Hurricane Harvey: Connecting the dots between climate change and more extreme events

Climate Code Red

The science of attributing extreme weather to climate change is complicated and developing every day. Here’s a guide of what we know about the links between climate change and Harvey to help unpack the elements that contributed to this historic and unfolding storm. For a complete annotated backgrounder, visit the related events page on Climate Signals.

Warmth

As seas warm, more water evaporates to the atmosphere. A warmer atmosphere can hold more water, fueling extreme rainfall and increasing flood risk. Record-breaking rainfall is a classic signature of climate change, and the fingerprint of climate change has been firmly identified in the observed global trend of increasing extreme precipitation.

• Many areas of Southeast Texas have received rain so extreme that historical data indicates it should only happen once every 1,000 years.
•  Houston is experiencing its third ‘500-year’ flood in 3 years.
•  Since the 1950s, Houston has seen a 167 percent increase in the frequency of the most intense downpours.
•  A rain gauge in Mont Belvieu, about 40 miles east of Houston, registered 51.88 inches of rain through late afternoon Tuesday. Once verified, this amount would break not only the Texas state rainfall record but also the record for the remaining Lower 48 states.
•  A formal attribution study of last year’s historic flood in Louisiana found that climate change to date had most likely doubled the frequency of the extreme rainfall that drove that flood.
• Kevin Trenberth, a senior scientist at the U.S. National Center for Atmospheric Research, said, “The human contribution can be up to 30 percent or so of the total rainfall coming out of the storm.”

Stalled weather

Another major contributor to the extreme rainfall totals was that Harvey stalled for many days over southeast Texas. Waves in the jet stream can stall in place (instead of moving eastward), leading to blocking and persistent weather patterns that fuel the intensity and duration of rainfall events.

• A study from March 2017 found that climate change is altering large scale weather patterns, such as the jet stream, which have the ability to dramatically amplify extreme weather events, such as extreme rainfall, during the summer.
• According to Michael Mann, the stalled weather pattern during Hurricane Harvey “is precisely the sort of pattern we expect because of climate change.”

Sea level rise

Sea level rise has significantly extended the reach of storm surge and coastal flooding driven by hurricanes.

•  While Harvey’s storm surge was not nearly as extreme as the observed rainfall, the two combined to create what is known as “compound flooding“. Rivers rushing toward the Gulf coast met the storm surge coming inland and water “piled up from both sides”. In Galveston, the sea surge was about 3 feet but the actual water surge was about 9 feet due to compound flooding.
•  A storm tide level of 7.0 feet above Mean Sea Level (MSL) was observed at Port Lavaca, Texas, during the storm—the highest storm surge in that location since Hurricane Carla in 1961.

Energy

The warmer the waters, the more energy available to passing storms, increasing the risk of major hurricane development. Climate change also affects other factors that shape and control hurricane development, such as wind shear. The balance of all these factors is not fully known. However, hurricanes have grown stronger over recent decades. And there is a significant risk global warming may be driving that trend.

•  Harvey rapidly intensified from a regenerated tropical depression into a Category 4 hurricane in the Gulf of Mexico, aided by sea surface temperatures up to 2.7 – 7.2°F (1.5 – 4°C) above average.
•  Last winter, the daily surface temperature of the Gulf of Mexico never dropped below 73°F for the first time on record.

Rapid intensification

There is an observable trend toward increasingly rapid intensification of hurricanes, leaving less time to prepare. There is a significant risk that this trend is driven by global warming.

• Harvey’s rapid intensification is consistent with the observed trendtoward rapidly intensifying tropical cyclones, particularly in the North Atlantic and Caribbean.

Source: Climate Code Red. Reproduced with permission.