Wow - Now experts are calling #Harvey a once in 500 year flood! We have an all out effort going, and going well! — Donald J. Trump (@realDonaldTrump) August 27, 2017

In the aftermath of Hurricane Harvey, President Trump tweeted that the storm had brought a “once in 500 year flood” to Houston, and expressed support for relief efforts. His words suggested the kind of catastrophic event that hadn’t been seen in the area for five centuries. But parts of Houston saw “500-year flooding” just last year. And in 2001, Tropical Storm Allison also delivered severe flooding to the area.

Harvey has been even more devastating, and officials say the flooding is the worst in state history. But is it really a once-in-500-years event?

Terms like “500-year flood” and “100-year flood” are used as shorthand by government officials and actuaries, but they are misnomers that can confuse the public, said Sandra Knight, a senior research engineer at the University of Maryland and a former official at the Federal Emergency Management Agency.

Rather than predicting the timing of a flood, these terms refer to the chance of it occurring at all.

A 500-year event has a 1 in 500 chance of occurring in a single year.

A 100-year event has a 1 in 100 chance of occurring in a single year.

A 10-year event has a 1 in 10 chance of occurring in a single year.

You get the idea.

“People think: ‘Well, it’s a one-in-100-year flood. We had one last year, so it won’t happen again,’” Dr. Knight said. But really, “they can happen back to back.”

A 100-year flood means there is a 1 percent chance — or 1 in 100 — that a flood of that magnitude will occur in that given location in any one year. In 500-year flood zones, the probability goes down to 0.2 percent. In other words: not very likely to happen — or, at least, not likely based on historical data, which helps scientists to determine these probabilities.

Here is a map of Houston’s flood zones as defined by FEMA. It shows the areas that have a 1-in-500 chance of flooding, and a 1-in-100 chance of flooding. (FEMA uses various scales for 100-year floods, which we’ve rolled into one below.)

Harris County flood zones in 2017 Lake Houston 100-year flood zone Addicks Reservoir HOUSTON Barker Reservoir Galveston Bay 500-year flood zone Lake Houston 100-year flood zone Addicks Reservoir HOUSTON Barker Reservoir Galveston Bay 500-year flood zone Lake Houston 100-year flood zone Addicks Reservoir HOUSTON Barker Reservoir Galveston Bay 500-year flood zone Lake Houston Addicks Reservoir 100-year flood zone HOUSTON Barker Reservoir 500-year flood zone 100-year flood zone Addicks Reservoir HOUSTON Barker Reservoir 500-year flood zone 100-year flood zone 500-year flood zone HOUSTON

But changing landscapes — and a changing climate — could alter the probability of flooding.

“We’re looking at historical data when really we have something that is called non-stationarity,” Dr. Knight said. “The world isn’t stationary anymore and the hydrology isn’t. The landscape isn’t. So why are we still presuming the future will look like the past?”

In Houston, factors like rapid urbanization over the past few decades have already affected how and where water flows, she said, and climate change will strengthen the likelihood and severity of future extreme events.

Devastating storms like Harvey are still relatively rare, “but the probability of them occurring has increased substantially because of climate change,” said Kevin Trenberth, a distinguished scientist at the National Center for Atmospheric Research in Boulder, Colo. “What used to be a 500-year event has become a 50- or 100-year event.”