As the climate changes, temperature, precipitation, and circulation changes are likely to change how the climate system deals with the greenhouse gases, aerosols, and surface modifications produced by humans, and this will affect the climate forcing. It is likely that climate change will evoke natural responses in the climate system that will magnify or mute human-produced climate forcing through alterations in greenhouse gases and aerosols.
The global carbon and sulfur cycles contain potentially important feedback processes. There are, however, major gaps in understanding. No definitive explanation has been given for the apparent vast uptake of CO2 by the terrestrial biosphere, and no confident prediction can be given of future biological uptake or release of CO2, particularly over the long term. Few observations are available to guide the necessary scaling of vegetation-climate feedbacks from the scale of an individual leaf to a landscape mosaic of vegetation and soils. In the marine realm the strengths of a wide variety of potential feedback mechanisms related to CO2 uptake and release of dimethylsulfide are yet to be determined.
Research into carbon uptake by the land and ocean as outlined in the U.S. Carbon Cycle Plan (Sarmiento and Wofsy, 1999) and North American Carbon Program (Wofsy and Harriss, 2002) should be undertaken to characterize and reduce the uncertainty associated with carbon uptake feedbacks. The goal is to characterize key atmospheric, biospheric, and oceanic processes that influence the abundance of CO2, with special attention given to observations that define large-scale, decadal, and longer-term sources and sinks, and to define the influences on these processes of climate, land use, and socioeconomic policies. A high priority is to understand the nature of the Northern Hemisphere carbon sink, so that the evolution of this sink and its relationship to the evolving climate can be better understood. Research outlined in the Surface Ocean Lower Atmosphere Study Science Plan (Liss et al., 2002) will improve understanding of climate-dimethylsulfide feedbacks.
Improved understanding of atmospheric chemistry feedbacks is important for producing future climate projections, for understanding the relationship between measured concentrations of greenhouse gases and their emissions, and for formulating control strategies. Both tropospheric and stratospheric chemical processes interact with temperature, humidity, circulation, and air composition changes and may in turn affect Earth's radiative balance. More research on atmospheric chemical feedback processes is required, with the goal of representing these processes more comprehensively in projections of future climate.
The physical and chemical processing of aerosols and trace gases in the atmosphere, the dependence of these processes on climate, and the influence of climate-chemical interactions on the optical properties of aerosols must be elucidated. A more complete understanding of the emissions, atmospheric burden, final sinks, and interactions of carbonaceous and other aerosols with clouds and the hydrologic cycle needs to be developed. Intensive regional measurement campaigns (ground-based, airborne, satellite) should be conducted that are designed from the start with guidance from global aerosols models so that the improved knowledge of the processes can be directly applied in the predictive models that are used to assess future climate change scenarios.
The key processes that control the abundance of tropospheric ozone and its interactions with climate change also need to be better understood, including but not limited to stratospheric influx; natural and anthropogenic emissions of precursor species such as NOx, CO, and volatile organic carbon; the net export of ozone produced in biomass burning and urban plumes; the loss of ozone at the surface, and the dependence of all these processes on climate change. The chemical feedbacks that can lead to changes in the atmospheric lifetime of CH4 also need to be identified and quantified.
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