Nearly a third of the world's population is exposed to lethal climate conditions in the form of deadly heatwaves for at least 20 days a year, and the risk is expected to cover 48 percent of the world's population by 2100 even if carbon gas emissions are drastically reduced.

"An increasing threat to human life from excess heat now seems almost inevitable," wrote the authors of a study in Nature Climate Change, a monthly peer-reviewed journal.  It says if current greenhouse gas emissions continue at the current rate, the risk widens to 74 percent of the world's population. 

"For heat waves, our options are now between bad or terrible," said Camilo Mora, an associate professor at the University of Hawaii and lead author of the study.

Mora and the researchers looked at 1,900 heatwave deaths in 36 countries over the past four decades.  This includes the 1995 scorcher that claimed hundreds of lives in Chicago in 1995, the European heat wave in 2003 that saw tens of thousands of heat-related deaths and lethal temperatures in Moscow in 2010 that killed more than 10,000.  

They determined that high humidity levels combined with heat play a major role in heat-related heath risks.  Human sweat doesn't evaporate in such conditions, making it difficult for people to regulate and release heat.

"People can then suffer heat toxicity, which is like sunburn on the inside of your body," Mora said.  "The blood rushes to the skin to cool you down so there's less blood going to the organs.  A common killer is when the lining of your gut breaks down and leaks toxins into the rest of your body."

High temperatures are currently baking large swaths of the southwestern US, with Phoenix, Arizona's high temperature bursting thermometers at 48.3 C degrees for two consecutive days.  And it's only June, the hot season in the desert tends to be in late July through early September.

"The impact of global climate change is not a specter on the horizon.  It's real, and it's being felt now all over the planet," said Amir AghaKouchak, UCI associate professor and co-author of another heatwave study from University of California, Irvine this month .  "It's particularly alarming that the adverse effects are pummeling the world’s most vulnerable populations."