activities in the past two centuries, just over half of that has remained in the atmosphere. Nearly half has been taken up by other mobile reservoirs, the biosphere, and surface ocean. Although initial estimates of carbon sinks failed to account for a large fraction of anthropogenic CO2 emissions, recent calculations suggest that a growing terrestrial biosphere may account for this “missing sink.” Uncertainties in estimates of biospheric activity, together with poor knowledge of the surface ocean's uptake, suggest that our knowledge of the fate of anthropogenic CO2 may not be inaccurate. Atmospheric CO2 has been increasing steadily since the mid-1800s, but interannual to decadal fluctuations in the rate of rise also are seen in the atmospheric record and may be due to biospheric or oceanic variability. Predictive knowledge of atmospheric greenhouse gas concentrations requires that we narrow the current uncertainties in evaluations of carbon sources, sinks, and fluxes.

  • How do mixed-layer water replacement rates interact with biological processes to produce changes in ocean carbon storage? The biological pump—that is, the sequestration of inorganic carbon by primary productivity in the surface ocean—virtually determines the partitioning of carbon between ocean and atmosphere. Without the biological pump, atmospheric concentrations of CO2 would be three to four times higher than they are today. The biological pump delivers carbon to the deep ocean, which exchanges with the atmosphere on timescales of centuries and longer. The pump's level of activity depends on the extent of nutrient utilization, which can change as a consequence of changing vertical mixing rates. The latter rates can alter both nutrient supply and the time available to surface phytoplankton to utilize these nutrients.

  • What are the uptake, pathways, and fate of anthropogenic carbon in the ocean on dec-cen timescales? In addition to the “biological pump” of carbon described above, the surface ocean takes up CO2 passively through gas exchange. The exchange of CO2 between atmosphere and surface ocean occurs partly as a function of the atmosphere-ocean concentration gradient; thus, as atmospheric CO2 rises from anthropogenic inputs, the ocean gas-exchange sink strengthens. Current best estimates of the oceanic uptake of anthropogenic CO 2 derive from numerical models. The paucity of accurate observations is still a significant obstacle to the quantification of oceanic carbon uptake and its pathways.

  • What are the contributions of various sources and sinks to the recent increase in methane? Although observations document that atmospheric CH4, a potent greenhouse gas, has increased steadily since the nineteenth century, the causes of this rise are not sufficiently quantified. Likely candidates for the new sources include expanding agricultural wetlands and livestock herds, biomass burning, fossil-fuel-related industry, and landfills. The total quantity of methane added to the atmosphere is well



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