Sir David King said that even the most optimistic projections suggested that carbon dioxide levels would reach double their pre-industrial levels, bringing temperatures 3C or more warmer than today.

Without action to mitigate the effects of this global warming, up to 400 million people would be at risk from starvation, as between 200 million and 400 million tonnes of cereal production were lost, he said. “If you ask me where do we feel the temperature is likely to end up if we move to a level of carbon dioxide of 500 parts per million — which is roughly twice the pre-industrial level and the level at which we would be optimistically hoping we could settle — the temperature rise could be in excess of 3C and yet we are saying 500 parts per million in the atmosphere is probably the best we can achieve through global agreement,” he told the Today programme on BBC Radio 4.

He said it was essential that the world started preparing now. “We don’t have to succumb to a state of despondency where we say that there is nothing we can do, so let’s just carry on living as per usual. It is very important to understand that we can manage the risks to our population.

“What we are talking about here is something that will play through over decades — we are talking 100 years or so. We need to begin that process of investment. It is going to be a major challenge for the developing countries.”

Sir David was scathing about politicians who believed that they could simply rely on new technologies to produce cleaner fuels and said that they must start listening to the scientists.

“What we have to do as we move forward with these discussions is see that this consensus position of the scientific community is brought right where the discussions are taking place.”

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