C. Le Quéré

Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks

The growth rate of atmospheric carbon dioxide (CO2), the largest human contributor to human-induced climate change, is increasing rapidly. Three processes contribute to this rapid increase. Two of these processes concern emissions. Recent growth of the world economy combined with an increase in its carbon intensity have led to rapid growth in fossil fuel CO2 emissions since 2000: comparing the 1990s with 2000–2006, the emissions growth rate increased from 1.3% to 3.3% y−1. The third process is indicated by increasing evidence (P = 0.89) for a long-term (50-year) increase in the airborne fraction (AF) of CO2 emissions, implying a decline in the efficiency of CO2 sinks on land and oceans in absorbing anthropogenic emissions. Since 2000, the contributions of these three factors to the increase in the atmospheric CO2 growth rate have been ≈65 ± 16% from increasing global economic activity, 17 ± 6% from the increasing carbon intensity of the global economy, and 18 ± 15% from the increase in AF. An increasing AF is consistent with results of climate–carbon cycle models, but the magnitude of the observed signal appears larger than that estimated by models. All of these changes characterize a carbon cycle that is generating stronger-than-expected and sooner-than-expected climate forcing.

Spatial distribution of air‐sea CO2 fluxes and the interhemispheric transport of carbon by the oceans

The dominant processes controlling the magnitude and spatial distribution of the preindustrial air-sea flux of CO2 are atmosphere-ocean heat exchange and the biological pump, coupled with the direct influence of ocean circulation resulting from the slow time-scale of air-sea CO2 gas exchange equilibration. The influence of the biological pump is greatest in surface outcrops of deep water, where the excess deep ocean carbon resulting from net remineralization can escape to the atmosphere. In a steady state other regions compensate for this loss by taking up CO2 to give a global net air-sea CO2 flux of zero. The predominant outcrop region is the Southern Ocean, where the loss to the atmosphere of biological pump CO2 is large enough to cancel the gain of CO2 due to cooling. The influence of the biological pump on uptake of anthropogenic CO2 is small: a model including biology takes up 4.9% less than a model without it. Our model does not predict the large southward interhemispheric transport of CO2 that has been suggested by atmospheric carbon transport constraints.

Simulating dimethylsulphide seasonality with the Dynamic Green Ocean Model PlankTOM5

[1] We study the dynamics of dimethylsulphide (DMS) and dimethylsulphoniopropionate (DMSP) using the global ocean biogeochemistry model PlankTOM5, which includes three phytoplankton and two zooplankton functional types (PFTs). We present a fully prognostic DMS module describing intracellular particulate DMSP (DMSPp) production, concentrations of dissolved DMSP (DMSPd), and DMS production and consumption. The model produces DMS fields that compare reasonably well with the observed annual mean DMS fields, zonal mean DMS concentrations, and its seasonal cycle. Modeled ecosystem composition and modeled total chlorophyll influenced mean DMS concentrations and DMS seasonality at mid‐ and high latitudes, but did not control the seasonal cycle in the tropics. The introduction of a direct, irradiation‐dependent DMS production term (exudation) in the model improved the match between modeled and observed DMS seasonality, but deteriorated simulated zonal mean concentrations. In PlankTOM5, exudation was found to be most important for DMS seasonality in the tropics, and a variable DMSP cell quota as a function of light and nutrient stress was more important than a PFT‐specific minimal DMSPp cell quota. The results suggest that DMS seasonality in the low latitudes is mostly driven by light. The agreement between model and data for DMS, DMSPp, and DMSPd is reasonable at the Bermuda Atlantic Time Series Station, where the summer paradox is observed.

Interannual variability in oceanic biogeochemical processes inferred by inversion of atmospheric O2/N2 and CO2 data

Atmospheric measurements of O2/N2 and CO2 at up to nine sites have been used to infer the interannual variations in oceanic O2 exchange with an inverse method. The method distinguishes the regional contributions of three latitudinal bands, partly the individual contributions of the North Pacific and the North Atlantic also. The interannual variations of the inferred O2 fluxes in the tropical band correlate significantly with the El Ni˜no/Southern Oscillation. Tropical O2 variations appear to be dominated by the ventilation of the O2 minimum zone from variations in Pacific equatorial upwelling. The interannual variations of the northern and southern extratropical bands are of similar amplitude, though the attribution to mechanisms is less clear. The interannual variations estimated by the inverse method are larger than those estimated by the current generation of global ocean biogeochemistry models, especially in the North Atlantic, suggesting that the representation of biological processes plays a role. The comparison further suggests that O2 variability is a more stringent test to validate models than CO2 variability, because the processes driving O2 variability combine in the same direction and amplify the underlying climatic signal