and their predictive powers beyond the next few decades are tenuous. Progress in understanding the driving forces of carbon storage on large spatial scales is especially hampered by the current inability to verify through observations how local processes and fluxes are extrapolated to large scales. For greenhouse gas management the highest priority is to learn more about the behavior and model representation of the terrestrial biosphere, since we exert considerable influence on it. A long sustained effort of observation and modeling, developed together, will be necessary. Uptake by the oceans needs to be closely monitored via repeated transects every 5 to 10 years and via measurements of the difference in CO2 partial pressure (pCO2) and isotopic ratio between the atmosphere and ocean surface. With respect to the latter, actual measurements of air-sea exchange flux over the open ocean would lead to a much better parameterization of the exchange process.

The injection of volcanic aerosols affects climate through stratospheric cooling (but warming in the tropics), lowering planetary albedo, and increasing surface area for chemical reactions that influence ozone levels, among other things. In fact, with respect to the latter, it has been demonstrated that changes in ozone cannot be properly interpreted without consideration of stratospheric aerosol loading. Volcanic eruptions and their stratospheric effects are necessarily unpredictable in occurrence and magnitude. Since the role of volcanic aerosols in climate and particularly their role in ozone trends is complex and perhaps longer lived than is generally understood, it is crucial to maintain the capability to depict global stratospheric aerosol loading (especially in terms of surface area density) with high vertical resolution.

Finally, the Sun is the driving force of climate, and even small variations in the amount of energy that the Earth receives can have significant impact. In general, a doubling of CO2 generates a radiative forcing that is equivalent to a 2 percent increase in solar irradiance. The changes in climate over the past several centuries are much smaller than expected from such a change, and we expect decade-to-century changes of about 0.25 percent. The relationship between solar wind and solar irradiance has been calibrated for the last solar cycles. Extrapolation for conditions outside this range, if applicable, would imply a decadal to century variability in solar irradiance with periods of lower irradiance by as much as 0.25 percent. The major difficulty in hindcasting solar irradiance over the past several centuries is the use of solar activity indices as surrogates for solar irradiance. Better understanding of solar physics and modeling of the solar cycle and the Earth's climate would allow greater confidence in this connection.

Key Scientific Questions About Atmospheric Composition and Radiation Budget
Greenhouse Gases
  • What are the changes in the spatial distribution of carbon storage and flux on dec-cen timescales? Of the carbon emitted by anthropogenic



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