The rain was still falling in Texas on Tuesday. But the question of whether human-caused climate change might have contributed to what is now the heaviest five-day rain in the history of the contiguous United States was already seeing broad circulation.
In discussing the Texas deluge, Martin Hoerling, a meteorologist at Boulder's National Oceanic and Atmospheric Administration, pointed by way of analogy to a study made on torrential rains that dumped 25 inches in southern Louisiana in August 2016.
Human-caused climate warming increased the intensity of the torrential Louisiana rains by at least 10 percent, according to a team of NOAA and partner scientists with World Weather Attribution. The same report stated that the likelihood of such an event had been raised 40 percent by global warming.
"They had rains of 25 inches or so, so 10 percent of that is 2 ½ inches" contributed by climate change, Hoerling said. "If you apply that to the Texas storm this year, you might be saying 4 inches of the 40 was related to warmer, more moist air from global warming. That is not definitive, though. That is sort of speculative at this time."
World Weather Attribution is considering whether to conduct a similar "attribution analysis" of Tropical Storm Harvey once all rainfall figures are recorded.
The unofficial five-day total from Harvey in some Texas locations reached 51 inches on Tuesday, Hoerling said. That easily tops the previous five-day record for the contiguous U.S., which was 48 inches from Tropical Storm Amelia in Medina, Texas, July into August 1978.
More intense, bigger, longer-lasting
Kevin Trenberth, distinguished senior scientist at Boulder's National Center for Atmospheric Research, has been examining this issue for a number of years and has been studying the Texas event in some detail in recent days.
He focused on sea surface temperatures that were running about 1 degree Fahrenheit above normal, which he said has "baked in" about another 1 degree from climate change, bringing sea surface temperatures above 86 degrees, providing "tons of warm water to fuel storms."
With climate change making for higher upper ocean temperatures, sea surface temperatures stay warmer even as the water is churned up by the hurricane.
"That is where the global warming plays a role in continuing to fuel the warm waters and the evaporation of moisture into the atmosphere that leads to the huge precipitation and flooding risk," Trenberth wrote in a brief assessment completed Monday.
He believes a 5 to 15 percent increase in rainfall from Harvey can be linked to climate change. Furthermore, he said a natural variability component could double those figures, and "nonlinearities" might possibly double those numbers yet again.
Nonlinearities, he said, refer to the fact that water-holding capacity of the atmosphere goes up exponentially, about 4 percent per degree Fahrenheit. As values go higher, that percentage remains the same, but it is a percentage of a higher number.
"So the storm is a bit more intense, bigger and longer lasting than it otherwise would be," Trenberth wrote.
The discussion has gone well beyond the Boulder climate science community.
Michael Mann, distinguished professor of atmospheric science at Pennsylvania State University, made a similar case in a piece for The Guardian, " It's a fact: climate change made Hurricane Harvey more deadly." Human-caused warming is penetrating down into the ocean, he said, creating deeper layers of warm water in the Gulf of Mexico and elsewhere.
"Harvey was almost certainly more intense than it would have been in the absence of human-caused warming, which means stronger winds, more wind damage and a larger storm surge," he wrote.
Gerald Meehl, a senior scientist at NCAR, on Tuesday would not put numbers to the percentage increase in Harvey's effects due to climate change, but supported the reasoning of his colleagues in the climate science community.
"We do know that the air is warmer. And because it is warmer it is holding more moisture, and so for any given storm there is more moisture available for rainfall," he said. "But for an individual hurricane, it is pretty difficult to say how much of that could be attributable to human influence."
'Bad things happen'
A phenomenon associated with Harvey is its having stalled out for several days over southeastern Texas, leaving no dry air over land, with the result, Trenberth said, that "Harvey feeds on the very rain it once deposited."
Phil Klotzbach, a research scientist in the Department of Atmospheric Science at Colorado State University, said the average survival time of a hurricane that remains a named storm once it makes landfall is 27 hours. Harvey lasted about 72, he said, before moving Tuesday back into the Gulf toward Louisiana.
Since making landfall, a "massive blocked high" to the west and a modestly strong high to the northeast of Harvey kept it from moving more than about 3 mph — extremely slow for a tropical system's movement over land, Klotzbach said. As for human's contribution to the disaster, he spoke mostly to what has happened once the rain started falling.
"There is certainly a human component in that you have a lot more people living in Harris County (where Houston is situated) than you used to," he said. "There is much more concrete that used to be bayou. So there is always a land-use component. When you put impermeable surfaces where you had permeable surfaces before, bad things happen."
As is often the case in discussions of climate change and its effects on individual weather events, there is dissent. Professor Roger Pielke Jr., director of CU's Sports Governance Center, formerly a scientist at NCAR and a vocal commentator on climate change issues in the past, has been active on Twitter in recent days.
"There is no reason to be debating Harvey and climate change in the context of an unfolding disaster, other than political opportunism and attention seeking," he tweeted on Monday.
"It's not a good look for scientists or journalists who are promoting this issue. The IPCC (International Panel on Climate Change) and other assessments are quite clear on this subject and one storm doesn't change that. A better focus in the short term is on those with expertise in disaster response and recovery. The politicized debate over climate change can wait."
Pielke declined comment Tuesday beyond what he has posted on Twitter.
Hoerling pointed out that Harvey has several comparable precedents in Texas, including Tropical Storm Allison (June 2001, 40 inches of rain), Tropical Storm Claudette (July 1979, 42 inches of rain in Alvin, Texas — all in one day) as well as 1978's Tropical Storm Amelia.
"We've seen these storms before," Hoerling said. "Sometimes we just forget that we live in these weather-prone disaster zones where the threat is high."