2. Carbon Cycle

P.P. Tans (Editor), P.S. Bakwin, L. Bruhwiler, T.J. Conway, E.J. Dlugokencky, D.W. Guenther, D.F. Hurst, D.R. Kitzis, P.M. Lang, K.A. Masarie, J.B. Miller, P.C. Novelli, C. Prostko-Bell, K.W. Thoning, B.H. Vaughn¹, J.W.C White¹, D. Yakir², and C. Zhao

¹Institute for Arctic and Alpine Research, University of Colorado

²Visiting scientist from Weizmann Institute of Science, Rehovot, Israel

2.1. Overview

It is the goal of the Carbon Cycle Group (CCG) to improve the understanding of the factors that determine the atmospheric burdens of major trace gases influencing the Earth's climate, in particular CO₂, CH₄, and CO. The anthropogenic impact on each of these species is large, but natural cycles are involved as well. The international climate change negotiations during December 1997 in Kyoto, Japan, highlighted the fact that the world has tentatively started to take steps to try to control the steadily increasing climate forcing by anthropogenic greenhouse gases. One of the factors required for effective policies is a quantitative understanding of what controls the atmospheric concentrations.

Our main tool for studying the global budgets of the trace gases is the measurement of atmospheric spatial concentration patterns and their changes over time. Two methods have been employed from the start of the Geophysical Monitoring for Climatic Change program, the forerunner of Climate Monitoring and Diagnostics Laboratory (CMDL). Continuous measurements are made in remote clean air locations, namely the four CMDL observatories, and weekly pairs of discrete flask samples are obtained. Initially the samples were analyzed only for CO₂, but gradually more species were added (Table 2.1). The isotopic ratio measurements are being carried out at the Institute for Arctic and Alpine Research of the University of Colorado in close cooperation with CCG. Anomalous ¹⁷O enrichments are measured in a small subset of the flasks by scientists at the University of California, San Diego. The global air samples provide a unique resource for narrowing uncertainties of greenhouse gas budgets as well as other atmospheric problems. We continue to investigate the feasibility of adding additional measurements.

TABLE 2.1. Species Analyzed in Samples of the Global Air Sampling Network

CU: University of Colorado

INSTAAR: Institute for Arctic and Alpine Research, University of Colorado, Boulder

UC: University of California

Information on sources and sinks of the trace gases is obtained from their rates of change and from their spatial distributions. The quantitative link is provided by numerical models of atmospheric transport, operating in both two and three dimensions. Since we are working "backwards" from observed concentrations to the sources causing them, this problem is in the class of so-called inverse problems. The greatest limitation is sparseness of data, especially in regions close to important sources and sinks. Therefore the Carbon Cycle Group has gradually expanded the spatial coverage of the cooperative air sampling network. We have added isotopic analyses because different sources and sinks may be characterized by different isotopic "signatures."

To overcome the limitation of only having measurements from the remote marine boundary layer, two new approaches were initiated. One is to continuously measure a number of chemical species and atmospheric physical parameters at different heights on very tall towers. The mole fractions of many chemical species in the continental boundary layer are highly variable, making them more difficult to interpret, requiring much more auxiliary data than the traditional marine air samples. The second new approach is to obtain discrete air samples from low-cost airplanes in automated fashion from the boundary layer up to about 8 km altitude. These samples are then sent back to the laboratory in Boulder for analysis. We hope to be able to greatly expand the use of this method, especially over North America, in order to provide significant regional-scale constraints on the budgets of the gases measured.

Since the global coverage of our sampling network is unmatched, CMDL plays an active role in bringing together the measurements from many different laboratories around the world. Toward this end, measurements of standard reference gases as well as actual field samples are being intercompared. The link with the Commonwealth Scientific and Industrial Research Organization (CSIRO), Melbourne, Australia, is particularly strong in this regard. For CO₂ and CO we provide calibrated reference gas mixtures under the auspices of the World Meteorological Organization (WMO).

We assembled a common database for CO₂ named GLOBALVIEW-CO₂. Its intended use is for three-dimensional (inverse) modeling. It is currently based on the measurements from laboratories in 12 countries, hopefully without significant calibration or methological discrepancies. The first release took place in 1996 and subsequent ones in 1997 and 1998. We plan to maintain and enlarge this database, as well as assemble similar ones for isotopic ratios, CH₄, CO, etc.

Full individual data records and monthly means can be obtained for each site from the CMDL World Wide Web page (www.cmdl.noaa.gov); the ftp file server's "pub" directory (ftp.cmdl.noaa.gov), from the WMO World Data Center for Greenhouse Gases in Tokyo, and from the Carbon Dioxide Information Analysis Center in Oak Ridge, Tennessee.

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