Wednesday 30 May 2018

Massive floods in Ellicott City, Maryland

How Climate Change Contributed to Ellicott City’s Back-to-Back Historic Flood Events

29 May, 2018

Back when climates were more stable during the 20th Century, we would have expected Ellicott City to see the type of severe rainfall that occurred during 2016 to happen once every thousand years or so. But with the present global climate amped up by human forced warming, just two years elapsed before another such ‘1 in 1,000’ year event hammered the region yet again.


(Climate change related factors that contributed to the second 1 in 1,000 year flood event to strike Ellicott City within two years.)

Back in 2016, a massive thunderstorm complex dumped 6 inches of rain over the Ellicott City region within just two hours. The storm crippled the downtown of this historic city, severely damaging more than 25 buildings, resulting in the loss of two lives, and spurring calls for a moratorium on development in such low-lying areas. This was the heaviest rainfall ever to hit Ellicott City on record. And though the city had experienced floods before, it had never seen so much heavy, short-duration, local precipitation.

Typically, Maryland does not see such severe rainfall amounts. In a normal climate, Ellicott City would expect to see such high rainfall totals once every 1,000 years. However, the climate is no longer normal. Atmospheric greenhouse gasses are now at higher levels than at any time in at least 15 million years. The Earth is warming up. Atmospheric moisture levels are rising. Severe rainfall events are proliferating across the world. And off the U.S. east coast, the Gulf Stream is slowing down as Greenland disgorges more of its ice.

(The second recent Ellicott City Flood dumped an amazing 9.6 inches of rain within just three hours. Climate change increases the ability of the strongest storms to generate more intense downpours by loading up the atmosphere with moisture and by increasing instability in certain regions. The Eastern U.S. is particularly prone to increasing rainfall intensity due to a warming Gulf of Mexico, a warming North Atlantic off the U.S. East Coast and due to an instability-generating cool pool off Greenland fed by glacial melt water. Image source: Radar Scope and The Washington Post.)

The Eastern U.S., in particular, is seeing increased potentials for heavy precipitation as a climate change related cool pool off Greenland is causing polar air masses to come into conflict with rising heat and increasing levels of atmospheric moisture streaming up from both the Gulf of Mexico and the middle Latitudes of the North Atlantic.

As a result, the chances that Ellicott City would see another historic severe storm of this kind were greatly increased. So much so that the region was hit again on May 27 of 2018. This time by a storm that was more severe than the one that occurred during July 30 of 2016.

(Due to human-forced climate change, the intensity of rainfall events, particularly in the strongest storms, is increasing across the U.S. with the greatest increase over the eastern half of the country. This record of increasingly severe rainstorms due to human-forced climate change isn’t just limited to the U.S. It is a global phenomenon. Image source: The National Climate Assessment and Katherine Hayhoe.)

This larger flood dumped 9.6 inches of rain just east of Ellicott City within only three hours. Hourly rainfall amounts during this event likely exceeded 4 inches at the storm’s peak intensity. And a new wave of massive flooding ripped through the historic district — re-damaging buildings that were on the verge of recovering and again resulting in loss of life.

Unfortunately, the climate dice are now loaded for more such ‘1 in 1,000 year events.’ So what happened in Ellicott City this weekend should not be attributed to some ‘fluke local weather event.’ Climate change increases the potential for these kinds of storms. So we’ll see more and worse such instances as we keep warming up our atmosphere, heating the oceans, and melting glaciers.

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