As scientists, engineers, and victims of Harvey try to grasp exactly how a storm got that big and destructive, some say that there might never be one answer — but that climate change, sea rise, sprawl, and randomness all converged with a vengeance on Houston.
"They are mysterious unto themselves," William Sweet, a National Oceanic and Atmospheric Administration oceanographer, said of storms like Harvey and Sandy.
"Climate change aside, rare events happen," Sweet says. "Oftentimes it's not very well understood that your area might be prone to these major events. Just because you haven't seen a storm like it in 30, 50 years doesn't mean they aren't prone to occur. If your region is prone to these kind of events, you need to be prepared."
So what are the factors suspected of making Harvey especially awful?
Most people think of sea-level rise as a component of climate change: Melting glaciers empty into the ocean and raise the sea level. But that's only partly correct. A portion of sea-level rise comes from what's known as subsidence — sinking land. Scientists consider more water from the oceans and subsidence when they look at sea-level rise for an area.
Subsidence is often caused by humans. Drilling for water and oil can contribute. Drawing large amounts of water from the ground causes certain types of rocks, such as fine-grained sediments, to compact. Subsidence is nearly irreversible. Subsidence has been a problem in the Houston area, which draws groundwater from three aquifers for municipal and commercial water supplies.
So, Houston not only has sinking land but more water in the Gulf Coast. That means overall sea rise is greater.
Indeed, sea rise is accelerating globally, adding about 8 to 9 inches since 1880 to the oceans. And scientists say sea levels rose at a faster rate in the 20th century than in any other century during the last 2,800 years.
The Insurance Journal took note of rising seas and their potential impact on Houston as recently as May, stating, "The rate of sea-level rise even under the lowest projection would increase the chances of severe flooding on the Texas Gulf Coast from storm surges or other causes from once every five years to once every two years by 2030 under the extreme projection, and 2060 under the low prediction."
A NOAA report from this year singled out the region that includes the New Jersey Shore in addition to Texas:
"Along regions of the Northeast Atlantic (Virginia coast and northward) and the western Gulf of Mexico coasts, sea level rise is projected to be greater than the global average for almost all future … rise scenarios."
As Harvey moved toward Texas, water in the Gulf of Mexico was nearly two degrees warmer than normal for this time of year, according to Weather Underground.
Warmer water is fuel for hurricanes.
Michael Mann, a prominent climate scientist at Pennsylvania State University, wrote on Facebook that warmer waters accounted for about 3 percent to 5 percent more moisture in the atmosphere. That contributed to the record 52 inches of rain, the most destructive force of all from Harvey.
"Not only are the surface waters of the Gulf unusually warm right now, but there is a deep layer of warm water that Harvey was able to feed upon when it intensified at near-record pace as it neared the coast," he said. "Human-caused warming is penetrating down into the ocean, warming not just the surface but creating deeper layers of warm water in the Gulf and elsewhere."
He added that Harvey may be part of a destructive phenomenon "favored" by climate change: weather anomalies that remain nearly locked in place for days. Harvey stalled over the coast and inland, retreating, then striking the coast again, adding to the rainfall and the misery.
"In conclusion, while we cannot say climate change `caused' Hurricane Harvey (that is an ill-posed question), we can say that it exacerbated several characteristics of the storm in a way that greatly increased the risk of damage and loss of life," Mann wrote.
Kenneth Kunkel of NOAA's Cooperative Institute for Climate and Satellites in Asheville, N.C., said the waters had been running warmer for months — as they have in most of the Northern Hemisphere.
"We can tie that pretty strongly to global warming and greenhouse gases in the atmosphere," he said.
Worse for Houston residents, the air was so moist that the storm had "this virtually unlimited supply of fuel."
One of the reasons Hurricane Sandy caused so much damage in 2012 along the East Coast was because of the tides.
Before October 2012, the highest water level recorded at Sandy Hook, N.J., was 4.86 feet above what's considered normally dry land. During Sandy, the water level reached 8.01 feet — and then a gauge was destroyed, making further measurements impossible.
Sandy dropped about 10 inches of rain near some parts of the coast, and that rain persisted during high tides, the worse possible time. At least seven feet of that record-high water level came from tidal surge.
The impacts of tides are not as great at Galveston Bay, but they are there. The highest water levels recorded during Harvey were at Manchester, which is next to Houston, at 9.33 feet. That figure includes storm surge, rainfall runoff, waves, and other non-tidal influence.
So, in Sandy there was much less rainfall but much greater impact from tides. In Harvey, there was much greater rainfall and might have been less impact from tides, but they still contributed.
The combination had a powerful effect. The Pine Island Bayou, part of a drainage network north and east of Houston, reaches flood stage at 25 feet, and major flood stage at 32 feet, according to National Weather Service data. On Wednesday it reached 39.42 feet. All that water has to go somewhere.
Houston was the fastest-growing city in the United States as of 2015, according to census figures, leading to dire warnings about what that means for flood control. The Houston Chronicle ran a five-part series on the issue last year.
"In the greater Houston area … the staggering increase of impervious surfaces — roads, sidewalks, parking lots, anything covered with asphalt and concrete — has exacerbated the effects of flooding as development in the region has exploded," one article said. "When land is covered by these surfaces, it loses ability to act like a sponge and soak up water. Things are further complicated in flat-as-a-pancake Houston, where much of the soil is heavily compacted and acts like pavement anyway, sending sheets of stormwater to the nearest low-lying area."
The article said that more than 337,000 of the 1.1 million acres in Harris County were covered by impervious surfaces in 2011. Developers rely on manmade ponds to hold runoff created by roads and concrete surfaces. But the ponds merely hold water instead of letting it be absorbed into soil.
In other areas they depend on levees, normally earthen dams, and bayous.
The Texas Tribune and ProPublica also teamed up with a project that had dire predictions for the unchecked growth and its impact on potential flooding if a hurricane struck the Houston area.
Harvey stalled largely because it got stuck between two high-pressure areas pushing it in opposite directions. Those fronts themselves became stuck and, Kunkel said, "had no net force to move it."
Meteorologists and climate scientists will spend weeks or months to perform calculations that could indicate climate change was a factor in heightening Harvey. Their work will likely be reviewed and checked by other scientists for years.