Feedbacks in the biosphere

As the greenhouse gases carbon dioxide and methane are added to the atmosphere because of human activities, biological or other feedback processes occurring in the biosphere (such as those arising from the climate change that has been induced) influence the rate of increase of the atmospheric concentration of these gases. These processes will tend either to add to the anthropogenic increase (positive feedbacks) or to subtract from it (negative feedbacks).

Two feedbacks, one positive (the plankton multiplier in the ocean) and one negative (carbon dioxide fertilisation), have already been mentioned in the text. Four other positive feedbacks are potentially important, although our knowledge is currently insufficient to quantify them precisely.

One is the effect of higher temperatures on respiration, especially through microbes in soils, leading to increased carbon dioxide emissions. Evidence regarding the magnitude of this effect has come from studies

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■ Emissions CO2 changes Land uptake

■ Emissions CO2 changes Land uptake

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Figure 3.5 The possible effects of climate feedbacks on the carbon cycle. Shown are (1) accumulated fossil fuel CO2 emissions from 1860 to the present and then projected to 2100 assuming the A2 SRES scenario (Figure 6.1) - in red (2) CO2 from (1) absorbed into the ocean - in blue, (3) CO2 taken up by the land (of the same sign as the other contributions but plotted below the axis for clarity) - in orange and (4) the residual CO2 from (1) added to the atmosphere - in green. The nine different ocean and land budgets resulted from a study with nine coupled atmosphere-ocean general circulation climate models (AOGCMs - see chapter 5) organised internationally as part of a climate model intercomparison project (CMIP). (a) Shows results assuming no feedbacks from climate change into elements of the carbon cycle. (b) Shows results when climate feedbacks into the carbon cycle are included.

Emissions CO2 changes Land uptake Ocean uptake

Emissions CO2 changes Land uptake Ocean uptake

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1950 2000 Year

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Figure 3.5 The possible effects of climate feedbacks on the carbon cycle. Shown are (1) accumulated fossil fuel CO2 emissions from 1860 to the present and then projected to 2100 assuming the A2 SRES scenario (Figure 6.1) - in red (2) CO2 from (1) absorbed into the ocean - in blue, (3) CO2 taken up by the land (of the same sign as the other contributions but plotted below the axis for clarity) - in orange and (4) the residual CO2 from (1) added to the atmosphere - in green. The nine different ocean and land budgets resulted from a study with nine coupled atmosphere-ocean general circulation climate models (AOGCMs - see chapter 5) organised internationally as part of a climate model intercomparison project (CMIP). (a) Shows results assuming no feedbacks from climate change into elements of the carbon cycle. (b) Shows results when climate feedbacks into the carbon cycle are included.

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of the short-term variations of atmospheric carbon dioxide that have occurred during El Niño events and during the cooler period following the Pinatubo volcanic eruption in 1991. These studies, which covered variations over a few years, indicate a relation such that a change of 5 °C in average temperature leads to a 40% change in global average respiration rate6 - a substantial effect. A question that needs to be resolved is whether this relation still holds over longer-term changes of the order of several decades to a century. A second positive feedback is the reduction of growth or the dieback especially in forests because of the stress caused by climate change, which may be particularly severe in Amazonia (see box in Chapter 7 on page 208).7 As with the last effect, this will increase as the amount of climate change becomes larger. The combined result of these two feedbacks is that less carbon is taken up by the biosphere and more remains in the atmosphere.8

Figure 3.5 shows this combined result as estimated for the twenty-first century by nine different climate models that incorporate the relevant processes in both the ocean and the land biospheres. Note that one of the models in the ensemble, from the Hadley Centre, predicts the strongest values for the climate/carbon-cycle feedbacks mentioned above and projects the highest atmospheric carbon dioxide level by 2100.9 The curve in Figure 3.5 for land uptake relating to this model begins to curve upwards from the middle of the century at which time the terrestrial biosphere changes from being a net sink of carbon (as in Table 3.1) to being a net source.

Taking the average of the nine models under the A2 scenario, about 50 Gt more carbon remains in the atmosphere in 2050 and 150 Gt in 2100 compared with what would occur in the absence of climate/ carbon-cycle feedback. For the Hadley Centre model the numbers are about 50 Gt and 350 Gt respectively for 2050 and 2100. In terms of carbon dioxide concentration an additional 100 Gt means an additional 50 ppm.

A third positive feedback occurs through the release of greenhouse gases into the atmosphere due to the increase of fires in forested areas because of the drier conditions as climate warms or because of the dieback due to climate stress mentioned above10.

The fourth positive feedback is the release of methane, as temperatures increase - from wetlands and from very large reservoirs of methane trapped in sediments in a hydrate form (tied to water molecules when under pressure) - mostly at high latitudes. Methane has been generated from the decomposition of organic matter present in these sediments over many millions of years. Because of the depth of the sediments this latter feedback is unlikely to become operative to a significant extent in the near future. However, were global warming to continue to increase unchecked for many decades, releases from hydrates could make a large contribution to methane emissions into the atmosphere and act as a large positive feedback on the climate.

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