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.

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.

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.

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.

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).

There is a growing recognition that the Earth itself is a single system within which the biosphere is an active component. Human activities are now so pervasive and profound in their consequences that they affect the Earth at a global scale in complex, interactive and apparently accelerating ways. The new IGBP project, Analysis, Integration and Modeling of the Earth System (AIMES) is charged with integrating human processes with Earth system processes.

In order to investigate the long-term and inter-annual variations in the atmospheric CO₂ concentration record obtained by aircraft measurements over Japan, we have conducted numerical experiments using a transport model with a process-based ecosystem model. The climate-induced anomalies of net biospheric flux account for a significant part of the inter-annual variations in the CO₂ growth rate. The results indicate that year-to-year change in observed vertical CO₂ gradient is mainly caused by the inter-annual variability in atmospheric transport, likely related to El Niño events.

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.