Cooperative Institute for Research in Environmental Sciences

Abstracts: 29

Carbon Cycle and Greenhouse Gases

Sara E. Mikaloff Fletcher (CIRES), Pieter P. Tans (NOAA/CMDL)

NOAA's Climate Monitoring and Diagnostics Laboratory, CIRES, and the Institute for Arctic and Alpine Research collaborate in maintaining a global network for atmospheric measurements of CO2, CH4, CO, H2, N2O, SF6, and isotopic ratios of CO2 and CH4. The goal of the measurements is to determine the time-dependent sources and sinks of each of these gases on very large spatial scales. Surface sources, if they are large enough, create a measurable concentration pattern in the atmosphere which is detected through the measurement network. Isotopic ratios improve the capability the ascribe sources because various source/sink processes are characterized by different isotopic ratios.

Measurements from the network led to the discovery of very large natural uptake of carbon dioxide by terrestrial ecosystems in mid-latitudes of the northern hemisphere.

The global budget of atmospheric methane is an example of how the measurements lead to estimates of large-scale fluxes. Simulations of atmospheric mixing ratios with an atmospheric transport model using process-based estimates of sources tend to overpredict the latitudinal gradient of methane compared to the observations. The specific source processes responsible for the discrepancy have not been identified with any confidence.

We use isotopic signatures of sources to attribute the discrepancy to a specific group of source processes, creating global and regional budgets of atmospheric methane that are in agreement with both the observed mixing ratios and the isotopic ratios of methane. We use an inverse modeling technique. In forward modeling, one defines source patterns and calculates the resulting trace gas distributions. In inverse modeling one starts with observed concentration patterns and deduces estimates of regional sources with an atmospheric transport/chemistry model that optimally reproduce the observations.

Initial results imply interesting differences from previous methane budget estimates. For example, the 13C/12C isotopic observations call for an increase in southern hemisphere sources with a bacterial isotopic signature such as wetlands, rice paddies, termites, and ruminant animals. The seasonal cycle of these fluxes suggests this source most likely to be primarily from wetlands.