Obasi said it is feared the same equation would hold true with respect to heatwaves.

CNN: Global warming may 'double heat deaths'

Heatwaves, droughts, wildfires, floods - all are now more frequent and more intense.

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The annual death toll from heatwaves in those big U.S. cities is expected to balloon to 3, 000 to 4, 000 by 2020, he said.

CNN: Global warming may 'double heat deaths'

It used to be that the first heatwaves of summer produced disproportionate mortality, and then, as people got used to the heat, a similar warm spell killed fewer.

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In the 15 biggest U.S. cities an average of 1, 500 people collapse and die from heatwaves each year, a significant increase over the past decade, Obasi said, without giving previous comparative figures.

CNN: Global warming may 'double heat deaths'

We can't be sure yet which, if any, were made worse by manmade climate change, although a study from researchers in Germany suggests that heatwaves are five times more likely due to current levels of heating.

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