Respiration, both on land and in the sea, is a key component of the global carbon cycle. On land, an estimated 60 Pg C/year is emitted to the atmosphere by autotrophic respiration. A similar amount, ~55 Pg C, is emitted as a result of heterotrophic respiration.

In the sea, autotrophic respiration is thought to account for ~58 Pg C/year of the dissolved inorganic carbon in surface waters, with the contribution of heterotrophic respiration being 34 Pg C. Most of the input of dissolved organic carbon (DOC) to oceans from rivers (~0.8 Pg C/year) is respired and released to the atmosphere. Physical air-sea exchange of this dissolved CO2 leads to an emission to the atmosphere of ~88 Pg C/year.

Although the worldwide human population has now grown to more than 6 billion,

Fig. 1.2. Main components of the natural carbon cycle. (Reproduced with permission from the Intergovernmental Panel on Climate Change (IPCC).)

our direct contribution to atmospheric CO2 concentrations via respiration is relatively insignificant. Of greater concern is the impact human-induced global warming could have on global respiration rates. As temperatures increase, rates of respiration also rise in many organisms; most microbes, for instance, double their rate of respiration with every 10°C increase in temperature. There exists therefore a danger of warming, inducing further increases in atmospheric CO2 concentrations, and thus more warming - an example of a positive feedback.

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