The global public health emergency involving deformed babies emerged in 2015, the hottest year in the historical record, with an outbreak in Brazil of a disease transmitted by heat-loving mosquitoes. Can that be a coincidence?

Scientists say it will take them years to figure that out, and pointed to other factors that may have played a larger role in starting the crisis. But these same experts added that the Zika epidemic, as well as the related spread of a disease called dengue that is sickening as many as 100 million people a year and killing thousands, should be interpreted as warnings.

Over the coming decades, global warming is likely to increase the range and speed the life cycle of the particular mosquitoes carrying these viruses, encouraging their spread deeper into temperate countries like the United States.

Recent research suggests that under a worst-case scenario, involving continued high global emissions coupled with fast population growth, the number of people exposed to the principal mosquito could more than double, to as many as 8 billion or 9 billion by late this century from roughly 4 billion today.