Ice core data show a strong correlation between atmospheric CO2 and global temperature over the glacial cycles, indicating that the climate system is closely coupled to the carbon cycle. During the four last glacial cycles, over the past 420,000 years, atmospheric CO2 had excursions from 200 ppmv during cold glacial periods, up to 280 ppmv during warm interglacial periods (Petit et al. 1999) (see fig. 1a). Other trace gases such as methane also show a strong temporal coupling with temperature over glacial cycles (Petit et al. 1999). Over the more recent history, atmospheric CO2 has been recorded to increase from roughly 280 ppmv at the dawn of the industrial revolution, up the 360 ppmv today (see fig. lb) (Etheridge et al. 1996). This sharp increase is due to a) the burning of fossil fuel for energy production (Andres et al. 1996) and b) the intense deforestation, essentially in the tropics, needed to meet the increasing food and fibre demand (Houghton 1995). Many forests have been cut or degraded, and today, a large fraction of the terrestrial ecosystems is directly influenced by human activities. Land use over the past 200 years has caused terrestrial ecosystems to release carbon (mainly to the atmosphere).
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