Ten coupled climate-carbon cycle models were forced by historical and SRES A2 anthropogenic emissions of CO₂ for the 1850-2100 time period to study the coupling between climate change and the carbon cycle. Each model ran two separate simulations in order to evaluate the climate-carbon cycle feedback. All models agree that future climate change will reduce the efficiency of the Earth system to absorb the anthropogenic CO₂. A larger fraction of CO₂ will stay in the atmosphere if climate change is accounted for.  By the end of the 21^st century, this ranges between 20 ppm and 200 ppm depending on the model, the majority of the models lying between 50 and 100 ppm. All models simulate a negative sensitivity for both the land and the ocean carbon cycle to future climate. However there is still a large uncertainty on the magnitude of these sensitivities. Also, the majority of the models attribute most of the changes to the land. Finally, most of the models locate the reduction of land carbon uptake in the tropics. However, the attribution to changes in net primary productivity versus changes in respiration is still subject to debate amongst the models.

Using discrete air sampling, atmospheric CO₂ and its stable carbon (d¹³C) and oxygen (d¹⁸O) isotopic ratios have been measured since 1994 in a cool-temperate deciduous forest in central Japan influenced strongly by the Asian monsoon. In this paper, the results are shown and the temporal variations on different time scales are discussed.

We present a comprehensive analysis of the recent inter-annual variation of the atmospheric CO₂ growth rate, with a special focus on the 2002-2003 period, using a state of the art atmospheric inversion, process driven model simulations (land and ocean), and recent biomass burning estimates. The inverse estimates compare favourably well with the model simulations over North Asia and indicate a large contribution of the fire anomaly to the total anomaly, for that region in 2003. Over Europe, the spatial distribution of the inverse and bottom-up flux anomalies for 2003 have similarities but the time evolution of the total fluxes still need to be reconciled.

Monthly time series of atmospheric carbon dioxide (CO₂), the relative amount of carbon-13 in CO₂ (¹³C), hydrogen (H₂) carbon monoxide (CO), and methane (CH₄) are examined and related to each other and to an index of the status of ENSO. Making use of simple 12-month running mean and difference filters isolates the year-to-year variability in the concentrations and apparent sources of these constituents.

The world is moving in a direction of managing the carbon cycle in order to limit the forcing of earth's climate by CO₂ as well as to limit acidification of the oceans.  We may expect limitations on emissions, sequestration of carbon and enhancements of natural sinks.  It would be important to be able to observe and quantify the impact of any such measures on the growth rate of CO₂. Until now it has been difficult to quantify changes of the growth rate of CO₂ with confidence due to the large year to year variations that are caused by climate variations.  A statistical method has been developed to predict the growth rate of CO₂ based on observed variations of climate parameters.

Organic carbon buried under the great ice sheets of the Northern Hemisphere is suggested to be the missing link in the atmospheric CO₂ change over the glacial-interglacial cycles. At glaciation, the advancement of continental ice sheets buries vegetation and soil carbon accumulated during warmer periods. At deglaciation, this burial carbon is released back into the atmosphere. In a simulation over two glacial-interglacial cycles using a synchronously coupled atmosphere-land-ocean carbon model forced by reconstructed climate change, I found a 547 Gt terrestrial carbon release from glacial maximum to interglacial, resulting in a 60 Gt (about 30 ppmv) increase in the atmospheric CO₂, with the remainder absorbed by the ocean in a scenario in which ocean acts as a passive buffer. This is in contrast to previous estimates of a land uptake at deglaciation. This carbon source originates from glacial burial, continental shelf and other land areas in response to changes in ice cover, sea level, and climate. The input of light isotope enriched terrestrial carbon causes atmospheric Δ¹³C to drop by about 0.3permil at deglaciation, followed by rapid rise towards a high interglacial value in response to oceanic warming and regrowth on land. Together with other ocean based mechanisms such as change in ocean temperature, the glacial burial hypothesis may offer a full explanation of the observed 80-100 ppmv atmospheric CO₂ change.

This research introduces the concept of a “CO₂ fertilization factor for soil carbon” (SigmaCF). The SigmaCF is a measure of an ecosystem’s capacity to increase soil carbon storage in response to elevated carbon dioxide levels. This research describes the mathematical derivation of SigmaCF and illustrates how SigmaCF can be determined experimentally, using data from three different CO₂ enrichment experiments. I have developed this concept to compare the results of carbon dioxide enrichment experiments having different soil carbon turnover times, different levels of CO₂ enrichment, and different lengths of exposure to elevated carbon dioxide levels. The SigmaCF can also be used to estimate increases in soil carbon uptake due to observed contemporary increases in atmospheric carbon dioxide levels. This approach approximates the extent to which elevated carbon dioxide levels increase soil carbon storage. I calculated SigmaCF for three experimental settings—a mixed forest, and stands of loblolly pine and white oak trees—by measuring changes in carbon inventories and radiocarbon ratios. The forest had a SigmaCF of 1.8, which would imply a global sequestration of 5.5 billion tons C/year during the 1990's (in the highly-unlikely event that all terrestrial vegetation shows this same response to elevated carbon dioxide levels). The loblolly pine stand had a SigmaCF of 0.9 (2.8 billion tons C/year) and the white oak stand had a SigmaCF of 1.18 (3.5 billion tons C/year). These results show that elevated carbon dioxide levels in the atmosphere are increasing the flux of carbon from the atmosphere to soil.

This study examines the potential for feedbacks between the carbon cycle, atmospheric carbon dioxide (CO₂) increases and climate change to affect the anthropogenic emissions that are required to stabilize future levels of CO₂ in the atmosphere. Using a coupled climate-carbon cycle model, I found that positive carbon cycle-climate feedbacks reduced allowable emissions by an amount that varied with the model’s climate sensitivity.  Emissions were further reduced if CO₂ fertilization was assumed to be inactive in the model, as this removed an otherwise important negative feedback on atmospheric CO₂.

The Amazon region represents a large stock of biomass as well as a potentially important sink for additional atmospheric CO₂. Climate change, land-use changes and their interaction present a risk to this role in the global carbon cycle. Both positive and negative feedbacks exist in the system that can lead to resilience but also to accelerated break-down of the carbon stocks and sinks. A set of linked projects will investigate elements of these processes in the coming years.