We’ve covered several studies seeking to clarify the role of human-caused climate change in the unbelievable amounts of water Hurricane Harvey dumped on Houston last year. The general approach of these studies was to simulate today’s climate and a pre-global-warming climate, and to then compare the behavior of hurricanes around Houston.

Since most people understand that hurricanes are fueled by warm ocean water, however, perhaps it would be conceptually simpler to focus on the seawater beneath Harvey.

That has now been done by a new study led by Kevin Trenberth of the National Center for Atmospheric Research. By analyzing the heat energy present in the Gulf of Mexico—and how much was lost as Harvey spun through—researchers get a fairly direct measure of Harvey’s fuel.

As long as wind speeds at higher altitude aren’t fast enough to tear a baby cyclone apart, sea surface temperatures over 26°C can grow that baby into a tyrant. Warm, moist air over the water gets pulled into the storm’s low pressure center, where it buoyantly rises. As it begins to cool, some of the water vapor condenses—which releases a considerable amount of heat, which keeps the air rising. Just as evaporating sweat carries heat energy away from your skin, evaporating seawater carries heat energy away from the ocean and up into the engine of the hurricane.

By measuring the change in ocean heat energy as a hurricane passes, you can work out how much energy went into its engine. The difficulty is that we only have so many floats and ships out there making measurements, so you rarely have enough data to reliably calculate that fairly rapid change. In this case, the researchers compared average heat energy in the upper ocean in the Gulf of Mexico for August 1-20 (just before Harvey) with September 1-20. Satellites now measure sea surface temperatures every day, although they can’t reach below the surface to really measure total heat energy.

When Harvey entered the Gulf of Mexico, it encountered record ocean heat energy (top graph in the image below). That wasn’t just random chance—average sea surface temperatures have increased about 0.6°C since 1960 (lower graph below). Global warming obviously includes warming seawater, which means greater ocean heat energy.

As Hurricane Harvey grew strong feeding on that energy, it left water about 1° to 2°C cooler in its wake (but still above the 26°C hurricane threshold). Doing the math, the researchers estimate this represents about 590 billion gigajoules of heat energy lost by the upper ocean. Want another big number you can’t really fathom for comparison? That’s like the ocean powered two 100 watt incandescent lightbulbs for every square meter of the Gulf of Mexico… for a month.

As that energy was released when rain condensed to fall back down, you can balance the energy ledger by adding up the total rainfall. At about a quarter of a quadrillion liters of water, this represents around 600 billion gigajoules of released heat energy—which compares pretty well to the 590 billion gigajoules of estimated heat energy lost by the ocean.

The point here is that you can tie Harvey’s extreme rainfall directly to the ocean heat it consumed. And with ocean heat energy at a record level after decades of global warming, it’s pretty difficult to argue that humans didn’t juice the storm.

The researchers also add a brief discussion of how well we are preparing for disasters like 2017’s many deadly Atlantic hurricanes. Efforts to respond to climate change are usually grouped into two categories: mitigation (slowing warming) or adaptation (protections to limit harm). The authors remind us that there’s a third option illustrated by these disasters: do little and “suffer the consequences.”

“Houston has been beset with three 500-year floods in 3 years prior to Harvey, and Miami regularly experiences ‘sunny day’ flooding with high tides,” they write. “Why was there reportedly only one in six with flood insurance in the Houston area and Florida? Why have various flood mitigation measures not been enacted?”

If we know the fuel for storms like Harvey is increasing, it becomes even more urgent to stop dragging our feet and protect the communities that could be next in harm’s way.

Earth’s Future, 2018. DOI: 10.1029/2018EF000825 (About DOIs).