Land Carbon Sink Research Articles

A first estimate of present and preindustrial air‐sea CO2 flux patterns based on ocean interior carbon measurements and models

The exchange of CO2 across the air‐sea interface is a main determinant of the distribution of atmospheric CO2 from which major conclusions about the carbon cycle are drawn, yet our knowledge of atmosphere‐ocean fluxes still has major gaps. A new analysis based on recent ocean dissolved inorganic carbon data and on models permits us to separately estimate the preindustrial and present air‐sea CO2 flux distributions without requiring knowledge of the gas exchange coefficient. We find a smaller carbon sink at mid to high latitudes of the southern hemisphere than previous data based estimates and a shift of ocean uptake to lower latitude regions compared to estimates and simulations. The total uptake of anthropogenic CO2 for 1990 is 1.8 (±0.4) Pg C yr−1. Our ocean based results support the interpretation of the latitudinal distribution of atmospheric CO2 data as evidence for a large northern hemisphere land carbon sink.

A possible global covariance between terrestrial gross primary production and13C discrimination: Consequences for the atmospheric13C budget and its response to ENSO

It is well known that terrestrial photosynthesis and13C discrimination vary in response to a number of environmental and biological factors such as atmospheric humidity and genotypic differences in stomatal regulation. Small changes in the global balance between diffusive conductances to CO2and photosynthesis in C3 vegetation have the potential to influence the13C budget of the atmosphere because these changes scale with the relatively large one‐way gross primary production (GPP) flux. Over a period of days to years, this atmospheric isotopic forcing is damped by the return flux consisting mostly of respiration, Fire, and volatile organic carbon losses. Here we explore the magnitude of this class of isotopic disequilibria with an ecophysiological model (SiB2) and a double deconvolution inversion framework that includes time‐varying discrimination for the period of 1981–1994. If the net land carbon sink and plant13C discrimination covary on interannual timescales at the global scale, consistent with El Niño‐induced drought stress causing a decline in global GPP and C3 discrimination, then less interannual variability in ocean and land net carbon exchange is required to explain atmospheric trends in δ13C and CO2as compared with previous studies that assumed discrimination was invariant.

Towards robust regional estimates of CO2 sources and sinks using atmospheric transport models

Information about regional carbon sources and sinks can be derived from variations in observed atmospheric CO2 concentrations via inverse modelling with atmospheric tracer transport models. A consensus has not yet been reached regarding the size and distribution of regional carbon fluxes obtained using this approach, partly owing to the use of several different atmospheric transport models. Here we report estimates of surface-atmosphere CO2 fluxes from an intercomparison of atmospheric CO2 inversion models (the TransCom 3 project), which includes 16 transport models and model variants. We find an uptake of CO2 in the southern extratropical ocean less than that estimated from ocean measurements, a result that is not sensitive to transport models or methodological approaches. We also find a northern land carbon sink that is distributed relatively evenly among the continents of the Northern Hemisphere, but these results show some sensitivity to transport differences among models, especially in how they respond to seasonal terrestrial exchange of CO2. Overall, carbon fluxes integrated over latitudinal zones are strongly constrained by observations in the middle to high latitudes. Further significant constraints to our understanding of regional carbon fluxes will therefore require improvements in transport models and expansion of the CO2 observation network within the tropics.

Contribution of increasing CO2 and climate to carbon storage by ecosystems in the United States.

The effects of increasing carbon dioxide (CO2) and climate on net carbon storage in terrestrial ecosystems of the conterminous United States for the period 1895-1993 were modeled with new, detailed historical climate information. For the period 1980-1993, results from an ensemble of three models agree within 25%, simulating a land carbon sink from CO2 and climate effects of 0.08 gigaton of carbon per year. The best estimates of the total sink from inventory data are about three times larger, suggesting that processes such as regrowth on abandoned agricultural land or in forests harvested before 1980 have effects as large as or larger than the direct effects of CO2 and climate. The modeled sink varies by about 100% from year to year as a result of climate variability.

Land and ocean carbon cycle feedback effects on global warming in a simple Earth system model

A simple Earth system model is developed by coupling a box model of the global carbon cycle to an energy-balance approximation of global temperature. The model includes a range of feedback mechanisms between atmospheric CO2, surface temperature and land and ocean carbon cycling. It is used to assess their effect on the global change being driven by anthropogenic CO2 emissions from fossil fuel burning and land-use change. When tuned to reach the 1990 level of atmospheric CO2, the model CO2 predictions for 1832–1990 are reasonably close to ice-core and instrumental records, observed global warming of ~0.6 K from 1860–1990 is accurately predicted and the land and ocean carbon sinks for the 1980s are close to IPCC central estimates. The ocean sink is reduced by ~0.3 GtC yr-1 when the ocean surface is assumed to warm at the same rate as global surface temperature. Land and oceanic carbon sinks are predicted to be growing at present and hence buffering the rate of rise of atmospheric CO2. In the basic model, the current land carbon sink is assumed to be due to CO2 fertilisation of photosynthesis. The slight warming that has occurred enhances soil respiration (carbon loss) and net primary productivity (carbon uptake) by similar amounts. When the model is forced with a “business as usual”(IS92a) emissions scenario for 1990–2100 followed by a linear decline in emissions to zero at 2200, CO2 reaches a peak of 985 ppmv in 2170 and temperature peaks at +5.5 K in 2180. Peak CO2 is ~135 ppmv higher than suggested by IPCC for the same forcing, principally because global warming first suppresses the land carbon sink then generates a land carbon source. When warming exceeds ~4.5 K, soil respiration “overtakes” the CO2 fertilisation of NPP, triggering a release of ~70 GtC from terrestrial ecosystems over ~100 years. When the effects of temperature on photosynthesis, respiration and soil respiration are removed, peak levels of CO2 are reduced by ~100 ppmv and peak temperature by ~0.5 K. Distinguishing separate soil carbon pools with different residence times does not significantly alter the timing of the switch to a land carbon source or its effect on peak CO2, but it causes the source to persist for longer. If forest re-growth or nitrogen deposition are assumed to contribute to the current land carbon sink, this implies a weaker CO2 fertilisation effect on photosynthesis and generates a larger future carbon source. Peak CO2 levels are also sensitive by about ±80 ppmv to upper and lower limits on the temperature responses of photosynthesis, plant respiration and soil respiration. By forcing the model with a range of future emission scenarios it is found that the creation of a significant land carbon source requires rapid warming, exceeding ~4.5 K, and its magnitude increases with the rate of forcing. The carbon source is greatest for the most rapid burning of the largest reserve of fossil fuel. It is concluded that carbon loss from terrestrial ecosystems may significantly (~10%) amplify global warming under “business as usual” or more extreme scenarios.