The increase in 1-in-1,000 year flood events
They're occurring basically every year at this point.
Welcome to another edition of ‘Geography is the News!’ I’ve done away with adding it to the title because, I don’t know, I just didn’t like it. Regardless, today we’re going to talk about a series of news events that have occurred multiple times over the last few years, but is also very relevant today, and that is the concept of the 1-in-1,000 year flood event. Today, the Northeast is suffering from extreme flooding caused by unprecedented rains. But before them it was south Florida, and before them it was St. Louis, and before them it was California and so on and so forth. Granted these are all in different places, but it still seems like quite a lot of extreme floods that are not supposed to happen this often.
But that’s part of the problem. The terminology makes it sounds precise. So let’s break down what a 1-in-1,000 year flood event actually means because it’s not really what it sounds like.
What 1-in-1,000 means exactly
A ‘1-in-1,000 year’ flood event is a term used in hydrology and floodplain management to express the probability of a flood event happening in any given year. In this way, it’s similar to any weather forecast. They’re predicting the probability of rain, not a binary “yes it will” or “no it won’t.” It doesn’t mean that such a flood will only occur once every 1000 years.
Instead, it means that each year, there is a 0.1% (1/1000) chance of a flood of that magnitude occurring in a given location. In other words, it's an event with a 0.1% annual exceedance probability (AEP). This is also sometimes expressed as a 1000-year return period, although this terminology can be misleading, as it might suggest such an event happens like clockwork every 1000 years, which is not the case. The concept is about probability, not timing. So just like how someone can win the lottery and then a year later win it again is unlikely, it’s not impossible. The same can be said for 1-in-1,000 year flood events.
To determine these probabilities, scientists look at historical flood data and create statistical models to estimate the likelihood of different flood sizes. This information is then used to guide building codes, flood insurance rates, and other risk management decisions.
But keep in mind that while these models can provide a useful guide, they are based on the assumption that future flood patterns will mirror those in the past. Changes to the landscape, such as urban development or climate change, can alter these patterns and make the actual flood risk higher or lower than the models predict. Also, even if the yearly chance is small, the cumulative probability of experiencing a "1-in-1000 year" flood increases over time. For instance, over the course of a 30-year mortgage, there is approximately a 3% chance of a "1-in-1000 year" flood occurring. That seems bizarre, but probability statistics are not straight forward.
The Northeast flood event
Today, the Northeast region of the United States, but mostly northern New York and Vermont, is under some pretty extreme flood events. Apparently the amount of rain that the region is getting has far surpassed that of 2011 when a literal hurricane bludgeoned the region. And this can’t be stressed enough: this is not a hurricane event. This is just pure rain. According to NBC News:
West Point Military Academy in New York had 6.96 inches of rain in three hours, Reading, Pennsylvania, had 5.35 inches of rain— its wettest July day ever; and almost 10 inches of rain fell yesterday in the Hudson Valley of New York.
The region on average receives about 40 inches of rain per year, so this to get anywhere from 15% to 25% of the yearly total over just a few hours is pretty insane. No amount of infrastructure that can properly prepare any region for this kind of rain in such a short period of time.
Climate change impacts on these flooding events
Unfortunately, as with most things that are impacting our world, the increase in these events appears to be connected to climate change at large. Which is probably not a huge surprise to anyone reading this newsletter, but it needs to be said anyway. And we can almost pinpoint exactly how climate change is exacerbating these weather events.
Greater rainfall intensity
As the atmosphere warms, which we know it is, its capacity to hold moisture increases, potentially leading to more intense rainfall during storms. This escalation can elevate the risk of flooding, meaning that 1-in-1000 year flood events may become more common than previously anticipated.
Altered snowmelt patterns
Climate change also influences snowmelt patterns. Warmer temperatures are causing snow to melt earlier in the year, leading to a 'double-whammy' effect on river systems - increased water from earlier snowmelt, followed by enhanced water from rainfall, thereby amplifying flood risks. Basically we’re seeing record snowfalls in areas, almost immediately followed by intense heat which melts that snow rather quickly.
Increased frequency of extreme weather events
Due to the previous two events, there is emerging research suggesting that climate change could heighten the frequency of extreme weather events, including heavy storms that cause flooding. This could potentially transform the 1-in-1000 year flood event from a rare occurrence into a regular threat.
Vegetation and soil moisture changes
Finally, the changes in regional climates can also impact vegetation and soil moisture levels. For instance, extended periods of drought can harden the soil, resulting in increased runoff when rain does fall, thereby exacerbating the impact of flooding. Have you ever watered a plant that hasn’t been watered in a while? For a bit the water just sort of sits at the top of the soil before sinking in. This is because the soil is so dry that it’s no longer as permeable as it was previously. This is what happens in dry, drought-like conditions as well. Instead of sinking into the soil, water will just roll right on through an area.
The cumulative effects of these climate change factors mean that areas previously considered safe from a 1-in-1000 year flood event might become susceptible, and regions already at risk may experience these extreme floods more frequently. Consequently, there is a growing concern among experts that existing flood risk maps, which primarily rely on historical data, may underestimate the future flood risks associated with climate change.
So what’s happening in the Northeast today is unfortunate, and uncommon, but unfortunately probably not as uncommon as it will be heading into the future. Only next time it may be Tennessee, or Idaho, or even historically very dry places such as Las Vegas. The frequency of 1-in-1,000 year flood events will only increase as climate change gets worse.
Just because these flood events are occurring seemingly more often these days, *does not* conclusively mean that fossil fuel emissions are the chief factor behind extreme weather events like these (or drought, or heatwaves, or hurricanes, for example). It's just as likely that variability in the Sun or cosmic rays or oceans or other natural factors are behind those extreme weather events, and thus those weather events would probably occur anyway even without our fossil fuel emissions, as has always been the case throughout Earth's history. If one looks at the past historical and especially geological record, our seemingly stable global climate is anything but stable. While each extreme weather event is devastating and must be dealt with, there's no climate emergency any more than there was in the past; when I say "the past", I mean not just the past 50-100 years but also the past 1000 years or 10,000 years or in between. It would be useful to do a Google search for someone like Judith Curry or something like Clintel, and you'll see what I mean.