stituents, including ozone and their chemical interactions, in the upper troposphere as well as the lower stratosphere. Human impacts on these coupled processes include a potential future fleet of high-flying aircraft and changes in the sources of ozone-destroying halogenated compounds. The feedbacks among human influences, chemical interactions, and atmospheric dynamics are largely unknown. The most serious uncertainties for prediction of ozone recovery during the next century may be potential changes in the chemistry and circulation of the stratosphere that put us well outside current experience. Such changes might include continued large increases in CH4 and stratospheric H2O or stratospheric circulation changes associated with global warming and greenhouse gases. The predictive models are based in part on first-principle physics and chemistry and should correctly account for these changes, but we must recognize that observations of the recent past are used to test and calibrate models.

Aerosols
  • How do the spatial distribution, chemical composition, and physical properties of aerosols vary on dec-cen timescales and how do they interact with climate variability? How do the composition and properties of aerosols determine their radiative effects? What are the regionally varying impacts of aerosols on the Earth's radiation budget? How do aerosols contribute to cloud formation, precipitation, and radiative interaction? While aerosols appear to present a critical radiative forcing on dec-cen timescales, the mechanisms and processes of this forcing remain poorly characterized. For example, the composition and properties of aerosols determine radiative effects in ways not yet well documented. The direct (reflective) effects of aerosols may change or reverse over high-albedo surfaces, where radiation absorption can contribute to overall atmospheric heating rather than cooling. The indirect effects of aerosols, related to cloud formation and radiative interactions, remain a major uncertainty but in fact may represent aerosols' primary impact on climate. Predicting future aerosol impacts requires understanding regional sources and transports and how these may vary with future climate scenarios. Human-initiated changes (e.g., through industrial emissions, biomass burning, and land use) are a key component in anticipating future aerosol variations; natural sources may be even less predictable (e.g., volcanic eruptions and natural vegetation emissions).

Solar Radiation
  • How do proxies for solar activity (e.g., sunspots, cosmogenic nuclides) relate to total solar irradiance on dec-cen timescales? Direct measurements of the Sun's radiative output span only the last solar cycle. Solar radiation appears to correlate well, however, with solar activity as repre-



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