NEW ORLEANS — Climate change made the torrential rains that flooded Houston after Hurricane Harvey last summer much worse, scientists reported Wednesday.

Two research groups found that the record rainfall as Harvey stalled over Texas in late August, which totalled more than 127 centimetres in some areas, was as much as 38 per cent higher than would be expected in a world that was not warming.

While many scientists had said at the time that Harvey was probably affected by climate change, because warmer air holds more moisture, the size of the increase surprised some.

“The amount of precipitation increase is worse than I expected,” said Michael J. Wehner, a senior scientist at Lawrence Berkeley National Laboratory and an author of a paper on his group’s findings, which included the 38 per cent figure. Based on how much the world has warmed, Wehner said, before the analysis he had expected an increase of only about 6 or 7 per cent.

The other study, by an international coalition of scientists known as World Weather Attribution, found that Harvey’s rainfall was 15 per cent higher than would be expected without climate change. Geert Jan van Oldenborgh of the Royal Netherlands Meteorological Institute and the lead author of the second study, said that climate change also made such an extreme rainstorm much more likely.

“The probability of such an event has increased by roughly a factor of three,” he said. While the likelihood of a Harvey-like storm was perhaps once in every 3,000 years in the past, he said, now it’s once every 1,000 years or so — which means that in any given year, there is 0.1 per cent chance of a similar storm occurring along the Gulf Coast.

Harvey developed in the Gulf of Mexico and made landfall near Corpus Christi, Texas, as a strong Category 4 hurricane. By the time it reached Houston it had weakened to a tropical storm, but it moved slowly over the region, rotating and picking up more moisture from the Gulf of Mexico. Thousands of homes and businesses in the region were flooded and more than 80 people died.

David Titley, a meteorologist at Pennsylvania State University who was not involved with either study, said the research showed that “while a storm of Harvey’s strength is still rare, it’s not as rare as it once was.

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“Communities all along the Gulf Coast need to adapt to a world where the heaviest rains are more than we have ever seen,” he added.

Aerial footage shows flooding in Port Arthur, Kountze, Beaumont and Sabine Pass, Texas after days of heavy rain from Harvey. (The Associated Press)

Antonia Sebastian, a researcher at Rice University and a co-author of the World Weather Attribution paper, said that Harvey was a much larger event than governments and developers normally plan and build for.

“What we see from this study is that the flood hazard zone isn’t stationary,” Sebastian said. “Precipitation is changing, and that’s changing the boundaries. That should be considered.”

In August, a week before Hurricane Harvey, U.S. President Donald Trump rolled back an Obama-era executive order that included climate change and sea-level rise in federal flood risk standards.

Both studies were released during the annual meeting here of the American Geophysical Union, a large gathering of leading climate researchers and other earth scientists.

The studies only looked at the impact of climate change on rainfall, not whether warming affected Harvey’s formation or strength. Those issues remain a subject of much debate among scientists, with some researchers suggesting that strong hurricanes — Category 4 and above — will become more frequent as the world continues to warm.

Teasing out the influence of climate change on hurricanes remains extremely problematic, van Oldenborgh said.

“The effect of climate change on hurricanes is horribly complicated,” he said. “We’re working on it, but it’s very difficult.”

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But the more limited analysis, determining the influence of warming on the rainfall of a huge storm like Harvey “turns out to be a solvable problem,” van Oldenborgh said.

The studies are the latest in a series of analyses that search for the fingerprints of climate change on individual weather events like storms or heat waves. Despite overwhelming evidence that the climate is changing overall because of greenhouse gases emissions, for years most scientists had said it was extremely difficult to link warming to specific events.

That has now changed, with studies in recent years that found that climate change affected Australian heat waves in 2013, downpours in Louisiana in 2016, floods in France that same year and many other events. In some cases — German floods around the same time as the French ones, for example — studies have been inconclusive or found no link to climate change.

Although there were some differences, the two current studies employed the same basic approach — making use of actual data from the storm, and comparing two sets of climate models, those that take into account existing conditions, in which rising carbon dioxide has warmed the planet, and those that assume CO2 emissions had never happened and the climate is as it was more than a century ago.

Titley, who served as chairman of a National Academies committee that looked at developments in the field of climate-change attribution, said that both studies were “carefully done and combine observations with the latest simulation techniques.”

Wehner’s study, in particular, raises the issue of whether climate change might have contributed to the slow motion of Harvey, a subject that Titley said was worthy of further research.

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