The USGCRP will develop and apply, using tools of molecular biology, gene probes for key enzymes linking the carbon and nitrogen cycles in marine microbes.

The USGCRP will develop methods that assess the invasiveness of nonindigenous species by combining the science of landscape ecology with the principles of risk assessment. The program will use these methods to identify those areas in the U.S. that may be vulnerable to nonindigenous species invasion due to climate change and variability.

Using ecosystem-scale experiments involving increased CO2 and other environmental factors, the USGCRP will determine how atmospheric change and potential climatic change may affect forest productivity, forest health, and species distributions.

Composition and Chemistry of the Atmosphere

A combination of human and natural processes can affect the chemical composition of the global atmosphere. These changes can have important implications for life on Earth, including such factors as biologically damaging ultraviolet (UV) radiation, radiative forcing of the Earth/atmosphere system (which in turn affects climate), and the global composition of the atmosphere, which can affect air quality in regions. Human activity that can affect atmospheric composition on a global scale includes the use of chlorofluorocarbons and other halogenated hydrocarbons, fossil fuel combustion and the associated release of air pollutants, and changes in agricultural practices that affect the concentration of gases such as nitrous oxide and methane, as well as that of smoke. Changes in climate driven largely by increases in greenhouse gases can also be expected to affect atmospheric chemistry in complex ways that are difficult to predict. Natural processes affecting global atmospheric composition include volcanic eruptions, variations in solar radiation, and normal weather. Particular questions addressed by this element of USGCRP include:

How are global ozone levels and surface UV fluxes changing, and how are they likely to change in the future, given expected changes in both human industrial activity and the underlying climate in which ozone chemistry takes place? What changes may take place in the concentrations of ozone, aerosols, and other chemically and radiatively active atmospheric constituents that may contribute to climate change, and what changes may take place in the background concentrations of trace gases that affect regional atmospheric chemistry?

How will global changes in surface UV flux and surface-level concentrations of ozone and other gases and particulate matter affect human health and the productivity of ecosystems?

Current USGCRP activity in these areas builds on the accomplishments of previous research. For example, significant reductions in the total amount of stratospheric ozone over most of the Earth have been demonstrated over the past 20 years. A combination of airborne-, ground-, balloon-, and space-based instruments have all shown that industrially-produced chlorine- and bromine-containing chemical species contribute significantly to the observed ozone depletion. Observations have shown that the surface concentrations of several of the compounds regulated under the Montreal Protocol on Substances that Deplete the Ozone Layer have been reduced significantly, while those of the longer

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lived chlorofluorocarbons have essentially reached a maximum and will soon begin to decline. It is expected that maximum levels of stratospheric chlorine will be reached around the turn of the century. The stratosphere should be most susceptible to ozone depletion at that time; recovery of the ozone layer could, in principle, begin shortly thereafter. It is possible, however, that global climate change (which is projected to cool the stratosphere as the lower atmosphere warms), or a large volcanic eruption, could delay the projected recovery.

Key research challenges include:

1. Stratospheric Ozone and UV Radiation: Defining and predicting trends in the intensity of ultraviolet exposure the Earth receives by documenting the distribution of stratospheric ozone and surface UV flux, the chemical species that control the destruction of ozone, and the meteorological variables that define the physical environment of the stratosphere; and describing the coupling between chemistry, dynamics, and radiation in the stratosphere and upper troposphere.

2. Photochemical Oxidants: Defining the global processes that control ozone precursor species, tropospheric ozone, and the oxidizing capacity of the global atmosphere; and developing better understanding of what determines the ability of the atmosphere to cleanse itself of pollutants, both now and in the coming decades.

3. Atmospheric Modeling: Improving atmospheric models to better represent the trace gas and aerosol composition of the global atmosphere, as well as its transport properties, and predicting the atmosphere's response to future levels of pollutants and to changes in climate at both global and regional scales.

4. Atmospheric Aerosols and Radiation: Documenting the chemical and physical properties of aerosols; and elucidating the chemical, microphysical, and transport processes that determine their size, concentration, and chemical characteristics. 5. Toxics and Nutrients: Documenting the rates of chemical exchange between the global atmosphere and ecosystems; and elucidating the extent to which interactions between the atmosphere and biosphere are influenced by changing concentrations and depositions of harmful and beneficial compounds.

6. Clouds: Documenting the role of clouds in the partitioning of trace gases in the global atmosphere between different chemical forms and in their removal from the atmosphere, as well as their contribution to surface deposition.

The USGCRP work in several of these areas, notably photochemical oxidants and toxics and nutrients, will be carried out in close collaboration with the more regionally focused work on air pollution, acid deposition, and airborne toxics carried out through other Federal research programs organized under the auspices of the Air Quality Research Subcommittee of the Committee on Environment and Natural Resources.

Focus for FY 2000:

• The USGCRP will examine the chemistry of the stratosphere at high northern latitudes in winter, to determine the potential for an Arctic ozone hole. The study will use combined balloon and airborne measurements together with observations from an instrument currently planned for launch in late 1999 aboard a Russian satellite.

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The USGCRP will carry out significant modeling work in support of the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report, to be completed in 2001. These modeling efforts will help to simulate prior evolution of atmospheric trace constituents and aerosol composition and to forecast its future evolution. The output from these model runs will be used by climate modeling groups in their simulations of the future climate.

The USGCRP will examine the atmospheric chemistry over Southern Africa using a combination of ground-based, airborne, and satellite-based measurements. This will help establish the influence of land-cover and land-use change on regional atmospheric composition, and the role of trace gases and aerosols in atmospheric warming.

The USGCRP will increase knowledge of the distribution of ozone in the troposphere and southern sub-tropics using an enhanced network of balloon-based measurements. The data should provide a unique capability for the validation of tropical ozone columns derived from satellite data.

The USGCRP will have obtained the first full year of global carbon monoxide vertical profiles. These data, obtained by an instrument scheduled for launch in mid-1999 as part of the Earth Observing System, should provide a significantly improved picture of carbon monoxide distributions. When analyzed together with data on smoke and aerosols obtained from other EOS instruments, these measurements should lead to new insights about the role of biomass burning and industrial emissions in global pollution.

The USGCRP will have obtained surface UV flux data from the fully-implemented USGCRP ground-based UV monitoring network. These data, making use of some 60 instruments at some 50 locations, will be provided to researchers investigating biological response to ultraviolet radiation. UV flux data for other regions of the earth will be available from satellite-based techniques.

The USGCRP will provide extended and updated data sets on the global methane budget, using a combination of long-term surface-based measurements showing unexplained interannual variations in growth rate and newly-obtained total column methane observations made from a space-based instrument launched in 1999 as part of the Earth Observing System.

The USGCRP will carry out detailed studies of new data on the distribution and composition of aerosols in the global troposphere, based on a combination of ground-, ship-, airborne-, and space-based data; and the program will integrate these data into global numerical models designed to simulate aerosol formation, transport, and interaction with surrounding meteorology.

Paleoenvironment/Paleoclimate

The Earth's climate and environmental history has been long, amazingly complex,and marked by enormous changes. The challenge to this element of the USGCRP is to provide a quantitative understanding of the Earth's past environment and to define the envelope of natural environmental variability within which the effects of human activities on the planet's biosphere, geosphere, and atmosphere can be assessed.

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Implementation of the USGCRP in FY 2000