As the planet rapidly warms towards this grim scenario-and beyond it, if no action is taken-the poorest countries are already facing the largest damages from extreme weather events, the report notes.

On the other hand, "we can't prove there is no fifth planet." No one yet has the required precision in their observations to prove the absence of such a small exoplanet, he notes.

Lewis notes that several of the computer models that calculate the effects of climate change do predict that parts of the planet are going to get drier.