||This project has the overall goal of producing datasets relevant for documenting changes in the global carbon cycle and improving understanding of how land ecosystems may influence and be influenced by future CO2 changes and climate changes. It also has a more focused goal of using these and other data to challenge models that depict the response of changing climate and human forcing (e.g. rising CO2) on northern extra-tropical ecosystems over multi-decadal time scales, using records extended over the past 50 years. Targets for model improvement include improved depiction of the response of northern extra-tropical ecosystems to rising CO2 levels and climate changes. The project provides partial support for continuing core elements of the program initiated by Charles D. Keeling in the late 1950's, to measure changes in the concentration of atmospheric CO2 at remote sites, and which expanded to include measurements of the stable isotopes and radiocarbon content of CO2. In addition to sustaining these observations, which have global importance, the project supports interpretive activities focused on improving the understanding of changing climate on northern extra-tropical ecosystems, via a model/data comparison spanning more than 50 years. The project will have the following benefits and outcomes: (1) Extension and dissemination of datasets relevant to documenting global and hemispheric changes in the carbon cycle and for unraveling the underlying mechanisms. (2) Improved depiction of the impact of climate changes and rising CO2 on northern terrestrial ecosystems (3) Extension of datasets of the isotopic composition of atmospheric CO2, relevant to improving estimates of gross primary production, canopy conductance, and water-use efficiency of northern extra-tropical ecosystems.