Climate change is contributing to heavier precipitation, a major factor in flooding.
by Li Zhou
Parts of the United States’s eastern seaboard have been hit with massive floods this year, a phenomenon that’s expected to grow more commonand worse — as climate change warms the planet.
“It’s worse than a new normal. I call it a new abnormal,” says University of Pennsylvania climate scientist Michael Mann.
Friday, New York City experienced intense rains that left parts of the city underwater. Over the summer, the Philadelphia suburbs were hit with heavy flash flooding that inundated roads and killed five people. Earlier this year, Vermont also experienced heavy flooding that trapped people in their homes and damaged roads and buildings. Nationally, tens of millions of people were under a flood watch in July, while globally, countries including South Korea, Pakistan, and Turkey have seen destructive flooding that has displaced millions of people and forced evacuations in the last year.
“Everywhere is susceptible to these impacts,” Mann said. “The western, central, and eastern US, Europe, and Asia — with one of the best examples being the Pakistan floods last year which displaced more than 30 million people.”
As the Earth gets warmer, the atmosphere is able to hold more water, leading to heavier precipitation when it rains, and a greater likelihood of flooding as a result. A 1 degree centigrade increase in the atmosphere’s temperature corresponds to a 7 percent increase in water vapor that it’s able to hold, according to the Center for Climate and Energy Solutions. And estimates suggest global temperatures could breach a 1.5 degree Celsius increase threshold sometime in the 2030s, meaning much more rain to come.  
We spoke with four climate scientists about the factors behind the rise of extreme weather, and how the government could respond to both combat it and alleviate its impact. These interviews have been edited and condensed for clarity. 
Richard Seager, Columbia: In general, we know that heavy intense precipitation is increasing pretty much everywhere around the world, as a result of rising atmospheric temperatures. 
There’s never a complete one-to-one relationship between heavy intense precipitation and flooding of rivers. But you could certainly say that heavy, more heavy, and intense precipitation is more likely to cause the kinds of flooding that we’ve seen. 
We know from climate models, and just simple theory, that the atmosphere can hold more moisture as it gets warmer. We’ve known that … this can happen for decades now and it’s really coming true. 
Daniel Swain, UCLA: Let’s back up a little bit. Before we get to flooding, let’s talk about extreme precipitation. That’s sort of what the proximal cause is with terrestrial flooding, as you get too much water too quickly from the sky, in the form of rain generally.
One of the signatures of climate change is an increase in both extreme heat and extreme precipitation. And this is true almost everywhere. 
Flooding is a bit more complicated, because obviously, you need extreme, heavy rain for some sort of flooding, but then also, the antecedent conditions matter a lot. So if you had gone into [the New England storms], say, in an extreme drought, the soil was super dry, it might have taken longer for all of this water to saturate and the rivers wouldn’t have risen quite as much. But … the soils were actually unusually wet going into [the heavy rains in the northeastern US]. So they were primed to respond quickly to the precipitation. 
Michael Mann, University of Pennsylvania: Climate change is leading to anomalous warmth around the planet in general, and warmer ocean waters mean more moisture in the atmosphere that is available to produce flooding rains. 
But climate change is also altering the behavior of the jet stream, and some of our work suggests that it is leading to a wavier, slower jet stream associated with stalled weather systems that remain stuck in place for days or even weeks on end — that’s when you see the worst flooding events.
Mohammed Ombadi, University of Michigan: In general, global warming is increasing the intensity of heavy storms. 
Up until recently, we thought in the climate community that the increase in rainfall intensity is constrained to about 7 percent per 1 degree of warming, but there is growing evidence in recent studies that the increase might be much higher than that. 
Michael Mann, University of Pennsylvania: It’s worse than a new normal. I call it a new abnormal, and these flooding events will continue to become more extreme unless we reduce carbon emissions and stop the ongoing heating of the planet.
Daniel Swain, UCLA: I usually try and shy away from the term “new normal,” not because this isn’t related to climate change, but because it suggests that we’ve reached some semblance of a stable plateau. 
Whereas really, the new normal is continued change, continued escalation, and extreme. So if we call it “a normal,” it sounds like, “All right, we’ve reached this tipping point, we need to adapt to it. And if we can deal with everything now, we’re okay.”
And unfortunately, the reality is, this is our temporary, new normal, but in 10 years, we’ll have a new normal that’s escalated beyond today’s. We will see events like this — and worse.
Richard Seager, Columbia: Increasingly, we’re going to be seeing events, whether they’re heat waves, flooding events, or droughts that we thought looking back at our climate records would never be possible. Because of human-driven climate change, that’s going to create situations where things that have never happened before are going to become routine.
[In] another 10 or 20 years, we won’t be thinking they’re so abnormal, because these things are going to be happening more and more frequently. 
Mohammed Ombadi, University of Michigan: It is somewhat a new normal. In the future, we should expect to see more of these events occurring. However, it is important to note that this does not mean we will experience such floods every year going forward. 
Richard Seager, Columbia: They are two sides of the same coin. So when the atmosphere can hold more moisture, it also transports more moisture, from one place to the other. 
So like, in the Southwest United States, where I do most of my work on drought, when there are high-pressure systems … [with] winds blowing out of that area, those are the conditions that set up droughts in those areas. And the atmosphere is extracting moisture from those regions. So when it warms up, [the atmosphere] can hold more moisture, and move more moisture out of those areas. So droughts can intensify. 
In the warmer atmosphere, you can get both more extreme droughts and you can also get more extreme precipitation and they’re connected by the ability of the atmosphere to hold more moisture and therefore move more moisture from one place to the other, thus creating extremes on both ends of the spectrum.
Daniel Swain, UCLA: I mean, absolutely critical. And it’s actually quite good for the most part. 
If you look at the NOAA predictions for [the floods in the northeastern US], several days in advance, there were [public predictions] that were like, “A significant flood event is possible.” The day before, it was like, “We’re highly confident this event will be potentially as bad as what occurred with [Hurricane] Irene, potentially even worse” — which is exactly what happened. 
So it’s hard to fault that level of accuracy and the consistency of messaging. I know it still doesn’t mean that everyone gets the message. But that’s not a forecasting problem so much as it is a … communication and mass messaging problem.
Daniel Swain, UCLA: Day to day, aside from being aware of the weather and taking warnings seriously, as an individual, it’s difficult to adapt to the increased risk of extreme precipitation, flash flooding. I guess, be mindful of where you are during events, know whether your home or your place of work or places you spend time for other reasons are at risk during these events, but I feel like that’s weak advice.
[The] local and regional government level is actually where the rubber really meets the road here, in terms of planning and adaptation.
Cities and counties and local governments do update emergency response plans, they do update infrastructure as things age out, rebuild new things, whether that’s storm drains, or culverts, or restoring floodplains so that there’s less risk to the adjacent populated areas. Whatever it is, all of these sorts of interventions need to be climate aware. 
No city, no regional government should be building anything, or updating any infrastructure at this point, without taking climate change into account. And taking future climate change into account as well, not just how much things have changed to date, but how much they’re likely to change in the coming decades.
When it comes to flooding, specifically, one example would be in urban areas: You have storm drains and culverts that have a fixed maximum capacity; there’s a certain cubic feet per minute. They max out and then things go haywire when you exceed it. Part of it is having building infrastructure that has higher capacity. 
But the other part of it is in some ways the opposite, which is allowing the water to kind of do its thing in a safe or at least a semi-controlled way. That’s why things like river and creek setbacks, levee setbacks, restoring floodplains [are important] because of course, if you build right up on the floodplain, guess what’s going to happen when there’s a flood, you’re gonna wash away the structures, that infrastructure you built right on the margins of this natural floodplain.
Michael Mann, University of Pennsylvania: Obviously better emergency response is critical, but we will exceed our adaptive capacity in dealing with these events if we don’t address the problem at its source, which is primarily the ongoing burning of fossil fuels for energy and transportation. 
Richard Seager, Columbia: The lesson looking forward is to start planning for how we’re going to adapt to things that we have just never seen before. 
[We need to think about] how populations are going to deal with extreme heat, how our infrastructure is going to deal with extreme precipitation and flooding. We’ve got to think through all of that and start making changes now based on what we expect will happen in the future.
What we’ve already done to the climate system is going to lead to more warming going forward. So a lot of these changes we’re talking about in the near term are pretty much inevitable.
Whatever we do in terms of prevention can certainly prevent it from getting as bad as it otherwise would. And the payoff in decades to come will be huge. 
But by getting serious about reducing emissions right now, we’re not going to get out of this problem immediately because we have so much heating of the climate system that’s currently in the pipeline. 
Mohammed Ombadi, University of Michigan: There is little that can be done at the individual level. Most of the efforts needed for adaptation must be taken at the local, state, and federal government levels. However, people can cope with those flooding events by avoiding building homes in regions that are vulnerable to landslides and hill slopes that can be severely impacted by such torrential downpours.
​​The big message here is that our infrastructure was designed for a climate that no longer exists. This is very clear with the ongoing floods in northeastern US as we hear news of washed-out roadways and bridges, damaged tracks in railroads, and swamped homes. We need to change the way we design and build infrastructure to be in line with the increase in rainfall extreme events predicted by climate scientists.
Update, September 29, 2:15 pm ET: This story was originally published on July 11 and has been updated to include news about new floods in the US and around the world.
You’ve read 1 article in the last month
Here at Vox, we believe in helping everyone understand our complicated world, so that we can all help to shape it. Our mission is to create clear, accessible journalism to empower understanding and action.
If you share our vision, please consider supporting our work by becoming a Vox Member. Your support ensures Vox a stable, independent source of funding to underpin our journalism. If you are not ready to become a Member, even small contributions are meaningful in supporting a sustainable model for journalism.
Thank you for being part of our community.
Swati Sharma
Vox Editor-in-Chief
We accept credit card, Apple Pay, and Google Pay.
You can also contribute via
Understand the world with a daily explainer plus the most compelling stories of the day.
© 2024 Vox Media, LLC. All Rights Reserved