potential for nitrous oxide emission or methane consumption to change rapidly?

Early work on biogenic trace gases focused on radiatively active trace species. Measurements made during the USGCRP and by IGBP projects have demonstrated that biogenic gases from microbes, plants, and biomass burning influence atmospheric photochemistry and aerosol formation, thereby impacting the atmospheric cycles of ozone, the hydroxyl radical, and reactive nitrogen. These projects have thus led to questions regarding reactive biogenic compounds, including aerosol precursors.78

  • What changes are occurring to the atmosphere-ecosystem exchange of reactive trace gas species (nitric oxide, ammonia, nonmethane hydrocarbons, dimethyl sulfide)?

  • What biological and pyrogenic processes control these exchanges, and how might they change in the future?

  • What is the role of changing biogenic trace gas emissions in the changing photochemistry of the troposphere and stratosphere?

  • Aerosols have become a major issue in climate. What roles do sulfur and organic compounds from biogenic sources, dust from agriculture and other soil disturbances, and biomass burning play in global aerosol forcing?

Multiple Stress Imperative

Work during the past decade has shown that climate and CO2 interact and that changes to the global nitrogen cycle likely affect this interaction. In addition, the susceptibility of regions to climate and biogeochemical change is greatly modified by changes to land cover. Research on global climate change impacts at first emphasized the direct effects of CO2 on climate but has increasingly emphasized interactions of CO2 and climate with the nitrogen cycle as well as interactions with other stresses.79 Research on climate effects on ecosystems should be conducted with the realization that climate change is only one of a number of simultaneous impacts in many systems.

For regional scales the focus is changing from single-factor stresses (e.g., ozone) to consideration of multiple impacts. In areas of dense human populations, where industry, urbanization, and agriculture coexist (the “metro-agro-plexes”), the interactive effects of environmental changes to climate, atmospheric chemistry, sea level, water quality, and land use may dominate, producing consequences different from those expected based on single-factor studies or models. The study of multiple stresses (such as changing climate, air pollution, or water quality) must come to the fore, and large-scale experimental studies (which today focus on, e.g., CO2, soil warming, and ozone exposure) must address interactive effects.

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