Louisiana, August 2016: “I’m going home to see if I have a home”.

Ellicot City, Maryland, July 2016: “Oh my god. There’s people in the water”.

West Virginia, June 2016: “23 dead, thousands homeless after devastating flood”.

What do these events (and 5 more since April 2015) have in common? They were all considered very low probability, and the National Oceanic and Atmospheric Administration’s Hydrometeorological Design Studies Center created maps of annual exceedance probabilities (AEPs) for all of them. AEP is the probability of exceeding a given amount of rainfall for a given duration at least once in any year at a given location. It is an indicator of the rarity of rainfall. These maps are created for significant storm events that typically have AEPs of less than 0.2% (i.e, exceed 500-year average recurrence interval amounts). For Louisiana, the probability analyzed was for the worst case 48-hour rainfall. For Ellicott City, it was for the worst case 3-hour rainfall. And for West Virginia, the June 23-24 event became a map for the worst case 24-hour rainfall.

In other words, just in the past 17 months, 8 rain events that are considered very low probability (i.e., less than 0.2%) occurred. Three happened in the past 3 months. Flooding like this should happen very rarely – there are AEP maps for only 18 more events, one of which was in 1913, all others having occurred since 2010. As our hearts go out to the families affected by the flooding, we may be asking; is this a series of unfortunate events? Certainly. The sheer loss of life and property is staggering, and heartbreaking. Totally unexpected? Unfortunately, the answer is hardly.

Global warming has a significant role in these flood events

NOAA and NASA just released their global temperature data for the month of July 2016, and again, it was a record warm month. Not only the warmest July on record, but also the warmest month ever on record. According to NOAA, this is the 15th record warm month in a row, starting with May 2015. One can’t help but notice that over these 15 months, 8 rain events were off the probability charts, so to speak. Yes, climate change fingerprint is on these events, including the Louisiana flood, considered the worst natural disaster in the US since hurricane Sandy. Special conditions mainly fueled by climate change were behind this record event.

As much as local conditions affect rainfall events individually, global warming is among the main reasons why we are seeing places that never flooded before, such as Baton Rouge areas and Ellicott City, being swallowed by not only deep but very fast rising waters. Development and urbanization also play a big role in these events, as rainwater swells rivers that no longer have wide, protective margins, and hits impervious surfaces that do not allow for ground penetration – water that has nowhere to go but along streets and between buildings down the easiest path it can find, based on topology and gravity. Here and here are some good resources that elaborate on the aforesaid factors.

These floods are devastating and shocking, but the writing has been on the wall for some time now

The 2014 National Climate Assessment stated: Heavy downpours are increasing nationally, especially over the last three to five decades. The heaviest rainfall events have become heavier and more frequent, and the amount of rain falling on the heaviest rain days has also increased. Increases in the frequency and intensity of extreme precipitation events are projected for all U.S. regions.

Even though Louisiana is not among the areas that had seen the most increase in heavy precipitation events, the Southeastern US saw an increase of 27 percent from 1958 to 2012. The straightforward explanation for heavier downpours is that warmer air can contain more water vapor than cooler air. Indeed, global measurements show that there is more water vapor in the air now. It follows that there is more water to come down when it rains. Other factors also affect precipitation patterns, which I have explored a bit further here. Even if one cannot directly attribute individual events specifically to climate change, the latter is behind an increased likelihood of them happening, and also of them becoming more extreme. Extreme rainfall is one type of extreme event on which the effects of climate change are better understood, according to a recent report by the National Academies of Sciences, Engineering, and Medicine that I explore in this blog.

A lot needs to be done in terms of insurance and preparedness

An estimated 70% of businesses hit by the Louisiana floods are reported to not have flood insurance. For residences, the percentage varies according to the Parish, according to FEMA data (see figure below). The National Flood Insurance Program (NFIP) covers from 12.5% of households in low to moderate-risk areas to 42% in high-risk areas, with an average of 21% coverage, according to a local news source. This means that a lot of people affected by the flood, some who are more vulnerable than others such as the lower income people, elderly, etc., will need disaster assistance at the cost of taxpayer’s pocketbooks. While this assistance is critical to get families and business back on their feet, the real need is for the Administration and Congress to reform disaster assistance, so that badly needed funding to fortify homes and communities comes prior to the next storm. Current policies do not allow for comprehensive mitigation without a disaster, which is really not the best strategy. Floodplain managers know the value of implementing mitigation strategies ahead of a disaster so communities can become better prepared and future impacts lessened. This is particularly important in light of existing resilience inequities in many areas – Louisiana included – where those most likely to be affected by natural disasters are also those with the least means to bounce back. You can read more about equitable climate resilience here.

The lack of coverage comes as a surprise to many, considering the history and geography of Louisiana, but a lot also lies on the shoulders of the NFIP itself, which is up for reauthorization in 2017, and has been the focus of many organizations and advocacy groups. You can read about it, and what can be done to improve it, on this blog.

As the Louisiana families and businesses work on making themselves whole again, we know that events like these recent floods are likely to worsen with continued global warming. This is why reduction of greenhouse gas emissions worldwide has to be a priority not only to reduce the rate of warming, but to minimize the chances of such events to happen so often.

Posted in: Global Warming Tags: climate-change, Extreme Events, extreme precipitation, Flood mitigation, Global warming, Inland flooding, National Flood Insurance Program, preparedness, record temperatures



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