If global warming sometimes seems like a distant or abstract threat, new research casts the phenomenon in stark, life-or-death terms. It predicts that in the absence of significant progress in efforts to curb emissions of temperature-raising greenhouse gases, extreme heat waves could claim thousands of lives in major U.S. cities.

If the global average temperature rises 3 degrees Celsius (5.4 degrees Fahrenheit) above pre-industrial levels — which some scientists say is…