Those storms, followed by Hurricane Katrina a year later, sparked a massive debate about the relationship between hurricanes and our changing climate system. And it was one that proved difficult for scientists to resolve, in part because of the inadequate records of hurricanes before the development of reliable satellite systems, which allowed better tracking.
Yet even as scientists debated, politics and popular culture didn’t wait for their answer. Hurricanes quickly became a symbol of a warming world. This was, of course, problematic — the science linking hurricanes and climate change was contested. There was little doubt that a changing climate would change hurricanes, given that they are fundamentally fueled by ocean heat — but precisely how that would play out and whether the changes had already begun was hard to say.
Hurricanes have — with a few key exceptions, like Sandy — been quieter for the U.S. since the bowling alley years of 2004 and 2005. Yet as Hurricane Matthew has devastated the Caribbean nations of Haiti and Cuba, as it thrashes the Bahamas, and now, barrels toward Florida, people (including some scientists) are drawing the connection again — and quickly getting criticized for it.
The conservative Newsbusters.org faulted NBC’s Ron Allen for saying, in covering President Obama’s celebration Wednesday of the Paris climate agreement entering into force:
“It’s very interesting that this is happening on a day when there’s a hurricane bearing down on the United States and in the Caribbean because these severe storms, beach erosion, intense weather episodes that we’ve had are perhaps the most practical sample of what the president is talking about as the threat that the planet faces.”
“Scientists have pointed to Hurricane Matthew as the sort of fierce lashing that will become more common due to climate change,” adds Oliver Milman in the Guardian.
Such is the tenor of the debate — but what can we really say about the matter?
The science on the hurricane-climate link has progressed slowly but surely since the mid-2000s. A host of new scientific publications have emerged. Meanwhile, the globe itself has done little to quell fears about worsening hurricanes, with the strongest storm (for wind speeds) ever recorded in the East Pacific basin and perhaps anywhere on the Earth, Hurricane Patricia, striking just last year.
Researchers now think that a warming climate, by heating the oceans, will indeed make hurricanes more intense (on average), even though it may not increase their overall numbers (in fact, those may decrease). These storms will also dump more damaging rain in the future, as the atmosphere holds more water vapor on a warming planet.
But are clear changes already upon us, specifically in the North Atlantic region? That’s more tricky. A recent scientific overview of what we know about the hurricane-climate connection put it this way:
“While no significant trends have been identified in the Atlantic since the late 19th century, significant observed trends in [tropical cyclone] numbers and intensities have occurred in this basin over the past few decades, and trends in other basins are increasingly being identified. However, understanding of the causes of these trends is incomplete, and confidence in these trends continues to be hampered by a lack of consistent observations in some basins.”
That’s murky — and no wonder. Trying to figure out what’s going on in the Atlantic with hurricanes involves not only understanding what potential role a warming planet might play, but also factors like the role of natural cycles (such as the so-called Atlantic Multidecadal Oscillation) and even the impact of reductions in atmospheric aerosols that occurred following tighter regulations on these emissions.
It is furthermore very different to talk about an individual storm, like Matthew, than it is to talk about trends for storms. A climate influence would be scientifically detected in the aggregate, not in the anecdote. As one has to endlessly repeat, no individual storm is causally attributable to a changing climate.
It’s also important to note that it hardly takes a changing climate for Florida to be at risk of hurricanes. That’s basically inherent to its location in the world. “It’s a fairly run of the mill hurricane, actually,” MIT hurricane expert Kerry Emanuel said.
And yet, when it comes to a storm like Matthew, there are reasons to think that the way we’re changing the planet is relevant to some attributes of the storm. As Kevin Trenberth, a researcher with the National Center for Atmospheric Research in Boulder, Colo., put it to me, there seems to be a combination of the overall warming trend and natural variability, such as the El Nino-La Nina cycle, behind what we’re seeing:
“The overall increase in moisture is about 5 to 6% from climate change, and in a hurricane that gets doubled because the storm intensifies and increases the convergence of moisture. But in the Atlantic, in the year following El Nino, the [sea surface temperature] tends to be higher in the subtropics (because with El Nino the winds are lighter and more sunny skies), and indeed in the subtropics east of Florida, Georgia and South Carolina, [sea surface temperatures] have been running 2 deg C (3 to 4F) above normal, and moisture 10 to perhaps 15% above normal. Indeed this was the region that fed the Louisiana floods (not so much the Gulf). So the potential has been there: a natural variability component on top of the global warming to produce a very strong storm.”
Matthew also showed a very rapid rate of escalation to Category 5 status, and a long persistence at very strong hurricane strength, that is noteworthy. Explosive intensification and long life at intense strength are certainly the kinds of attributes we’d expect to see more often as a warming climate heats the oceans and provides more fuel for the most intense hurricanes.
“The rapid intensification certainly is up there,” said Greg Holland, a hurricane expert at the National Center for Atmospheric Research. He called the rate of intensification for Matthew “in the very top echelon.”
“Its rapid intensification to Category 5 when it was in the southern Caribbean was not forecast by any model, including my own. Right now, it looks a little bit mysterious,” added Emanuel.
These storm traits aren’t proof of anything, of course — they’re merely consistent with the notion of warming making storms worse.
In truth, though, perhaps the most direct way in which a changing climate affects the storm involves sea level rise, several researchers said. It is hard to dispute that, as sea level ticks steadily upward year after year, a place like Florida grows more imperiled by storms that can hurl large parts of the ocean inland.
In just the time between Hurricane Andrew, which hit South Florida as a Category 5 storm (but a small one) in 1992, and today, a “conservative” estimate would be that seas around Florida have probably risen about 2.5 inches, says Ben Strauss, a specialist on sea level rise at Climate Central. That may not sound like much, but even such a small sea level rise has a big effect.
“Every inch of sea level rise matters, much more than intuition would suggest,” Strauss said. “In Florida alone, more than 30,000 people live on land for each vertical inch above the high tide line, averaged across the first six feet. And if you’re a homeowner, there’s a huge financial difference between a flood which just reaches your lowest electrical outlet, and one which doesn’t; or which just floods your first floor, or doesn’t.”
And if you were to go back even earlier than 1992 — say, to 1940 or 1950 — the sea level rise between then and now would, naturally, be considerably larger. Sea level rise “makes a difference for the susceptibility of the coastline for surges,” said Emanuel, referring to the difference in sea level between the middle of the 20th century and today.
So in sum — even as people will inevitably invoke climate change to discuss Matthew, any precise attribution remains complex and the science isn’t settled on precisely what is happening with hurricanes in the Atlantic.
Still we’re living in a warming world with more moisture and higher seas, and it’s hard to dispute that that matters.
Read more at Energy & Environment: