The impacts of Hurricane Harvey continue to be felt in the southern U.S., where at least nine people have died after unprecedented flooding in Houston, Texas.

The events have sparked early debate over the links between the hurricane and climate change. Commentary from scientists suggests that warming is likely to have intensified its impact.

Above-average sea surface temperatures in the Gulf of Mexico provided more energy and more moisture for the developing hurricane, they said, while sea level rise ensured a larger storm surge at the coast and prevented floodwater from draining more quickly.

Nevertheless, many other factors are likely to have played a role. These include Houston's population explosion, continued building in flood-prone areas and subsidence due to groundwater over-extraction, media reports suggested.

Mobile homes are destroyed at an RV park after Hurricane Harvey landed in the Coast Bend area, 26/08/2017, in Port Aransas, Texas. Tribune Content Agency

What has happened?

At 10 p.m. on Friday, Aug. 25, the category 4 Hurricane Harvey made landfall near Corpus Christi on the southern coast of Texas.

It had developed over the previous week, according to an Associated Press timeline, after being officially named on Aug. 17. During that time, it subsided, before rapidly strengthening, gathering energy from the above-average warm waters of the Gulf of Mexico.

Harvey is the first category 4 hurricane to hit the mainland U.S. since Charley in 2004, reported the Washington Post, and the first to make landfall in Texas since Carla in 1961.

Harvey later slowed and then hovered over Houston—the fourth largest city in the U.S.—dumping "unprecedented" rainfall of as much as 40 inches (100cm) by Monday morning.

Meanwhile a storm surge of more than six feet (1.8m) was recorded at some coastal sites on Friday, reported the Washington Post—and as much as 13 feet had been expected in shallow estuaries.

The governor of Texas had already declared a state of emergency in anticipation of flooding, with mandatory evacuations in some areas. More than 3,000 national and state guard troops were deployed to aid the response effort, the Washington Post reported, a figure later raised to 12,000, noted the New York Times.

Tragically, this has not been enough to prevent loss of life: the LA Times reported at least nine people dead, as of Monday evening. The rain has caused Houston's worst-ever flooding, one meteorologist told the Houston Chronicle.

Some 30,000 people have sought refuge in temporary shelters, Reuters reported. So far, more than 300,000 homes have been left without power, reported the Times. The Federal Emergency Management Administration expects some 450,000 to seek disaster assistance, Reuters added.

In a series of maps, the New York Times has tracked Harvey's path of destruction, showing the major infrastructure that has been hit and the places where flooding has been recorded.

Harvey is already being listed as one of the ten costliest storms in U.S. history, the Financial Times said, with the energy and insurance industries expecting heavy losses. Insurer payouts could reach $10 billion to $20 billion, the paper said, citing a JP Morgan Chase "best guess." Thousands of homeowners lack adequate insurance cover, meaning these totals underestimate the true costs.

The final costs of the ongoing damage remain highly uncertain, with one insurance analyst telling Bloomberg it could pass $100 billion. This can be compared to Hurricane Katrina, the most expensive to hit the U.S., which cost about $118 billion in 2005. Hurricane Sandy cost $75 billion.

Meanwhile, Harvey, now downgraded to a tropical storm, continues to wreak havoc.

In its latest advisory, issued early on Tuesday morning, the US National Hurricane Center (NHC) said "catastrophic and life-threatening flooding" is affecting "large portions of southeastern Texas and southwestern Louisiana." It said these areas face a further 7 to 13 inches (18-33cm) of rain.

President Trump visited Texas on Tuesday, Reuters reported.

What role does climate change play?

Alongside the swath of news coverage on Hurricane Harvey and the devastation it has caused, much of the reporting has delved into the question of where climate change fits in.

Most articles highlighted that the impact of climate change on hurricanes has many different strands. As the New York Times pointed out, "the relationship between hurricanes and climate change is not simple. Some aspects are known with growing certainty. Others, not so much."

To help unpick the details, journalists have been quizzing climate scientists, quoting them widely in recent days. Some scientists have also penned guest articles.

Because of year-to-year weather fluctuations, it is not possible to say that climate change "caused" an extreme event such as Hurricane Harvey, Kerry Emanuel, professor of atmospheric science at the Massachusetts Institute of Technology, told the Washington Post:

"My feeling is, when there's a hurricane, there's an occasion to talk about the subject ... But attributing a particular event to anything, whether it's climate change or anything else, is a badly posed question, really."

Instead, scientists look at how different aspects of climate change can affect the likelihood or strength of a hurricane, or the amount of rainfall it brings when it arrives.

The principal link between climate change and more intense storms comes down to the amount of heat in the atmosphere. In a warmer world, more moisture evaporates from the earth's oceans and the atmosphere can hold more water vapor.

The relationship is exponential, professor Ben Kirtman, an atmospheric scientist at the University of Miami, told National Public Radio:

"For a small change in temperature, you get a huge amount of evaporation."

This means that when storms occur, they can dump more rainfall on a region. And the amount of rainfall that Hurricane Harvey brought to parts of Texas was unprecedented, Eric Fisher, chief meteorologist at WBZ-TV in Boston, told the Washington Post:

"It's fair to say it will produce more rain than we have ever seen before in the U.S. from a tropical system and over the fourth-largest city in the country."

Speaking to the Atlantic, Dr. Kevin Trenberth, distinguished senior scientist at the National Center for Atmospheric Research (NCAR), said the extra heat in the atmosphere has the potential to make storms like Harvey more costly and more powerful:

"The human contribution can be up to 30 percent or so of the total rainfall coming out of the storm ... It may have been a strong storm, and it may have caused a lot of problems anyway—but [human-caused climate change] amplifies the damage considerably."

Some quickfire calculations by Emanuel suggest that climate change made the huge downpours in Texas more likely. Seth Borenstein reported Emanuel's quotes for the Associated Press:

"The drenching received by Rockport, Texas, used to be maybe a once-in-1,800-years event for that city, but with warmer air holding more water and changes in storm steering currents since 2010, it is now a once-every-300-years event."

And as a reminder, Climate Denial Crock of the Week on Saturday posted a short video made last year of Emanuel discussing on climate change and strengthening hurricanes.

(The Washington Post has a helpful explanation of the meaning of once-in-500 year events: "A 500-year flood isn't necessarily something that happens once every five hundred years. Rather, a 500-year flood is an event that has a 1 in 500 chance of occurring in any given year.")

Climate change has also boosted sea levels, wrote Michael Mann, distinguished professor of atmospheric science at Pennsylvania State University, in a piece for the Guardian:

"That means the storm surge was half a foot [15cm] higher than it would have been just decades ago, meaning far more flooding and destruction."

Hurricane Harvey commentary—Michael Mann in the Guardian, Aug.28, 2017

And the seas are not only higher, they're also warmer. Sea surface temperatures in the region have risen about 0.5C over the past few decades, noted Mann:

"Not only are the surface waters of the Gulf of Mexico 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. Human-caused warming is penetrating down into the ocean. It's creating deeper layers of warm water in the Gulf and elsewhere."

More heat means more fuel for hurricanes like Harvey, Trenberth told the Atlantic:

"Although these storms occur naturally, the storm is apt to be more intense, maybe a bit bigger, longer-lasting, and with much heavier rainfalls [because of that ocean heat]."

Scientists have high confidence that ocean warming will also means stronger hurricane wind speeds, Dr. James Done, a research fellow at NCAR, told Think Progress:

"In recent decades, we have seen an increase in the proportion of hurricanes that reach category 4 or 5. It looks like this trend will continue. So, for every hurricane that comes along it will be more likely to be a category 4 or 5 than in past decades."

But it's worth noting that the impact of climate change is not just about warming, wrote Dr. Friederike Otto, a senior researcher at the Environmental Change Institute at the University of Oxford, in a guest article for Climate Home:

"In a changing climate, two effects come together: not only does the atmosphere warm up (thermodynamic effect) but the atmospheric circulation, which determine where, when, and how weather systems develop, can change as well (dynamic effect)."

Changes to the weather patterns can increase the thermodynamic effect, or counteract it, Otto said, which makes attributing extreme events more complicated:

"Hence, while it is very likely that climate changes played a role in the intensity of the rainfall, it is far from straightforward in practice to quantify this role."

Hurricane Harvey commentary—Climate Home, Aug. 28, 2017

"Climate science has repeatedly shown that global warming is increasing the odds of extreme precipitation and storm surge flooding," Noah Diffenbaugh, professor of earth systems science at the Stanford Woods Institute for the Environment, wrote in the New York Times, but "we can't yet draw definitive conclusions about the influence of climate change on Hurricane Harvey."

That said, it is "well established that global warming is already influencing many kinds of extremes, both in the U.S. and around the world", Diffenbaugh noted, "and it is critical to acknowledge this reality as we prepare for the future."

Getting that definitive answer to how climate change affected Hurricane Harvey "needs to be answered by carefully estimating the likelihood of such hurricanes developing in a warming world as well as how much rain they bring," wrote Otto.

This "requires a dedicated study," Otto added, but "it is a question scientists now can answer."

What about other factors?

Of course, the amount of damage caused by storms and flooding can also be made worse by building homes, offices and infrastructure in harm's way on floodplains.

As Grist noted, "People keep building in flood-prone places like Houston." Propublica has a series of articles on why Houston has become more vulnerable to flooding, with explosive growth as well as climate change to blame. Last year, it published a joint feature with the Texas Tribune, introduced with the prescient words:

"Climate change will bring more frequent and fierce rainstorms to cities like Houston. But unchecked development remains a priority in the famously un-zoned city, creating short-term economic gains for some while increasing flood risks for everyone."

Population growth also plays a role in the impact of extreme weather events, said Dr. Andrew King, a climate extremes research fellow at the University of Melbourne, in a piece for the Conversation. Houston is the second-fastest growing city in the U.S., and the fourth most populous overall, he wrote:

"As the region's population grows, more and more of southern Texas is being paved with impermeable surfaces. This means that when there is extreme rainfall the water takes longer to drain away, prolonging and intensifying the floods."

The rising population also changes flood risk in some unexpected ways. Parts of Houston are subsiding rapidly as a result of people extracting too much groundwater, reported the Houston Chronicle. It recounted the staggering rate of recent change:

"Spring Branch, where Interstate 10 and Beltway 8 meet, has dropped 4 feet since 1975. Jersey Village, along Route 290 and to the west of Beltway 8, is almost 2 feet lower than it was in 1996. And Greater Greenspoint, where Interstate 45 intersects with Beltway 8, has given up about 2 feet in the last decade alone, according to USGS data."

And flooding is not just a scientific problem—it is also one of policy. Politico has a piece on how the U.S. government was warned 20 years ago, in a National Wildlife Federation report, that its flood insurance program was encouraging homes to be built, and rebuilt, in flood-prone areas of the country.

More than half of U.S. homes that have been flooded multiple times are in Houston, Politico noted. Two decades on, the author of the report told Politico that a flood event on the scale of Hurricane Harvey "was inevitable."

Business Insider is among those reporting that President Trump revoked flood risk regulations 10 days before Hurricane Harvey. The Obama-era rules would have required the federal government to account for climate risk and sea level rise when building new infrastructure, or rebuilding after disasters.

What are the opinion columns saying?

Many of the opinion pieces responding to Hurricane Harvey take a strong line on climate links. It's time to "shed some of the fussy over-precision about the relationship between climate change and weather," wrote David Leonhardt in the New York Times:

"Yes, I know the sober warning that's issued whenever an extreme weather disaster occurs: No individual storm can be definitively blamed on climate change. It's true, too. Some version of Harvey probably would have happened without climate change, and we'll never know the hypothetical truth."

Meteorologist and writer Eric Holthaus is even more unequivocal. "There's an uncomfortable point that, so far, everyone is skating around," he wrote in a piece for Politico:

"Now is the time to say it as loudly as possible: Harvey is what climate change looks like. More specifically, Harvey is what climate change looks like in a world that has decided, over and over, that it doesn't want to take climate change seriously."

Similarly, Ryan Cooper in This Week said, "Hurricane Harvey is America's climate future." He wrote:

"This destruction is a window into the future of climate change. This is what happens when humanity fails to either meaningfully restrict greenhouse gas emissions or prepare for the damage that is certainly coming."

Taking a similar line, Eugene Robinson wrote in the Washington Post, "Pay attention to what happened to Houston. It is rare to be given such a vivid look at our collective future." He wrote:

"Global warming did not conjure the rains that flooded the nation's fourth-largest city, but it likely did make them more torrential. The spectacle of rescue boats plying the streets of a major metropolis is something we surely will see again. The question is how often."

It is true that scientists don't know if climate change is making hurricanes more likely, noted David Roberts in Vox. Yet articles on what can be said about Hurricane Harvey and climate change "are all saying too little," he argued. The question of whether climate change "caused" recent events is "malformed." Instead, warming has increased the severity of storms, he said.

Hurricane Harvey commentary—Vox, Aug. 29, 2017

In Time, Justin Worland picked up this theme, saying "scientists [now] have a better answer" on the attribution of extreme events. The science of attribution—covered in a recent in-depth Carbon Brief article—evaluates how climate change has affected the odds of a given event.

In a separate piece for Grist, Holthaus noted that record-breaking rainfall brought by Hurricane Harvey is more likely in a warmer climate:

"There's a clear climate connection when it comes to higher rainfall. All thunderstorms, including hurricanes, can produce more rain in a warmer atmosphere, which boosts the rate of evaporation and the water-holding capacity of clouds."

Indeed, intense downpours measuring at least 10 inches (25cm) have already doubled in frequency over the past three decades, the Associated Press noted.

Flooding elsewhere

Amidst the wall-to-wall coverage of events in Texas, it's worth remembering another, arguably more catastrophic flooding crisis, taking place halfway around the world in South Asia.

Some 41 million people across India, Nepal and Bangladesh are being affected by monsoon flooding, reported the Hindustan Times on Friday. Tens of thousands of homes, schools and hospitals have been destroyed, the paper said. The situation is worsening, with CNN reporting earlier last week that just 24 million people were affected.

The latest information from the UN Office for the Coordination of Humanitarian Affairs catalogues the numbers of people displaced, homes destroyed and people killed by the floods.

As of Friday, the death toll due to flooding in recent weeks had risen above 1,200, reported Reuters, describing it as the "worst monsoon floods in years." Rains have brought India's financial centre, Mumbai, to a virtual standstill on Tuesday, reported the Gulf Times.

However, both authorities and citizens in Bangladesh have been reluctant to attribute the crisis to climate change, according to an article at New Security Beat.

Reposted with permission from our media associate Carbon Brief.