(See list of Stanford experts available to discuss impacts of Hurricane Harvey.)

Houston, the fourth largest city in the nation with an economy the size of Sweden's, and many other cities and towns in southeastern Texas have been devastated by the torrential rains and flooding caused by Hurricane Harvey. Estimates put total rainfall in some areas as high as 52 inches, shattering the record for highest rainfall from a single storm in the continental United States. Casualties continue to mount as heartbreaking stories emerge of the human toll of the catastrophic event. Before it’s over, Harvey could impact as many as 13 million people along the Texas and Louisiana coastlines.

Stanford researchers share their perspective on the causes and impacts of Harvey and what the future holds.

Noah Diffenbaugh is a professor of Earth system science in the Stanford School of Earth, Energy & Environmental Sciences and the Kimmelman Family Senior Fellow at the Stanford Woods Institute for the Environment.

Newsha Ajami is director of urban water policy at Stanford’s Water in the West program and co-leads the Urban Water Systems & Institutions Thrust at the NSF-ReNUWIt Engineering Research Center.

Charles Kolstad is a senior fellow at the Stanford Woods Institute for the Environment, the Stanford Institute for Economic Policy Research and the Stanford Precourt Institute for Energy.

How is climate change influencing the frequency and severity of extreme events like Hurricane Harvey?

Diffenbaugh: In terms of Hurricane Harvey, we’re not going to know how much influence climate change had on this particular event until we can do a formal scientific analysis of the conditions and circumstances. However, there are a number of testable hypotheses. We know ocean temperatures have warmed and sea level has risen, so we can analyze how much these changes contributed to the strength and severity of the storm. We know that the atmosphere has warmed and is holding more moisture, and we can analyze whether the increasing water vapor in the air had an effect on the amount of rainfall this storm produced. The path the storm took was unusual – we’ve recently developed a framework to test whether or not the odds of those kinds of specific atmospheric conditions have changed as a result of global warming.

As scientists, we tend to be very conservative when prescribing influence of global warming on extreme events. Erring on the side of caution for us means assuming that climate change has no effect on extreme events, and then rejecting that assumption only in the face of an abundance of scientific evidence. So at this stage you will hear lots of scientists being very conservative about drawing any conclusions. But when it comes to planning for the future for cities like Houston, it is important to keep in mind that we already have mountains of evidence that extremes are increasing, and that global warming is increasing the odds of events that fall outside of our historical experience.

How did stormwater management or the lack of it contribute to the high flooding in Houston and surrounding areas?

Ajami: The engineering of our cities and urban areas are quite rigid and not flexible enough to handle and recover from natural disasters easily. Houston is no exception. Conventional urban design and development have altered our land use, hardening the natural landscape by replacing trees, vegetation and natural environment with buildings, roads, highways and other infrastructure ultimately affecting natural infiltration rates, increasing polluted urban runoff and leading to urban flooding during extreme weather events. Developing in flood zones and covering cities with impervious surfaces such as asphalt, concrete and other building materials does not help and turns our cities into bathtubs with very limited absorption capacity to rely upon.

While the magnitude of flooding caused by Hurricane Harvey is massive – a 500-year flood that brought 9 trillion gallon of water with it – if the city had a more strategic stormwater management plan that better fits our 21st-century climatic and hydrologic reality, it could have helped reduce its social and economic impacts. This was not the first time Houston was affected by an extreme flooding event and will not be the last time. Hopefully we will not approach it as a passing event but rather as an opportunity to change the conversation around urban water development and how it needs to reflect our “new normal.”

Will the economic impact of Hurricane Harvey on Houston be similar to what we saw after Katrina?

Kolstad: The economic impact will be similar to Katrina, though Houston is a larger city. There are temporary displacements where people are not working and having to shelter elsewhere, as well as permanent displacements, which include job losses and having to actually move out of the area. There are also physical damages to property, which could mean immense cost to repair, and some property may ultimately have to be abandoned. The loss to individuals will be great because much of the damage is uninsured.

The private insurance market has been responding to the increased risk from climate change and that will continue. But flood damage is not generally covered by private insurance. It is the government insurance market, often subsidized, which will take a big hit. It is due for a politically tortuous overhaul.

How are probabilities of extreme events determined? What is a “500-year” or “100-year” flood if it happens more often than that?

Diffenbaugh: These numbers are really probabilities. When you hear this is a 100-year flood, what that really means is that scientists and engineers have looked back at the observed history of flooding, and they’ve calculated that this level flooding is the 99th percentile of the statistical distribution.

What our research focuses on is testing whether changes in the climate system have affected what the probability of exceeding that distribution is. What we found is that climate change has already increased the odds of record-setting hot events in 80 percent of the global area with reliable data, and increased the odds of record-setting wet and dry events in about half of the area.

Observational records are often pretty short. Calculations of the likelihood of flood events like this one are often made with the assumption that the climate conditions are stationary. But we now have substantial evidence that the climate is not stationary, and the odds of encountering events that are more extreme than our historical experience are in fact increasing. Being resilient and prepared requires planning around that “new normal.”

What do other cities in the U.S. need to do to prepare for stormwater flooding from rain events like this?

Ajami: There are so many solutions that can be embraced by cities and urban areas to help make our communities more resilient to future extreme rain events. Our cities and urban areas channel stormwater out of the area as quickly as possible through impervious landscape toward storm drains rather than letting it absorb locally, which can not only avoid or dampen flooding but also provide an opportunity to store water during extreme rain events that can be reused later as an alternative water supply source.

Cities should be designed as sponges – introducing enough green and natural spaces in their current land use to create more flexibility in managing the next big storm. Implementation of green and living infrastructure solutions, such as green roofs, permeable pavements and green spaces which turn public spaces into multipurpose absorbing grounds and infiltration basins, can introduce more resiliency to the urban areas and reduce the impacts of future extreme events. The floodwater can ultimately recharge the groundwater basins or be stored in cisterns for future use. It is important to remember that this strategy also requires engaging the community closely in the process by encouraging them to implement some of these practices on their property and in future development projects.