Since smashing into the coast of Texas on Friday as a Category 4 hurricane, tropical cyclone Harvey has submerged streets, homes and vehicles by dumping up to a metre or more of rain on some parts of the state. At least eight people have died, according to some media reports, and nearly seven million people have been affected by Harvey, the most powerful hurricane to hit Texas in more than 50 years.
It's extreme and unusual, and yet it comes just a few years after Sandy slammed the U.S. east coast in 2012, causing $75 billion US in damage, and little more than a decade after Katrina devastated Louisiana and nearby states, along with Cuba and the Bahamas, becoming the costliest storm in U.S. history at $108 billion.
You may wonder: is climate change to blame for these extreme storms, making them bigger, stronger and more frequent than before? Could nature have brewed up such powerful disasters without global warming?
'Under global warming, we should see stronger storms, especially the strongest ones.' - James Elsner, climatologist
Studies have already shown that we should expect stronger hurricanes and other tropical cyclones in the future — although perhaps fewer of them. Climate change has also been found to have played a role in some cyclones in the Pacific.
"The reason hurricanes form to begin with is there's a tremendous amount of energy stored in the ocean in the form of heat," says Bob Robichaud, a meteorologist with the Canadian Hurricane Centre in Dartmouth, N.S. "Nature doesn't like these imbalances, so what it tries to do is dissipate or re-balance some of that heat. That creates the environment for hurricanes."
A warmer climate, with warmer oceans, could provide an energy boost that creates the right conditions for big hurricanes.
Because of that, says James Elsner, a scientist who studies the relationship of hurricanes to climate factors at Florida State University, "Under global warming, we should see stronger storms, especially the strongest ones" — though perhaps not as many storms overall.
Elsner says another way that climate change may be having an effect is by raising sea levels, leaving less room for any extra water that might be poured on top by a big storm.
A warmer atmosphere can also hold more water, roughly six to seven per cent more for each degree Celsius increase in temperature.
Robichaud says that means "when we get a hurricane squeezing all the moisture out of the air, it's going to result in more rainfall."
There's some debate about how significant those factors are for a given hurricane.
Kevin Trenberth, a scientist in climate and global dynamics at the U.S. National Center for Atmospheric Research, says an increase in temperature boosts the water-holding capacity of the atmosphere, which in turn boosts the intensity of the storm. That can lead to other effects that further increase the strength of the storm in some computer modelling experiments.
"Some of the experiments that have been done suggest that the climate change effect total can be easily be up to about 30 per cent or something like that," Trenbeth tells CBC News.
But Chris Landsea, a researcher at the U.S. National Hurricane Center, says the intensity and rainfall effects are minimal compared to other factors.
He says climate models show big storms could get two to five per cent stronger and produce 10 per cent more intense rainfall by the end of the century if humans continue emitting large quantities of the greenhouse gases that drive the increase in global temperatures. He was a co-author of a 2010 report predicting that climate change will lead to fewer but stronger tropical storms.
He notes that a bigger factor in massive destruction from tropical storms in the past 15 years is the fact that the population living in coastal areas is doubling in the United States every 20 to 40 years — laying out more coastal property and infrastructure that the storms can ravage.
"Even a 10-per-cent increase in rainfall is very small in comparison."
Many of the experts who spoke to CBC News note that Harvey's most unusual quality is not how strong or intensely rainy it is, but the fact that it has "stalled" and keeps pouring water over the same, already overflowing region of Texas instead of advancing inland — something the researchers say isn't clearly linked to climate change.
"We've seen other storms that have been just as strong, but have blown through fairly quickly and have not given the same rainfall amount," Robichaud says.
Meanwhile, natural climate variability independent of climate change is likely what's boosting the number of big storms in recent years. Robichaud says that includes an Atlantic hurricane "cycle": "We're in a period of high activity right now."
That period started in 1995 and is expected to last 20 or 30 years before being followed by 20 or 30 years of less hurricane activity.
In fact, there are so many factors that contribute to a storm, he says, that "it's tough to tie any one event to climate change."