Studies: Warming made Harvey’s deluge 3 times more likely

Researchers told the annual meeting of the American Geophysical Union here in New Orleans that the heavy rainfall seen in Harvey was very likely exacerbated by the extra warming associated with increased greenhouse gases in the atmosphere

New Orleans: Global warming’s fingerprints were all over the record rainfall from Hurricane Harvey this year, confirming what scientists suspected, according to new research. Scientists have weighed the water that fell on Texas during the record-breaking Hurricane Harvey in August.

They calculate, by measuring how much the Earth was compressed, that the Category 4 storm dropped 127 billion tonnes, or 34 trillion gallons.

“One person asked me how many stadia is that. It’s 26,000 New Orleans Superdomes,” said Adrian Borsa from the Scripps Institution of Oceanography, according to BBC.

His numbers were released as other scientists stated that this year’s big hurricanes had a clear human influence.

Harvey, Irma and Maria ripped through the US Gulf states and the Caribbean, leading to widespread flooding and wind damage.

Researchers told the annual meeting of the American Geophysical Union here in New Orleans that the heavy rainfall seen in Harvey was very likely exacerbated by the extra warming associated with increased greenhouse gases in the atmosphere.

Sea surface temperatures were particularly high in the Atlantic and the Gulf of Mexico this hurricane season. Warm ocean water acts as a fuel for the storms.

While scientists say man-made climate change didn’t trigger Harvey, new studies calculate that a warmer, wetter world made it at least three times more likely that the stalled storm over Houston would flood the fourth most populous US city. Researchers also said global warming often goosed aspects of two other destructive hurricanes this year, Irma and Maria.

Findings were discussed Wednesday at the American Geophysical Union meeting in New Orleans.

The Harvey studies used a combination of established and new techniques. An international team used computer simulations and decades of past observations to estimate the odds for the record rainfall that came with Harvey over a period of three days in August with and without global warming. By comparing those two, they concluded global warming tripled the likelihood for the deluge, which reached more than 127cm in one location.

Based on Houston’s weather history, researchers said a storm like Harvey would occur about once every 9,000 years.

“Did climate change make this event more likely than in the past? Yes,” said Karin van der Wiel of the Royal Netherlands Meteorological Institute.

Another study by the Lawrence Berkeley National Laboratory used computer simulations to analyse Harvey’s downpours over a week and calculated that global warming increased rainfall by nearly 20 per cent.

Kevin Trenberth of the National Center for Atmospheric Research linked Harvey’s downpours to the heat content in the ocean. Most of the heat trapped by greenhouse gases gets absorbed by oceans and that energy serves as fuel for hurricanes and other storms. The ocean heat content was record high in the Gulf of Mexico and elsewhere before Harvey hit. If it had been lower, there would have been much less rain, he said.

Changes in the jet stream — the rivers of air that steer weather — led to Harvey’s stalling over Houston, unleashing rain, said Kerry Emanuel of the Massachusetts Institute of Technology who presented two studies.

While Emanuel wouldn’t directly blame climate change on Harvey, he said destructive hurricanes will be more likely in a warmer world.

“I think this is a window into the future. I think nature is giving us a foretaste of the future,” Emanuel said in an interview.

Definitive change

Meanwhile another team of scientists released a blockbuster report on extreme weather in 2016, saying that for the first time they could declare that three separate weather events — the weirdly warm “blob” of water off the Alaska coast, a heat wave in Asia and the highest-ever global temperature — would have been impossible without human-caused climate change.

“This is the first time we’ve ever had statements like that,” said Stephanie Herring, a climate scientist with the National Oceanic and Atmospheric Administration who spoke here at the fall meeting of the American Geophysical Union Wednesday. And about that “blob”: “The blob is an ongoing phenomenon. It’s still sitting there.”

“Attribution” research, as it’s known, seeks to find and quantify the influence of climate change on a weather event, which has always been problematic. There’s a truism: Climate is what you expect and weather is what you get. Weather events emerge from chaotic forces and elements, and there is variability from place to place and year to year.

The result has been an ongoing issue for scientists studying extreme weather and journalists reporting on the subject. Definitive statements about causality, or the magnitude of an effect, are hard to come by. The discussion gets mired in caveats, because extreme events can happen with or without a changed climate.

That’s changing.

There’s a buy-more-sandbags message lurking amid the sessions here at the AGU meeting. It’s a sprawling science meeting, physically and intellectually: As of Wednesday morning 22,500 people had registered and more are coming in, including experts on volcanoes, earthquakes, glaciers, the atmosphere, Mars, Jupiter and so on.

When a keg exploded Tuesday and shot a geyser of beer 20 feet high in the poster hall, a voice called out, “Can someone model that?”

Extreme weather is a familiar topic here in the Crescent City. At one panel Wednesday a city planner warned that the flood control infrastructure is nowhere near adequate for the perils ahead. Many scientists have urged that the government improve flood maps — they’re out of date and no longer capture the new reality of the warmer world.

“They have not been improving the maps as they should. They’re treating that as static,” said Columbia University research professor Suzana Camargo, an expert on extreme weather.

And flood maps are just maps, by the way: “I’ve never met a molecule of floodwater that could read a flood map,” said George Homewood, a planning director for the city of Norfolk.

Astonishing year

The meeting had numerous sessions Wednesday devoted to late-breaking research on hurricanes Harvey, Irma and Maria. Scientific research usually takes longer to cohere, but 2017 was an astonishing year of natural disasters and many people dropped what they were doing to tease out early findings about the hurricanes and other tumult, including western US wildfires.

Hurricane Harvey hit the Texas coast near Corpus Christi on August 25 after it intensified rapidly in the Gulf of Mexico. The storm then stalled and dropped record rains for the better part of a week on southeast Texas before finally drifting north and dissipating. The storm flooded Houston and much of the region and was one of several hurricanes that slammed the United States during a volatile 2017 season, including Hurricane Irma in Florida and Hurricane Maria in Puerto Rico.

“Climate change made this event more likely and heavier,” said Karin van der Wiel, a researcher at the Royal Netherlands Meteorological Institute who co-wrote one of the papers published Wednesday.

There are uncertainties here — the boost in rainfall could have been somewhere between 8 and 19 per cent, according to the scientists based in the Netherlands. That 19 per cent figure is at the lower end of the range calculated by the scientists from the Lawrence Berkeley National Laboratory. They said their best estimate was 37 per cent but cautioned that this is science done on the fly after a major natural disaster.

“There’s a clear human fingerprint. The numbers will undoubtedly change as more researchers look at this with different techniques, and perhaps different data sets and different methods. But our numbers are kind of big.” He added: “We were stunned.”

The teams worked independently and used different methods — for example, examining different geographical areas, different time periods during the week that Harvey struck Texas, and framing their findings with different standards of certainty. Though their numbers are not identical, the scientists on the two teams emphasised that each study bolsters the other, with strikingly similar conclusions and lessons for the future.

“We have two independent efforts with essentially the same answer,” said Wehner.


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