The carbon cycle has undergone changes from 1998-2003 as a result of extensive droughts.  The CO₂ seasonal amplitude at MLO halted its increase, and the CO₂ growth rate accelerated as a result of a slowing down of the North American carbon sink.  In a series of coupled carbon-climate model experiments, we show a greater probability of drier soils in the 21^st century, especially in the tropics and in mid-latitude summers as temperature-driven evapotranspiration exceed precipitation, and a positive feedback between the carbon cycle and climate. This positive feedback reduces the land and ocean’s capacity to store fossil fuel CO­_2­ and accelerates the warming. A fossil fuel emission accelerating rapidly as the sink capacities decrease leads to further increases in the airborne fraction of fossil fuel CO₂.

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.

We have developed a multiple element (C, N, P, Si, Ca, Fe) biogeochemical model of marine ecology that includes small, large and diazotrophic phytoplankton as well as explicit ballast-driven sinking and remineralization of detrital organic matter and cycling of dissolved organic matter. Phytoplankton growth is described through a new formulation including co-limitation by N, P, Si, Fe and light to reproduce broad observational trends.  Phytoplankton grazing is described through different power laws in the closure terms for small and large phytoplankton to reproduce observed augmentation of large phytoplankton with increasing production. Detritus production is assumed to be a temperature dependent fraction of small and large phytoplankton. This model has been imbedded in a 1-degree; global ice/ocean general circulation model (MOM4) forced by a 43-year atmospheric reanalysis forcing from the Common Ocean Reference Experiments (CORE) program to quantify the relationship between food web structure, biogeochemical cycles and the atmospheric CO₂ signature on inter-annual timescales. Novel aspects in the model structure are described, the impact of the formulation of ecosystem structure on biogeochemical cycling are discussed, and results of the atmospheric reanalysis forcing experiment presented. Of particular interest are the dynamical roles played by equatorial ENSO variability and polar sea ice dynamics on air-sea CO₂ fluxes.

A new three-dimensional global coupled carbon-climate model is presented in the framework of the Community Climate System Model (CSM-1.4). A 1000-year control simulation has stable global annual mean surface temperature and atmospheric CO₂ with no flux adjustment in either physics or biogeochemistry. At low frequencies (timescale > 20 years), the ocean tends to damp (20-25%) slow, natural variations in atmospheric CO₂ generated by the terrestrial biosphere. Transient experiments (1820-2100) show that carbon sink strengths are inversely related to the rate of fossil fuel emissions, so that carbon storage capacities of the land and oceans decrease and climate warming accelerates with faster CO₂ emissions. There is a positive feedback between the carbon and climate systems, so that climate warming acts to increase the airborne fraction of anthropogenic CO₂ and amplify the climate change itself. Globally, the amplification is small at the end of the 21st century in our model because of its low transient climate response and the near-cancellation between large regional changes in the hydrologic and ecosystem responses.

Despite their relative small extension, wetlands are important as sources or sinks of C. But, due to their intermediate position between land and permanent water, they have been modified in the name of “health” or “productivity.” Such changes have altered substantially their ability to store/produce C greenhouse gasses but the main point is to establish until which point this changes are “structural” (implying the intrinsic environmental mechanisms), and therefore unrecoverable, or “casual” (implying not the environment processes but its “external”–not directly implied in the C storage/emission- components), and consequently recoverable. Temperate wetlands are strongly dependant on water availability due to their position but, on the other hand, use to be occupied by resistant species able to survive hard conditions. The example shown below presents a case of intense human activity on a Mediterranean wetland that has caused very intense changes in the flooded area but not so evident and perdurable in the main ecological relations implied in the C cycle.

The Siberian permafrost carbon stock has been studied using a newly developed soil model, which takes into account soil freezing/thawing and organic matter decomposition in the form of soil respiration and methanogenesis. The results show that the soil response to a rapid external warming can be a self-sustaining process involving permafrost melting, deep-soil respiration with associated heat generation, and methanogenesis. Most of the soil carbon is thus consumed until there is not enough of it to feed intense respiration and/or methanogenesis. This behavior is manifested only at sufficiently warm climate established after the warming. Carbon consumption in the extremely carbon-rich Yedoma Ice Complex region appears to be moderate due to cold climatic conditions.

Future climate warming is expected to enhance plant growth in temperate ecosystems and to increase carbon sequestration. But although severe regional heatwaves may become more frequent in a changing climate, and their impact on terrestrial carbon cycling is unclear. Europe experienced a particularly extreme climate anomaly during 2003, with July temperatures up to 6°C above long-term means, and annual precipitation deficits up to 300 mmy^-1, that is 50% below the average. We used the 2003 heatwave as a ‘laboratory assistant’ to estimate the impact on terrestrial carbon cycling.

Because vegetation growth in the Northern Hemisphere is typically nitrogen-limited, increased nitrogen deposition could have attenuating effect on rising atmospheric CO₂ by stimulating the accumulation of biomass. Given the high carbon to nitrogen ratios and long lifetimes of carbon in wood, a most significant effect of nitrogen fertilization is expected in forests. Forest inventories indicate that the carbon content of northern forests have increased concurrently with increased nitrogen deposition since the 1950s [Spiecker et al., 1996]. In addition, variations in atmospheric CO₂ indicate a globally significant carbon sink in northern mid-latitude forest regions [Schimel et al., 2001]. It is unclear however, whether elevated nitrogen deposition or other factors are the primary cause of carbon sequestration in northern forests. We argue that the elevated nitrogen deposition is unlikely to enhance vegetation carbon sink significantly because of its differentiating effect on the carbon sequestration capacity of uneven aged forests and climatic limitations on carbon sequestration in the Northern Hemisphere. We estimate the potential of forests with lifted nitrogen limitation to decelerate CO₂ concentrations rise in the atmosphere and therefore to mitigate climate warming. We also outline areas of the Northern Hemisphere which are most sensitive to increased nitrogen deposition.

We have developed a Climate-Carbon coupled model based on the IPSL OAGCM and on two biogeochemical models, ORCHIDEE for the continent and PISCES for the ocean, to investigate the coupling between climate change and the global carbon cycle. We have performed four climate-carbon simulations over the 1860-2100 period in which atmospheric CO₂ is interactively calculated. They are :

§ A control coupled simulation with no anthropogenic emissions.

§ A coupled simulation with anthropogenic emissions.

§ A coupled simulation with anthropogenic emissions including non-CO₂ greenhouse and sulfate aerosols.

§ An uncoupled carbon simulation with the same anthropogenic emissions as second simulation but for which atmospheric CO₂ change has no impact on climate.

Compared to the first IPSL Climate-Carbon coupled model [Dufresne, et al., 2002], the simple carbon models have been replaced by IPSL advanced ocean and land biogeochemical models, respectively PISCES and ORCHIDEE. CO₂ is transported in the atmosphere and compared with observations. Comparison with satellite data is also done. We then analyze the coupled and uncoupled simulations, highlight the importance of the climate change both on the oceanic and biosphere sink and estimate the climate-carbon feedback. The results are also compared to the outputs of other models participating in the C4MIP inter-comparison project. Finally, off-line simulations are carried out to perform sensitivity tests (fire, dynamics of land and ocean ecosystems, soil respiration) in order to identify the key processes which govern the simulated response.

Observations suggest the global reflectivity of Earth changed during recent decades. Although there is some ambiguity surrounding these findings, it is clear that, should there be changes in clouds or scattering aerosols, a change in the total solar radiation received at the surface and the fraction of diffuse light could result. Intriguingly, the d¹⁸O of CO₂ time series measured at Mauna Loa shows variability during the 1990s that does not match secular trends in CO₂ concentration or d¹³C. While a decrease in total solar radiation alone would reduce biospheric productivity, an increase in diffuse light can increase productivity, as has been argued for the period following the eruption of Pinatubo. Moreover, since the changes in radiation affect the surface latent energy exchange, the isotopic composition of terrestrial water with which CO₂ interacts (specifically leaf and soil water) will be modified and can thus drive a change in isotopic fluxes.