NEW YORK • How much does Hurricane Harvey, or any particular storm, have to do with climate change? The relationship between hurricanes and climate change is not simple. Some things are known with growing certainty. Others, not so much.

The most recent draft of a sweeping climate science report pulled together by 13 federal agencies as part of the National Climate Assessment suggested that the science linking hurricanes to climate change was still emerging. Looking back through the history of storms, "the trend signal has not yet had time to rise above the background variability of natural processes", the report stated.

Temperatures have been rising, and theory and computer modelling suggest an increase in storm intensity in a warmer world. And the models "generally show an increase in the number of very intense" storms.

While the science of attributing weather events to climate change is advancing, "studies of individual events will typically contain caveats", the report stated.

Climate scientist Katharine Hayhoe at Texas Tech University, an author of the report, said that even if global warming does not change the number of storms - and, she noted, there could even be fewer hurricanes overall - tropical storms and hurricanes do gain energy from warm water, so the unusually warm water that has accompanied climate change could "have a role in intensifying a storm that already exists".

More moisture in the atmosphere, she said, means the amount and intensity of rain associated with hurricanes and other storms is growing.

Ms Hayhoe noted that scientists are not saying that hurricanes are necessarily caused by climate change, but that they are being affected by them.

"We care about a changing climate because it exacerbates the natural risks and hazards that we already face," she said. "People always want to know, is it climate change or is it not? The answer is it's in between."

NYTIMES