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Recent studies show that for a particular choice of climate stabilization temperature, there would be only a certain range of allowable cumulative carbon emissions. Humans have emitted a total of about 500 billion tons (gigatonnes) of carbon emissions to date. The error bars account for estimated uncertainties in both the carbon cycle (how fast CO2 will be taken up by the oceans) and in the climate responses to CO2 emissions. Source: National Research Council, 2011a
Source: National Research Council. 2011. Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia. p.101. Washington, DC: The National Academies Press.