Early in his career, a scientist named Mario J. Molina was pulled into seemingly obscure research about strange chemicals being spewed into the atmosphere. Within a year, he had helped discover a global environmental emergency, work that would ultimately win a Nobel Prize.

Now, at 70, Dr. Molina is trying to awaken the public to an even bigger risk. He spearheaded a committee of the American Association for the Advancement of Science, the world’s largest general scientific society, which released a stark report Tuesday on global warming.

The report warns that the effects of human emissions of heat-trapping gases are already being felt, that the ultimate consequences could be dire, and that the window to do something about it is closing.

“The evidence is overwhelming: Levels of greenhouse gases in the atmosphere are rising,” says the report. “Temperatures are going up. Springs are arriving earlier. Ice sheets are melting. Sea level is rising. The patterns of rainfall and drought are changing. Heat waves are getting worse, as is extreme precipitation. The oceans are acidifying.”

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