Nature World: With climate change causing much of the world's hot extremes, heat waves are likely to become more common. Well, now new research shows that, at least in the United States, exposure to extreme heat could increase four- to six-fold by 2050, due to both a warming climate and a population that's growing especially fast in the hottest regions of the country. Surprisingly, extreme heat kills more people in the United States than any other weather-related event. And with blazing heat becoming the new...