Rising temperatures and humidity will make the world's tropics increasingly unliveable by pushing more people to the thresholds of their physical tolerance and beyond, a new international study finds.

As of 2000, about 30 per cent of the world's population lived in regions where the climate exceeds deadly threshold levels – based on temperature and relatively humidity levels – for at least 20 days a year, researchers publishing in the Nature Climate Change journal estimate.

Even with the most optimistic scenario for greenhouse gas emissions reductions, that share will rise to about 48 per cent by the end of the century. If so-called business as usual emissions continue, that share would climb to 74 per cent by then, the paper found.

"You are going to have all of those people in the tropics 'cooking' there because they are not going to have any possibility to cope with this [increase in heat and humidity]," said Camilo Mora, the paper's lead author and an associate professor in the Department of Geography at the University of Hawaii, Manoa.