High humidity will magnify the effects of rising heat from South America to India, affecting people's ability to work and even survive, unless greenhouse gas emissions are substantially cut in coming decades, according to U.S. researchers.

Heat remains underestimated as a threat by governments, aid agencies and individuals, and muggy heat is even more oppressive than the "dry" kind, because it stops people from sweating which takes away excess heat.

A study from the Earth Institute at Columbia University found areas along the coast and others that experience humid-weather patterns will be most affected by higher temperatures unless governments curb greenhouse gas emissions that can raise temperatures and put in place measures to tackle the heat.

The areas likely to be affected include the Amazon, southeastern United States, western and central Africa, parts of the Middle East, northern India and eastern China.