Terrestrial Carbon Cycle Variability.

For perspective, atmospheric CO 2 increased from about 180 to 280 ppm since the last glacial period, adding about 220 Pg-C to the atmosphere over a 10,000-year period 6 ; this pre-industrial change was associated with a positive trend in the net global carbon exchange rate of about 20 Tg-C y -1 .

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It remains controversial whether or not there is a significant trend in global primary productivity (due to rising CO 2 , temperature, nitrogen deposition, changing land use, and preponderance of wet and dry regions).

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