Terrestrial ecosystem carbon dynamics and climate feedbacks
Martin Heimann & Markus Reichstein
Recent evidence suggests that, on a global scale, terrestrial ecosystems will provide a positive feedback in a warming world, albeit of uncertain magnitude.
It has only been recognized relatively recently that biological processes can control and steer the Earth system in a globally significant way. Terrestrial ecosystems constitute a major player in this respect: they can release or absorb globally relevant greenhouse gases such as carbon dioxide (CO2), methane and nitrous oxide, they emit aerosols and aerosol precursors, and they control exchanges of energy, water and momentum between the atmosphere and the land surface. Ecosystems themselves are subject to local climatic conditions, implying a multitude of climate–ecosystem feedbacks that might amplify or dampen regional and global climate change. Of these feedbacks, that between the carbon cycle and climate has recently received much attention. Large quantities of carbon are stored in living vegetation and soil organic matter, and liberation of this carbon into the atmosphere as CO2 or
methane would have a serious impact on global climate. By definition, the carbon balance of an ecosystem at any point in time is the difference between its carbon gains and losses. Terrestrial ecosystems gain carbon through photosynthesis and lose it primarily as CO2 through respiration in autotrophs (plants and photosynthetic bacteria) and heterotrophs (fungi, animals and some bacteria), although losses of carbon as volatile organic compounds, methane or dissolved carbon (that is, non- CO2 losses) could also be significant. Quantifying and predicting these carbon-cycle–climate feedbacks is difficult, however, because of the limited understanding of the processes by which carbon and associated nutrients are transformed or recycled within ecosystems, in particular within soils, and exchanged with the overlying atmosphere.
There is ample empirical evidence that the terrestrial component of the carbon cycle is responding to climate variations and trends on a global scale. This is exemplified by the strong interannual variations in the globally averaged growth rate of atmospheric CO2, which is tightly correlated with El Niño–Southern Oscillation climate variations (Fig. 1). Many lines of evidence show that the variations in the CO2 growth rate are mainly caused by terrestrial effects, in particular the impacts of heat and drought on the vegetation of western Amazonia and southeastern Asia, leading to ecosystem carbon losses through decreased vegetation productivity and/or increased respiration. These interannual variations reflect short-term responses of the carbon cycle to climate perturbations, however, and cannot be expected to hold over longer timescales. Conversely, the close correlation between atmospheric concentrations of CO2, methane and nitrous oxide and global climate during the last glacial cycles1 indicates that ecosystem–climate interactions are also operating on timescales of millennia and longer.
Unfortunately, empirical evidence for global carbon-cycle–climate interactions on the timescale pertinent to current global climate change, that is, decades to centuries, is much scarcer. Hence the assessment on these timescales has to be attempted by means of comprehensive, coupled carbon-cycle–climate models. A recent comparison of different model simulations for the industrial epoch (the past ~150 years) and the next 100 years, made on the basis of a standard model of CO2 emissions, has shown a variety of responses2. Almost all the models show terrestrial CO2 sequestration in the early phase of industrial expansion in the nineteenth and twentieth centuries but a substantial decrease in sequestration as the world warms (Fig. 2) (see page 297). In some models, the terrestrial carbon cycle even becomes a substantial source of atmospheric CO2 and thus strongly amplifies global climate change. The rather wide spread of results from the different model simulations demonstrates on the one hand genuine differences in the simulated climate change, and on the other hand the very poor understanding of processes in functioning ecosystems as represented in these models.
NATURE|Vol 451|17 January 2008|doi:10.1038/nature06591
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