The report foresees ever-worsening damage to the planet as temperatures gain, including rising seas that could swamp low-lying Pacific island states or declining crop yields that could mean hunger for millions.
“The longer we go without action (to curb greenhouse gases) the more likely it is that some of the big feedbacks will kick in,” Richard Betts, manager of the climate impacts research team at the British Met Office.
“We can make a big difference by either choosing a low emissions scenario or a high emissions scenario,” said Gunnar Myhre, of the Centre for International Climate and Environmental Research in Oslo.
In the scenarios, the biggest temperature gain comes if the world stays dependent on fossil fuels, with 70% of energy in 2100 from sources such as coal and gas, and sharply raises greenhouse gas emissions.
The scenario with the smallest temperature gain assumes that carbon emissions will dip by 2100 by when the world will get about half its energy from renewable sources.
The draft report will build on the first part and lay out the regional impacts of climate change, such as a drying of the Amazon basin or a sharp contraction of vast Himalayan glaciers that feed rivers in Asia.