Ice cores are unique archives of past climatic and atmospheric conditions through the isotopic composition of the ice and the analysis of the air bubbles trapped. In 1999 Petit et al published the reconstruction of the Antarctic climate and atmospheric composition over the last 420 000 years from the Vostok ice core. This record covered the last four glacial inter glacial cycles back to the end of the marine interstadial 11 (MIS 11). It has revealed the close relationship between the atmospheric part of the carbon cycle and the climate. With CO₂ concentration oscillating between 180 and 280 ppmv during the last 4 climatic cycles. In a similar way the methane concentration followed closely temperature on glacial interglacial time scales, with millennial-scale structures during glacial times which appear out of phased with Antarctic temperature but, at least for the last glaciation, in phase with the Greenland rapid climatic oscillations, as revealed by the GISP and GRIP ice cores.

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Compared to the first IPSL Climate-Carbon coupled model [Dufresne, et al., 2002], the simple carbon models have been replaced by IPSL advanced ocean and land biogeochemical models, respectively PISCES and ORCHIDEE.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.

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.

Paleo-climate records in ice cores revealed high variability in temperature, atmospheric dust content and carbon dioxide. The longest CO₂ record from the Antarctic ice core of the Vostok station went back in time as far as about 410 kyr BP showing a switch of glacials and interglacials in all those parameters approximately every 100 kyr during the last four glacial cycles with CO₂ varying between 180-300 ppmv [Petit et al., 1999]. New measurements of dust and the isotopic temperature proxy deuterium of the EPICA Dome C (EDC) ice core covered the last 740 kyr, however, revealed glacial cycles of reduced temperature amplitude [EPICA community members, 2004]. These new archives offer the possibility to propose atmospheric CO₂ for the pre-Vostok time span as called for in the EPICA challenge [Wolff et al., 2004]. Here, we contribute to this challenge using a box model of the isotopic carbon cycle [Köhler et al., 2005] based on process understanding previously derived for Termination I. Our results show that major features of the Vostok period are reproduced while prior to Vostok our model predicts significantly smaller amplitudes in CO₂ variations.

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.

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.

Is the massive Amazon forest a CO₂ sink, a source or is it in equilibrium?  

There is a large uncertainty in carbon fluxes estimates for the tropics as a whole and in particular for the Amazon region in South America, bringing the attention to the lack of information to call the region a carbon source or sink. The production of scientific consistent and long term data series for the region is a process that has to advance step by step.

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.