The land and ocean absorb on average over half of the anthropogenic emissions of carbon dioxide (CO2) every year. These CO2 “sinks” are modulated by climate change and variability. Here we use a suite of nine Dynamic Global Vegetation Models (DGVMs) 5 and four Ocean Biogeochemical General Circulation Models (OBGCMs) to quantify the global and regional climate and atmospheric CO2 – driven trends in land and oceanic CO2 exchanges with the atmosphere over the period 1990–2009, attribute these trends to underlying processes, and quantify the uncertainty and level of model agreement. The models were forced with reconstructed climate fields and observed 10 global atmospheric CO2; Land Use and Land Cover Changes are not included for the DGVMs. Over the period 1990–2009, the DGVMs simulate a mean global land carbon sink of −2.4±0.7 PgCyr−1 with a small significant trend of −0.06±0.03 PgCyr−2 (increasing sink). Over the more limited period 1990–2004, the ocean models simulate a mean ocean sink of −2.2±0.2 PgCyr−1 with a trend in the net C uptake that 15 is indistinguishable from zero (−0.01±0.02 PgCyr−2). The two ocean models that extended the simulations until 2009 suggest a slightly stronger, but still small trend of −0.02±0.01 PgCyr−2. Trends from land and ocean models compare favourably to the land greenness trends from remote sensing, atmospheric inversion results, and the residual land sink required to close the global carbon budget. Trends in the land sink 20 are driven by increasing net primary production (NPP) whose statistically significant trend of 0.22±0.08 PgCyr−2 exceeds a significant trend in heterotrophic respiration of 0.16±0.05 PgCyr−2 – primarily as a consequence of wide-spread CO2 fertilisation of plant production. Most of the land-based trend in simulated net carbon uptake originates from natural ecosystems in the tropics (−0.04±0.01 PgCyr−2), with almost no 25 trend over the northern land region, where recent warming and reduced rainfall offsets the positive impact of elevated atmospheric CO2 on carbon storage. The small uptake trend in the ocean models emerges because climate variability and change, and in particular increasing sea surface temperatures, tend to counteract the trend in ocean uptake driven by the increase in atmospheric CO2. Large uncertainty remains in the magnitude and sign of modelled carbon trends in several regions, and on the influence of land use and land cover changes on regional trends

Satellite-derived Normalized Difference Vegetation Index (NDVI), a proxy of vegetation productivity, is known to be correlated with temperature in northern ecosystems. This relationship, however, may change over time following alternations in other environmental factors. Here we show that above 30°N, the strength of the relationship between the interannual variability of growing season NDVI and temperature (partial correlation coefficient R NDVI-GT ) declined substantially between 1982 and 2011. This decrease in R NDVI-GT is mainly observed in temperate and arctic ecosystems, and is also partly reproduced by process-based ecosystem model results. In the temperate ecosystem, the decrease in R NDVI-GT coincides with an increase in drought. In the arctic ecosystem, it may be related to a nonlinear response of photosynthesis to temperature, increase of hot extreme days and shrub expansion over grass-dominated tundra. Our results caution the use of results from interannual time scales to constrain the decadal response of plants to ongoing warming.

The influence of CO(2) transported in the transpiration stream on measurements of leaf photosynthesis and stem respiration was investigated. Measurements were made on trees in a temperate forest in Scotland and in a tropical rain forest in Cameroon, and on shrubs in the Sahelian zone in Niger. A chamber was designed to measure the CO(2) partial pressure in the gas phase within the woody stems of trees. High CO(2) partial pressures were found, ranging from 3000 to 9200 Pa. Henry's Law was used to estimate the CO(2) concentration of xylem sap, assuming that it was in equilibrium with the measured gas phase partial pressures. The transport of CO(2) in the xylem sap was calculated by multiplying sap CO(2) concentration by transpiration rate. The magnitude of aqueous transport in the studied species ranged from 0.03 to 0.35 &mgr;mol CO(2) m(-2) s(-1), representing 0.5 to 7.1% of typical leaf photosynthetic rates. These values strongly depend on sap pH. To examine the influence of aqueous transport of CO(2) on stem gas exchange, we made simultaneous measurements of stem CO(2) efflux and sap flow on the same stem. After removing the effect of temperature, stem CO(2) efflux was positively related to sap flow. The apparent effect on measurements of stem respiration was up to 0.7 &mgr;mol m(-2) s(-1), representing ~12% of peak stem respiration rates.

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