Yet he says the temperature rise likely would be enough to spur killer heat waves in highly populated regions of the planet.
"The health and economic impacts of such warming would be vast and unprecedented in human history,” he says.
Allen agrees with the authors of the new study that their work bolsters the case for a "carbon budget" approach to climate policy: setting a cap on cumulative emissions. In contrast, the current approach of reducing emission rates simply defers dangerous warming.
A carbon budget approach implies that carbon emissions must reach zero—either by "keeping it in the ground," as some activists put it, or by capturing carbon or storing it.
The Paris Accord avoided mention of a carbon budget, but Allen says the science should point policy in that direction.
"The first thing we need to do to get to zero [emissions] is acknowledge we need to get there," he says.