Burning issue (Image: Plainpicture/Design Pics)

Global warming is irreversible without massive geoengineering of the atmosphere’s chemistry. This stark warning comes from the draft summary of the latest climate assessment by the Intergovernmental Panel on Climate Change.

Delegates from national governments are discussing the draft this week, prior to its release on Friday morning.

According to one of its lead authors, and the latest draft seen by New Scientist, the report will say: “CO 2 -induced warming is projected to remain approximately constant for many centuries following a complete cessation of emission. A large fraction of climate change is thus irreversible on a human timescale, except if net anthropogenic CO 2 emissions were strongly negative over a sustained period.”


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 CO 2 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.

Since the last assessment, published in 2007, the IPCC has almost doubled its estimate of the maximum sea-level rise likely in the coming century to about 1 metre. They also conclude that it is now “virtually certain” that sea levels will continue to rise for many centuries, even if warming ceases, due to the delayed effects of thermal expansion of warming oceans and melting ice sheets.

The draft report says the available evidence now suggests that above a certain threshold of warming, the Greenland ice sheet will almost disappear within approximately 1000 years, which will result in 7 metres of global sea-level rise. It estimates that the threshold may lie between 1 °C and 4 °C of warming, but is not confident of this figure.