Why is the trillionth tonne of carbon important?

Emissions of carbon dioxide since 1750 comprise just over half a trillion tonnes of carbon, and are estimated to have caused just under 1°C of global warming. Emitting a further half trillion tonnes will commit us to a most likely warming of around 2°C, widely regarded as the threshold for the most serious global impacts of climate change. The trillionth tonne could be released in less than 40 years time, or, if we take the measures necessary to avoid dangerous climate change, it could never be released. The fate of this tonne of carbon sums up the climate challenge.

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