Data from long-term measurements of carbon balance in boreal, mid-latitude and tropical ecosystems are used to assess the mechanisms that drive changes in ecosystem carbon balance in response to a changing climate. We find that most model parameterizations overestimate the temperature sensitivity of ecosystem respiration and underestimate the role of soil water balance in controlling respiration and flammability. We conclude that model assessments of climate—carbon feedbacks must carefully simulate regional precipitation, evaporation, evapotranspiration, and water balance, including factors leading to fires (e.g. sources of ignition), in addition to assessing changes in temperature. Covariances among these drivers of ecosystem respiration and vegetation change may be critically important for these simulations.

Results from recent models in the coupled carbon cycle climate model intercomparison project (C4MIP) indicate a positive feedback to global warming from the interactive carbon cycle, but the magnitude varies widely. A typical model simulates an additional increase of 90 ppmv in the atmospheric CO₂, and 0.6 degree additional warming due to this feedback, but some model can be as large as 250ppm. Using a liner perturbation framework, we analyze what might have caused such large discrepancy in the models, with a focus on land where the largest uncertainties lie. Change in NPP such as different sensitivity to the CO₂ fertilization effect is one where in some models it is modest largely due to the multiple limiting factors constraining terrestrial productivity and carbon loss. The large differences among the models are also manifestations of other poorly constrained processes such as the turnover time and rates of soil decomposition.

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.

Increased winter temperatures associated with the observed positive trend in the winter Arctic Oscillation can partially explain trends towards earlier spring leafout in the northern hemisphere.  Increased spring drawdown associated with earlier leafout, coupled with increased winter respiration due to warmer temperatures, indicate the trend in the winter Arctic Oscillation can help explain observed increases in the seasonal amplitude of atmospheric CO₂ concentration.

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.

The MC1 Dynamic General Vegetation Model (DGVM) was used to assess the impacts of global warming on North American ecosystems, north of Mexico, under 6 future climate scenarios (3 General Circulation Models X 2 emission scenarios).  The simulations were begun in 1900 using observed climate and CO₂ until 2000, then transferring to the future scenarios to 2100.  Carbon sequestration over the continent occurred in the late 20^th century and for a short period into the 21^st century, being fostered largely by increased precipitation, enhanced water-use efficiency and mild temperature increases.  However, these ‘greening’ processes were overtaken by the exponential effects of increasing temperature on evaporative demand and respiration, producing a subsequent decline. Simulation experiments suggested that fire suppression could significantly mitigate the carbon losses, yet many ecosystems were still forced to a lower carrying capacity.

An idealized general circulation model is constructed of the ocean’s deep circulation and CO₂ system that reproduces the main features of glacial-interglacial CO₂ cycles, including the tight correlation between atmospheric CO₂ and Antarctic temperatures, the lead of Antarctic temperatures over CO₂ at terminations, and the shift of the ocean’s ¹³C minimum from the North Pacific to the Atlantic sector of the Southern Ocean. The model is based on a new idea about the nature of the glacial-interglacial cycles in which the driving force is independent of the orbital forcing and is not in the ocean. The key to glacial-interglacial transitions, we claim, is a relationship between the mid-latitude westerly winds, atmospheric CO₂, and the mean state of the atmosphere. Cold glacial climates seem to have equatorward-shifted westerlies, which allow more respired CO₂ to accumulate in the deep ocean. Warm climates like the present have poleward-shifted westerlies that flush respired CO₂ out of the deep ocean.

Using a coupled 3D hydrodynamic-biogeochemical model system for the Northwest-European shelf we simulated the years 1993-96, which exhibit an extremely strong transition from a NAOI-high to a NAOI-low regime. The induced temperature-shift had two consequences for the carbon budget of the North Sea: Firstly it increased the CO₂ solubility and secondly it destabilized the water column in spring 1996. The latter effect was the precondition for mixing events which brought new nitrogen for primary production into the upper layer. Consequently the air-sea flux was 540 Gmol C a^-1 in 1996, the NAOI-low year, and it was 203 Gmol C a^-1 in 1995, the year with the highest NAOI.

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.