How did climate change affect Hurricane Harvey?

Reference:
Emanuel, 2017: Assessing the present and future probability of Hurricane Harvey’s rainfall, Proc. Natl. Acad. Sci., 114 (48), 12681-12684.  doi:  https://doi.org/10.1073/pnas.1716222114

 

It finally feels like summer, and you know what that means: it’ll be hurricane season soon!  You might be wondering how climate change affects the severity of hurricane damage. To address this question, let’s revisit the impact of climate change on Hurricane Harvey, which devastated the Houston area last August.

 

Did climate change “cause” Hurricane Harvey?

In the wake of 2017’s destructive hurricane season, many of us have wondered if climate change “caused” disasters like Hurricane Harvey.  Unfortunately, hurricanes are so complex that this question is practically impossible to answer. A more robust approach is to shift our focus from a single hurricane to many, and to ask how climate change affects the hurricanes on average. To understand how climate change affects hurricane season, we need to answer these two questions:  

 

  1. How does climate change affect the probability of a hurricane occurring?  
    For example: Do we expect a storm like Harvey to strike once a century, or once a year?

 

  1. Given that a hurricane makes landfall, how does climate change affect the damage it causes?
    For example: If a hurricane strikes Texas, how does climate change affect how much it rains?

 

In a study published last year, Dr. Kerry Emanuel from the Massachusetts Institute of Technology tackled these questions in relation to Hurricane Harvey.

 

Why was Harvey so destructive?

Harvey is both the most costly hurricane on record and the wettest hurricane to strike the United States.  Its rainfall totals were so extreme that the National Weather Service had to revise the color scheme of its rain maps!  This is partly a result of Harvey’s movement stalling over Texas, causing some areas to received multiple days of hurricane-strength precipitation.

Interstate 45 in Houston, before and after Hurricane Harvey. Image at left courtesy of Shutterstock, image at right courtesy of REUTERS/Richard Carson; images paired by Business Insider.

It’s no coincidence that Harvey broke records for both cost and rainfall.  Historically, hurricane damage has been most severe in the context of flooding, so it’s important to know how flooding will be affected by climate change.  In this study, Emanuel focuses on the implications of climate change for one major contributor to hurricane flooding: how much rainfall occurs .

 

How do you model hurricane rainfall in a changing climate?

It is widely expected that hurricanes will become wetter as temperatures rise.  Warmer air can hold more water vapor, so a warmer hurricane contains more water that can eventually become rain.  To quantify expectations from this and other possible effects of climate change, Emanuel combines climate predictions with a hurricane model.

Predictions for future climates are usually carried out using general circulation models (GCMs), which are global models that simulate the evolution of the atmosphere and oceans over long periods of time.  GCMs have low resolution because they cover a global domain. This means that while GCMs are very useful for predicting climate trends over large areas, they’re not quite as good at simulating something small, like a single thunderstorm or hurricane.

Example of climate change predictions based on a GCM. GCMs are usually run under several different scenarios for atmospheric CO2 concentrations. Each of these scenarios is known as an RCP, or Representative Concentration Pathway. RCP 2.6 is the most optimistic climate change scenario, whereas RCP 8.5 is the least optimistic. Image credit: IPCC AR5 Working Group I, Summary for Policymakers.

To use GCM predictions for climate change and also simulate hurricanes realistically, Emanuel and his colleagues used their own hurricane model but incorporated the GCM climate predictions to describe the hurricane’s environment.  A hurricane’s environment is extremely important for how it develops – for instance, we only really see hurricanes in very warm, humid regions.

All the hurricane tracks in the Atlantic and East Pacific. You can see how all the hurricanes seem to start from within the white box, which is known as the Main Development Region because the environment there is very favorable for hurricane development. Image Credit: National Hurricane Center (slightly modified).

Climate change may alter temperature, humidity, and winds, so Emanuel took GCM predictions for these and plugged them into his hurricane model.  He simulated thousands of hurricanes for each climate scenario, and then examined how the statistics of the simulated hurricanes were different between one climate scenario and the next.  These differences can tell us a lot about how different climate conditions can impact hurricane strength.

 

How will climate change affect hurricane rainfall?

Emanuel’s study demonstrates that Harvey-strength hurricanes will become more likely to strike Houston as climate change progresses.  He measured this likelihood by using a concept called a return period, which is the time before you expect another event with the same severity to strike again.  A high return period means that the event doesn’t happen very often, whereas a low return period means that it happens all the time.

Emanuel found that the return period for Houston to receive rainfall on the scale of Hurricane Harvey is going to drop by 20 times over the coming century.  This is a very sobering result, especially for those who belong to coastal, hurricane-prone communities. It can take years to recover from hurricane damage.  As you probably know, our fellow citizens in Puerto Rico are still struggling to get back power.

 

What does this mean?

Emanuel’s analysis suggests that, as a result of climate change, a Harvey-like deluge will become more likely by the end of the century.  In fact, Harvey may already have been more likely in 2017 than it was in the 1980s. Hopefully these findings will help people prepare for hurricanes in the future.

 

Title Image: Harvey on August 24, 2017, shortly after re-consolidating into a tropical storm.  Image courtesy of GOES-16, displayed by the NOAA RAMMB/CIRA slider.

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Rohini Shivamoggi

I'm a PhD student studying atmospheric sciences at MIT. I study the formation of secondary eyewalls in hurricanes, which hopefully will help us improve our forecasts of hurricane intensity! Before I got to MIT, I grew up in Florida and studied Chemistry and Physics at Harvard University. My other interests include weather forecasting, photography, and encouraging diversity in STEM! You can find me on Twitter @RShivamoggi.

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