forcings. Much of the current understanding of radiative forcing and other forcing concepts has been obtained from climate models. To improve this understanding, routine observations of climate forcings will be essential, both as a record of change in the climate system and as a critical constraint for climate models.

The major long-lived greenhouse gases (carbon dioxide [CO₂], methane [CH₄], nitrous oxide [N₂O], and halocarbons) are all extensively observed by surface networks such as the National Oceanic and Atmospheric Administration (NOAA) Climate Monitoring and Diagnostics Laboratory (CMDL) and the Atmospheric Lifetime Experiment (ALE)/Global Atmospheric Gases Experiment (GAGE). All have sufficiently long lifetimes to be well mixed in the atmosphere. Their spectroscopy is also well established. Radiative forcings can thus be assessed with confidence.

There is, however, a strong impetus to improve the observational system for these gases in order to constrain inverse model analyses of their regional budgets. For example, many analyses have used the large-scale gradients of CO₂ measured from the surface networks to constrain the global carbon budget and quantify the terrestrial sink at northern midlatitudes. However, they have not been successful in determining the longitudinal distribution of the carbon sink among the three northern mid-latitude continents. The International Geosphere-Biosphere Programme (IGBP) TransCom activity (http://transcom.colostate.edu/) has provided a forum for standardizing and comparing these inverse model analyses, but model transport errors ultimately limit their ability to exploit the relatively sparse surface air observations in terms of regionally resolved source and sink constraints (Gurney et al., 2002).

Better understanding of terrestrial uptake is critically needed for future projections of CO₂ concentrations (IPCC, 2001). An extensive network of CO₂ flux measurement towers has been deployed worldwide in recent years and is coordinated through the FLUXNET activity (Baldocchi et al., 2001). It includes in particular the AmeriFlux network in North America (http://public.ornl.gov/ameriflux/). These measurements provide direct observations of the terrestrial component of the carbon budget and also the biogeochemical constraints needed to interpret these observations. However, it has not been clear how to integrate them into large-scale inverse model analyses. The North American Carbon Program (NACP) outlines a strategy for doing so, involving in particular the use of aircraft observations to scale up the tower flux observations and providing a linkage to the global observation network (Wofsy and Harriss, 2002; Denning et al., 2003).

Global mapping of CO₂ concentrations from space would greatly im-