In August 2017, Hurricane Harvey dumped more than four feet of rain on Houston, Texas, in a matter of days, sending unprecedented floods through one of the largest cities in the U.S.
In the deluge’s aftermath, climate scientists noted that storms like Harvey are rare—but cautioned that unusually warm waters, made likelier by human activity, may have supercharged the hurricane’s extreme rainfall. Now, two separate teams of scientists have found humans’ fingerprints all over the storm.
One research team’s results, accepted for publication in Geophysical Research Letters (GRL), found that in comparison to a typical 1950s hurricane, climate change likely increased Harvey’s seven-day rainfall by at least 19 percent. A separate study, published today in Environmental Research Letters (ERL), found similar results, showing that climate change boosted Harvey’s three-day rainfall by about 15 percent.
Both studies also found that climate change roughly tripled the odds of a Harvey-type storm.
“It is not news that climate change affects extreme precipitation, but our results indicate that the amount is larger than expected,” said Michael Wehner, a climate scientist at the Lawrence Berkeley National Laboratory who coauthored the GRL study, in a press release.
Both studies were announced at the American Geophysical Union’s fall meeting in New Orleans, Louisiana.
Loading the Dice
Just as a gambler can’t predict how a single roll of the dice will go, scientists can’t determine whether climate change definitely caused any one weather event. However, in the same way that casinos might catch a gambler using loaded dice, scientists can quantify weather events’ natural odds and see how today’s weather unnaturally differs.
In Wehner’s case, he and his colleague Mark Risser examined data from Houston-area weather stations between 1950 and 2016, to determine the area’s heavy rainfall events. They then coupled these data to climate records, accounting for CO2 levels and El Niño, and compared Harvey against these trends.
When stacked against a 1950s-style hurricane, climate change boosted Harvey’s rainfall by as much as 38 percent, and it may have made Harvey roughly three times likelier.
The ERL study takes a different approach but comes to similar conclusions. Using U.S. Gulf Coast rainfall records going back to 1880, researchers found that rare, extreme rainfall events have gotten more intense. The effect is no mere fluke: The team’s climate models show that warming from human activity likely drove the uptick.
When the researchers, members of the World Weather Attribution group, turned their gaze to Harvey, it found that climate change made a Harvey-like storm in the Gulf Coast anywhere from 1.5 to five times likelier. What’s more, the rainfall in such a storm increased as much as 19 percent.
Both teams were surprised by this strong signal. In the industrial era, the Gulf of Mexico has seen one degree Celsius (1.8 degrees Fahrenheit) of warming, in line with global averages. Generally, this warming makes the air above a body of water about six to seven percent wetter—far less than the boost Harvey likely received.
Human Impact on Our Changing Planet
The announcements are the latest results pouring out of the American Geophysical Union’s ongoing meeting that underpin scientific consensus: that humans are profoundly reshaping Earth’s climate.
At a press conference on Tuesday, the U.S. National Oceanic and Atmospheric Administration released its latest Arctic Report Card and announced that today’s extent and rate of melting has no precedent in the last 1,500 years.
On Wednesday, scientists also announced that Arctic warming threatens the 1,300-mile Alaskan Highway—a quarter of which is built over permafrost—and that in the last four decades, New Zealand’s glaciers have shrunk by more than 30 percent.
World leaders aim to hold Earth’s warming by 2100 to two degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial temperatures, despite the U.S. federal government’s recent climate reversals under President Donald Trump.
However, even if humankind hits the two-degree target, future generations will see Harveys of their own. The ERL study estimates that by the end of the century, storms with extreme rainfall like Harvey’s will become three times likelier than they are today.
However, if we stay on a “business-as-usual” path, relying on fossil fuels as we do today, Harvey-like hurricanes in 2100 could have rainfall boosted by as much as 50 percent.
“If we miss those targets, the increase in frequency and intensity could be much higher,” said ERL study coauthor Karin van der Wiel, a postdoctoral researcher at the Royal Netherlands Meteorological Institute, in a press release.