This new epidemic of extreme heat represents one of the gravest threats to humanity, scientists say, but it won’t affect the world in a uniform way. While certain parts of rich countries will see a surge in days, most of the danger will come in poor countries in already hot regions such as South Asia and sub-Saharan Africa that lack widespread air conditioning and other advantages like advanced health-care systems. “The resources just look vastly different,” said Tamma Carleton, an assistant professor of environmental economics at the University of California at Santa Barbara and with the Climate Impact Lab. “The story of heat is inequality.”