By the end of Tuesday, the city of Houston, Texas will have seen 50 inches of rain since Storm Harvey made landfall on Friday night. At least nine people have died and floodwaters have forced 30,000 locals to flee their homes.

President Donald Trump, among others, has described the storm as a “once in 500 year flood”:

Reading President Trump’s tweet, you’d be forgiven for thinking that a flood of this magnitude hasn’t happened in the last 500 years and won’t be seen again for 500 years.

But you’d be wrong.

Houston has been battered by three 500-year storms since 2015

Crucially, when climate and weather experts describe a “500-year” flood or storm, they are talking about probability rather than frequency. We spoke to Professor Kerry Emanuel, Cecil and Ida Green Professor of Atmospheric Science at the Massachusetts Institute of Technology (MIT). He explained:

“It is not correct to interpret a 500-year event as the first time something has happened in 500 years. The interpretation is simple: It is an event whose annual probability is 1/500. If that event occurs today, it in no way changes the probability of the same event happening next year; it still has a probability of 1 in 500.

“To take a simple analogy:  Suppose you roll a die once a day. Each time you roll it, there is a probability of one in six that you will roll a five (or any other particular number). We could say that rolling a five is a ‘6-day event’, which means that the daily probability of rolling a five is one in six. It does not mean that it has been six days since you last rolled a five.”

Indeed, including the rains brought by Storm Harvey this week, there have been three 500-year floods in the Houston area in as many years. 2015 saw the Memorial Day Flood bring 12 inches of rain in 10 hours, and the following year, the city saw over 15 inches in 12 hours.

Are these types of event becoming more common?

So we know the maths underneath the 500-year claim doesn’t exclude the chance of several consecutive 500-year storms.

But that fact alone doesn’t rule out the possibility of extreme weather events becoming more likely. The dice analogy that Professor Emanuel uses assumes that there is always a one in six chance of rolling a five every time you throw it. But what if we’re loading the dice so that throwing a five actually becomes more likely?

We asked Professor Emanuel whether there are any factors that might make events like the 500-year flood more likely over and above the probability explanation. He said: “All of us who do research on the influence of climate change on hurricanes project that hurricanes will rain more in the future, as the planet warms. This is simply because warm air can hold more water vapor than cold air.”

Channel 4 News’ own weatherman, Liam Dutton, explains what that means for future weather prospects: “As the climate changes, rising sea level, warmer oceans and a warmer atmosphere will all contribute to worsening the impact of hurricanes.

“A warmer atmosphere can hold more water, increasing their rain-making potential. Warmer oceans provide more energy for more powerful storms and thus bigger storm surges, which when coupled with rising sea level, makes the future picture for coastal locations even more bleak.”

Michael E Mann, Distinguished Professor of Atmospheric Science at Pennsylvania State University drew this link even more explicitly to Storm Harvey.

Writing in Monday’s Guardian, Mann said: “[Storm] Harvey was almost certainly more intense than it would have been in the absence of human-caused warming, which means stronger winds, more wind damage and a larger storm surge.”

He estimates that “the storm surge [caused by Storm Harvey] was half a foot higher than it would have been just decades ago” because of “sea level rise attributable to climate change”.

Mann concludes: “while we cannot say climate change ‘caused’ Hurricane Harvey (that is an ill-posed question), we can say is that it exacerbated several characteristics of the storm in a way that greatly increased the risk of damage and loss of life. Climate change worsened the impact of Hurricane Harvey.”

A 2012 study by researchers from MIT and Princeton University created 45,000 synthetic storms using a computer model to test the effects of climate change on storm intensity. The study found that overall, global warming will increase the frequency of intense storms.

So while it may or may not be the sole cause of individual storms, for as long as global warming continues, we can expect it to worsen their effects.

FactCheck verdict

500-year floods are much more common than they sound – in fact, Houston has seen three in as many years.

And leading climate experts anticipate more intense storms as sea levels and temperatures rise. So what is now classed as a 500-year storm could become a much more common occurrence.