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.

It is investigated how abrupt changes in the North Atlantic (NA) thermohaline circulation (THC) affect the terrestrial carbon cycle. The Lund-Potsdam-Jena Dynamic Global Vegetation Model is forced with climate perturbations from freshwater experiments with the ECBILT-CLIO ocean-atmosphere model. A reorganization of the marine carbon cycle is not addressed. Modeled NA THC collapsed and recovered after about a millennium in response to prescribed freshwater forcing. The initial cooling of several Kelvin over Eurasia causes a reduction of extant boreal and temperate forests and a decrease in carbon storage in high northern latitudes, whereas improved growing conditions and slower soil decomposition rates lead to enhanced storage in mid-latitudes. The magnitude and evolution of global terrestrial carbon storage in response to abrupt THC changes depends sensitively on the initial climate conditions. These were varied using results from time slice simulations with the Hadley climate model for different periods over the past 21,000 years. Terrestrial storage varies between -67 and +50 PgC for the range of experiments with different initial conditions. Simulated peak-to-peak differences in atmospheric CO₂ and d¹³C are 6 and 18 ppmv for glacial and early Holocene conditions. Simulated changes in d¹³C are between 0.18 and 0.30 permil. The small CO₂ changes modelled for glacial conditions are compatible with available evidence from marine studies and the ice core CO₂ record. The latter shows CO₂ variations of up to 20 ppmv broadly in parallel with the Antarctic warm events A1 to A4.

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.

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.

A coupled Biogeochemistry-Ecosystem-Circulation (BEC) ocean model is used to examine the sensitivity of ocean biogeochemical cycling and air-sea CO₂ exchange to variations in mineral dust deposition from the atmosphere.  Mineral dust deposition estimates from four different climate regimes are used to force the ocean model.  Our estimated climate-induced changes in dust deposition to the oceans significantly modify phytoplankton community composition, and global-scale rates of nitrogen fixation, export production, and air-sea CO₂ flux.  Dust driven variations in air-sea CO₂ exchange exceeding 1 PgC/yr are of similar magnitude to present net oceanic anthropogenic uptake.  Dust deposition directly modifies rates of export production and CO₂ flux over large regions where iron is the primary growth-limiting nutrient.  Dust deposition also indirectly influences these rates by modifying the rates of nitrogen fixation in the tropics and subtropics where nitrogen is the primary limiting nutrient.  Initially the direct pathway dominates the ocean biogeochemical response to dust variations, but over multi-decadal timescales the indirect response may be equally important.  Our predicted decrease in mineral dust deposition over the next century would significantly slow oceanic uptake of CO₂ and act as a positive feedback mechanism for the ongoing global warming.

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.

Various patterns of large-scale climate variability have exhibited trends over the past few decades. These patterns of variability are known to have contributed substantially to recent trends in, for example, surface temperatures and precipitation. However, it is less clear to what extent the climate impacts of these patterns extend to the carbon cycle. Here we summarize the observed relationships between monthly and daily mean variations in concentrations of atmospheric carbon dioxide and the dominant pattern of variability in the extratropical circulations, the so-called Northern and Southern Hemisphere Annular Modes. The observed relationships are compared with results derived from surface flux estimates from the Atmospheric Tracer Transport Model Intercomparison Project (TransCom).

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.