Scientists Update Climate Scenarios, Narrow Range of Future Warming

Scientists preparing the next series of United Nations climate reports have replaced two long-used future scenarios with seven new ones. The update removes the highest and lowest emissions pathways that were previously considered plausible. The previous highest scenario assumed heavy reliance on coal and projected about 4.5 degrees Celsius of warming by 2100.

The new highest scenario projects roughly 3.5 degrees Celsius of warming by the end of the century. The previous lowest scenario assumed rapid emissions cuts that would have kept warming near 1.5 degrees Celsius. The new lowest scenario projects a peak near 1.7 degrees Celsius before a possible later decline.

Scientists said the world is currently following a middle scenario that would produce about 3 degrees Celsius of warming by 2100. The planet is already about 1.3 degrees Celsius warmer than pre-industrial levels. Even small additional increases in temperature are expected to intensify extreme weather, reduce fresh water availability, and increase species loss.

The new scenarios focus only on emissions from fossil fuel burning. They do not include potential additional warming from natural climate feedbacks such as carbon release from oceans and forests. Nine of ten scientists interviewed said it is now too late to avoid passing the 1.5 degree Celsius threshold set in 2015.

They said any later return below that level would require large-scale carbon removal technology that does not yet exist. "There is kind of a narrowing of the futures. It cannot be as bad as we thought, but it cannot be as good as we hoped," said Johan Rockström, director of the Potsdam Institute for Climate Impact Research in Germany.