Just a few weeks ago, Betty Martin looked into purchasing homeowner’s insurance for her Houston condo, since she was considering selling the unit where she’s lived for 18 years. Neither she nor the insurance agent she talked to even thought of discussing flood insurance: The condominiums where she lived aren’t even situated in the 500-year flood plain, and no one can remember the area ever flooding, not even during hurricanes Camille, Ike, or Katrina.

But when the rains from Harvey hit, Houston’s Memorial Drive became a river, and her condo at The Pines – along with treasured recipes, mementos, and family heirlooms – became part of the nearby Buffalo Bayou. When Ms. Martin left, even before the U.S. Army Corps of Engineers approved a controlled release at nearby Addicks Reservoir, the water was already to her waist.

As many residents like Martin start to take stock of their losses, a number of experts are pointing to hurricane Harvey as the latest example of what could be a new reality for many parts of the world, due at least in part to climate change.

In Houston, it wasn’t the wind or the storm surge that caused the damage in the end. It was the massive amounts of rain, that fell and fell over multiple days, long after the system had been downgraded from a hurricane to a tropical storm. At least 30 inches of rain fell over an area the size of Maryland, so much that the National Weather Service had to add two new colors to their map. And while there is significant debate among climate scientists about how climate change will affect hurricanes, there is strong consensus about its link to heavy and intense rainfall.

Those changing weather patterns – combined with human factors like city zoning, where homes are built, and the loss of porous surfaces, all of which can affect flooding and a storm’s impact – mean it's time for a major shift in how we think about flood risk and planning, say experts.

“Our entire civilization is built on an obsolete assumption,” says Katharine Hayhoe, director of the Climate Science Center at Texas Tech University, noting that all the mapping that tends to drive requirements for building, engineering, and insurance is based on past observations about a static climate that means little for the future. “The smartest thing to do at this point,” she says, is to “be building resilience to the risks we face. And in many cases, those risks are being exacerbated by a changing climate.”

An outdated model?

While detailed studies and analyses about the role climate change may have played in Harvey's severity will take time to complete, most scientists say they are confident about at least a few factors. Among the clearest: Sea surface temperatures are higher, the atmosphere above oceans is warmer, and the warmer air can hold more moisture.

In addition, warmer subsurface water in the oceans means that storms can last longer than they might otherwise, says Kevin Trenberth, a senior scientist in the Climate Analysis Section at the National Center for Atmospheric Research (NCAR) in Boulder, Colo. Research he and others have done leads him to expect that climate change may actually lead to fewer hurricanes, but ones that are more intense and longer-lasting. Relatively few storms, and the huge variability among storms makes, the observational record tricky. However, there is significant debate and not a lot of consensus among scientists about just how hurricanes will be affected.

That’s not the case when it comes to the linkage between global warming and heavy rainfall. “That’s where the greatest agreement exists,” says Dr. Trenberth. “It’s not just Harvey, and it’s not just tropical storms. In general, when it rains it rains harder.”

But while rainfall is a weather- and climate-driven phenomenon, flooding also has a lot to do with human factors.

Even before Harvey hit, many scientists and engineers in Houston had been warning how ill-prepared the city was for a big rainfall event, and the ways in which planning decisions – like the significant loss of green space – can exacerbate floods.

Back in December, The Texas Tribune and ProPublica ran an article, part of a larger series about Houston’s flood risk, about the ways in which development planning decisions were combining with climate change to increase risk. It documented a number of housing developments that were seeing repeated “urban flooding” that occurred outside of any known flood plain. It also noted that since the county began keeping data for 24-hour rainfall totals, in 1989, there have been eight storms that qualify as either 100-year storms or rarer.

With Harvey – which has shattered records for rainfall, dropping nearly 52 inches at one location just east of Houston – scientists are talking about it as a 1,000-year event, meaning it has a 0.1 percent chance of occurring in any given year. But increasingly, experts say all those numbers are fairly meaningless.

“Those numbers are based upon stationary statistics,” says Trenberth. “What used to be a 1,000-year event is now more like a 100-year event. A 500-year event is a 50- or 70-year event.”

While scientists can agree that a changing climate is making heavy precipitation events more common, they still don’t know exactly where and how those changes will play out, creating challenges for planners and engineers who in most cases are still relying on outdated maps.

The key, say experts, is more resiliency planning – which a number of cities and states are already doing – that takes worst-case scenarios into account and sets standards well beyond where they currently are.

Building resilience

Rather than relying on the principle that buildings and towns need to be “fail-safe” and designed to keep extremes out, “we need to start to think about how to make it safe to fail instead,” says Mari Tye, a project scientist at NCAR’s Capacity Center for Climate and Weather Extremes. When the 2013 floods hit Colorado, she notes, cities like Boulder and Arvada escaped much of the damage that hit other towns because they had designed routes along creeks and detention basins that were able to absorb the flooding when it occurred. Those areas have park spaces and bike paths in normal times, and could accommodate the floods – resulting from a 1,000-year rain event – with minimal damage to infrastructure and homes.

Jennifer Jacobs, an engineering professor at the University of New Hampshire and director of the Infrastructure and Climate Network, says that it’s becoming clear to a growing number of states just how vulnerable their existing systems are. Just in the first six months of 2017, she notes, there were five major incidents of flooding in the United States that closed interstate highways, including in California, Idaho, and Missouri.

“This is an issue that’s happening everywhere,” she says. And if there’s any upside to disasters like Sandy or Harvey, it’s that “it opens the door to the question of what extremes are we preparing for.”

Putting that thought into resiliency planning, and designing for more extreme weather, can mean saving about $10 for every $1 spent, says Professor Jacobs, since such planning results in less devastation when a disaster does hit.

And it’s certain to be a big topic as the waters subside from Houston and attention turns to where and how to rebuild.

Professor Hayhoe notes that she works with a number of people who are skeptical about the idea of human-caused climate change, but says all of them – anyone who works with data and risks and hazards – accept that things are changing and that planning has to change as a result.

That made President Trump’s decision, 10 days before Harvey hit, to rescind an Obama-era rule requiring that the federal government consider stricter flood-risk standards when building all the more infuriating to many engineers and scientists. “It’s like a step backward into the last century,” says Hayhoe. “It’s a short-term decision that might save money now but will cost more money later.”

Laura Lightbody, who directs the Pew Charitable Trusts’ project on Flood-Prepared Communities, says that rule was an important tool for communities trying to be more resilient.

“It’s a resource communities no longer have when they think about rebuilding,” she says.

Ms. Lightbody says many cities – including Nashville, Tenn., which had flooding in 2010, and the Charlotte-Mecklenburg area in North Carolina – have taken the initiative themselves to heighten their standards and be better prepared for big storms in the future.

But it’s a discussion that needs to happen at the federal level as well, she says.

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“It’s unfortunate that an event like hurricane Harvey has to bring that to the table,” says Lightbody. “But it does force the hand of Congress to have a conversation about flooding.”

Carmen Sisson contributed reporting to this article from Houston, Texas.