This paper examines two key feedbacks that operate between the terrestrial carbon cycle, atmospheric carbon dioxide (CO₂) and climate: the positive carbon cycle-climate feedback and the negative CO₂ fertilization feedback.  Both feedbacks affect strongly the growth rate of future atmospheric CO₂, and interact in such a way that the effect of one is notably modified in the absence of the other.

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.

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.

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.

Different CO₂ stabilization scenarios and CO₂ emission scenarios have been carried out with an earth system model to investigate feedbacks between future climate change and carbon cycle. The model predicts a sensitivity of 1.6±0.1 K for an increase of 280 ppm in atmospheric CO₂ concentration. The decrease of the thermohaline circulation is predominantly controlled by an enhanced atmospheric moisture transport to high latitudes by global warming. Overall, the simulated effect of atmospheric CO₂ concentration on climate change reduces the total carbon uptake of the ocean and the land is reduced by 24-29%.

Using the CSIRO global climate model (CCAM) coupled with a terrestrial carbon cycle model, we carried out two simulations using the protocol of C4MIP (Coupled Carbon Cycle Climate Model Intercomparison Project) Phase I to study the influences of increasing atmospheric CO₂ concentration and changes in sea surface temperature over the last 100 years on CO­₂ between atmosphere and 11 biomes. It was found that the inter-annual variation of net ecosystem prediction of global terrestrial biosphere is significantly correlated to the variation of land surface temperature from 1980 to 1999, and the increase in net ecosystem production can be largely explained by the increase in net primary production from CO₂ fertilization from 1970 to 1999 in our model. The response of net ecosystem production to CO₂ fertilization is strongest in tropical rainforest and not significant in tundra. Our estimates of net ecosystem production of global terrestrial biosphere in 1990’s agree well with the results from an inversion study by Allison et al. [this volume].

The effect of changes in iron supply to the ocean on CO₂ uptake is examined. Dust deposition fields from a dust model driven by output from a future climate simulation of a coupled general circulation model (GCM) were used as input to an ocean GCM with an embedded ecosystem model. In simulations using dust produced in a future climate the primary productivity of the ocean increased by 56% compared to simulations using dust from the present climate. The sinking particle flux of carbon at 100 m depth increased by 46%. The net air-to-sea flux of CO₂ was 4.1 PgC/y greater in the future dust simulation. Most of these changes occurred in the Equatorial Pacific Ocean, where the model ecosystem was iron-limited with present-day dust inputs but which received a large increase in the dust supplied from the Amazon Basin. These perturbations to the marine biogeochemical system are large compared to other potential climate effects that have been observed in the model. Although these results are preliminary, they could form a large negative feedback on global warming.