Carbon emissions: Deciding how fast and how soon to cut

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Scientists estimate that to avoid the more damaging impacts of climate change, the total amount of extra carbon put into the atmosphere would have to be limited to 1 trillion tonnes. Human activities have already emitted half this amount. Staying within the limit would involve reducing global emissions over time, so that no more than another 500 billion tonnes of carbon are emitted in total. If global emissions are reduced soon, the rate of reduction can be more gradual. But if emissions continue to rise, faster reductions would be necessary later on in order to stay within the trillion-tonne limit, which would be more challenging both economically and technologically.