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 May 09, 2018
Because of climate change, hurricanes are raining harder and may be growing stronger more quickly

 Two studies published in the past week have troubling implications for the effects hurricanes have on society because of climate change, now and in the future.

One directly links Hurricane Harvey's disastrous rains to the amount of heat stored in the ocean, which was record-setting before the storm plowed into Texas last year. The other shows an increasing trend in storms that are becoming really strong, really fast.

Storms that unload more rain and explosively intensify cause more destruction and suffering, as the 2017 Atlantic hurricane season painfully made clear. Harvey, Irma and Maria each ranked among the five costliest hurricanes on record.

These studies suggest that future storms will carry even greater damage potential.

Record-setting ocean heat content linked to Harvey's epic rains

Hurricanes are fueled by heat, and in late August, before Harvey struck, ocean heat content reached record high levels in the western Gulf of Mexico.

The study determined that the energy released into the atmosphere from Harvey's rainfall matched the amount of energy which was removed from the ocean in the storm's wake. In other words, the study found that the amount of heat stored in the ocean is directly related to how much rain a storm can unload.

The implication is that if climate change, driven by increasing greenhouse gases from human activity, increases the heat content of the ocean, storms passing over it will be able to draw ever more moisture that they can unload as rain.

Harvey dumped more than 60 inches of rain in parts of Southeast Texas, the most ever recorded from a single storm in the United States in recorded history.

"Record high ocean heat values not only increased the fuel available to sustain and intensify Harvey, but also increased its flooding rains on land," the study said. "Harvey could not have produced so much rain without human-induced climate change."

The study, published in the journal Earth's Future, said the added ocean heat content not only increases a storm's rainfall but also "invigorates and enlarges the storm," turning it into an even greater rain-producer. Two independent studies found climate warming boosted Harvey's rainfall by about 20 to 35 percent.

Kevin Trenberth, lead author of this latest study, called "Hurricane Harvey links to Ocean Heat Content and Climate Change Adaptation," is a climate scientist at the National Center for Atmospheric Research who has published other studies that connect human activities and increasing hurricane intensity.

Rapid intensification events on the rise

During the 2017 hurricane season, Harvey, Irma, Jose and Maria all underwent what is known as "rapid intensification." This means their peak wind speed rose at a torrid pace, by at least 29 mph in 24 hours, according to one definition (alternative definitions vary).

Maria, for example, strengthened from a Category 1 to Category 4 storm on the Saffir-Simpson wind scale in 12 hours. The storm would go on to decimate the island of Dominica and, ultimately, St. Croix and Puerto Rico.

A study in the journal Geophysical Research Letters found that the magnitude of these rapid intensification events increased from 1986 to 2015 in the central and eastern tropical Atlantic Ocean.

From 1986 to 2000, the average storm that rapidly intensified saw its peak winds increase by 32 mph in 24 hours, but the increase was 36 mph in 24 hours from 2001 to 2015.

The study analysis did not detect a trend in these events in the western tropical Atlantic, which includes the Caribbean Sea and Gulf of Mexico.

Even so, the study said its results "have substantial implications" for the eastern Caribbean Islands, because they can lie in the path of hurricanes that form over the central and eastern Atlantic and head west. And it is in this region where the study detected an uptick in rapid strengthening events and also where Irma, Jose and Maria became such behemoths.

The increase in these events occurred during a period when upper ocean temperatures heated up. The study concluded that it could not "rule out" that the buildup of greenhouse gases from human activity was to blame. But it mostly attributed the change to a cycle known as the Atlantic Multidecadal Oscillation, which is in its warm phase.

Scientists differ on whether this cycle or human activity is driving trends in tropical Atlantic Ocean temperatures. Michael Mann, a climate scientist at Penn State who was not involved in the study, has published work that demonstrates a dominant man-made role in the temperature increase. In an email, he said the study's attribution to this cycle, rather than human activity, was "not justified."

A need to plan and adapt

The study on the connection between Harvey's rains and the high ocean heat content devotes an entire section on the importance of planning for even more "supercharged" hurricanes. "[T]he risk is clear, and preparations for expected effects of climate change on hurricanes and more generally are woefully inadequate," the study says.

It calls for vulnerable zones to "build resilience and plan for inevitable impacts" or "suffer the consequences." It suggests:

*Increasing engineering mitigation measures (such as levees and sea walls, flood control)

*Adhering to building codes

*Preventing building in flood plains

*Stopping unbridled growth

*Hardening infrastructure

*Managing water and drainage systems

*Developing emergency response plans including evacuation routes and their implementation along with emergency shelter and power supplies

*Providing property flood insurance that matches the true and changing risk

"Given the price tag with units of hundreds of billions of dollars for the recent hurricanes, a modest (two orders of magnitude less) investment in building resiliency may well have saved billions and a lot of grief," the study concluded.

Atlantic hurricane season officially begins in less than three weeks.