In other words, even if all the world ran on carbon-free energy and deforestation ceased, the only way of lowering temperatures would be to devise a scheme for sucking hundreds of billions of tonnes of carbon dioxide out of the atmosphere.

Much of this week’s report, the fifth assessment of the IPCC working group on the physical science of climate change, will reaffirm the findings of the previous four assessments, published regularly since 1990.

It will point out that to limit global warming to 2 °C will require cumulative CO2 emissions from all human sources since the start of the industrial revolution to be kept below about a trillion tonnes of carbon. So far, we have emitted about half this. Current emissions are around 10.5 billion tonnes of carbon annually, and rising.