With the torrential rains of Hurricane Harvey, the historic winds of Irma, and Jose still meandering slowly through the Caribbean, the last few weeks have been full of powerful and frequent hurricanes. If the 2017 hurricane season continues like this, it could join 2005 and 2010 as one of the most intense hurricane seasons in recorded history.
Scientists say climate change has probably played a role in shaping this year’s big storms and those previous monster seasons. When scientists build models of the climate system — digital worlds where the physics of a warming atmosphere can spin out thousands of possible futures — they see a clear connection between a hotter planet and hurricanes. In those model Earths, a warmer world makes storms stronger and increases the rainfall they leave behind. In the real world, the planet has gotten warmer, the oceans have gotten warmer, and here we have these intense storms — it all seems to add up.
But, frustratingly, it’s harder to look at the actual hurricanes that have happened and find evidence of those changes playing out the way we’d expect. The planet’s average global temperature has increased, but its hurricanes don’t seem to have changed much. We can tie hurricanes to climate change in theory, but we don’t see a statistical signal of those theories playing out in practice. It could be that the climate hasn’t changed enough yet. It could be that hurricanes have changed, just not in a way that’s apparent statistically. We can’t be sure.
What would it take to change that? Is there data we don’t have now that could make the connections between climate change and hurricanes more explicit? I asked several scientists who study these issues about the data they wish they had. They told me that the hole in our present understanding is tied to a lack of knowledge about what happened in the past. We don’t know if climate change is altering hurricanes now, because we know very little about what hurricanes were doing 100 years ago.
We’re not lacking a historical record of hurricanes. We’re just lacking a complete one. The Atlantic Hurricane Database goes back to 1851, and this raw data suggests that the number and intensity of Atlantic hurricanes has significantly increased over time, and that the increase is correlated with rising surface water temperature. But there’s a catch, said Thomas Knutson, leader of the climate impacts and extremes research team at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory.
Prior to the 1960s, hurricane records were primarily made in two ways: either a storm made landfall in a place that had a weather station reporting back to the U.S. government, or the hurricane was spotted by ships at sea. But not every hurricane strikes the shore, and even if it does it’s not guaranteed to be at its peak intensity. Nor are there sailors in every storm’s path. That means the database doesn’t document all the hurricanes that occurred, or their true strengths. Knutson was part of a group of scientists who analyzed historical ship traffic patterns to estimate how many hurricanes went unnoticed by the official record books. They compared recent hurricane years with those old ship routes and counted how many modern storms wouldn’t have been seen by the old methods of hurricane detection. They found that the difference between reported hurricanes and the likely number of actual hurricanes was big enough to wipe out that trend in increasing hurricane frequency.
After 1965, hurricane data is based on satellite imagery, said Gabriel Vecchi, professor of geosciences at Princeton University. And it’s no big surprise that, after that date, the number of hurricanes on record dramatically rises, as do accurate measurements of wind speed and pressure. This change in tools drastically improved our understanding of the storms and our ability to predict their paths, he said. But the methodological change means the data is contaminated, and can’t easily be used to compare a modern hurricane season to the past. “If I could fix that issue, if I could magically put up a satellite 150 years ago over the Atlantic, that would go a long way towards answering the question about whether these trends are actually real,” he said.
There are some ways to tease more knowledge of hurricanes out of the distant past. Paleotempestology is a field that documents patterns in historic hurricane landfall by looking for evidence in the lands they hit. For instance, Vecchi told me, scientists might look for recurring layers of marine sediments near a freshwater pond on a hurricane-prone island — a possible echo of ocean water driven inland by hurricane forces. That can tell you when an island was hit by a hurricane, and how long it took before another one struck. In some cases, this data can flesh out hurricane strike patterns back 1,000 years — but only for very specific locations. Insurance companies are very interested in this, Knutson said. But the data doesn’t necessarily tell you a lot more about how many hurricanes were in the Atlantic each season. The mysteries of the past still remain.
Filling in the gaps in historical data is not a problem unique to hurricanes, of course, but it is especially difficult — at least compared to some other aspects of the climate system and how those are affected by anthropogenic climate change. For instance, what we know about global average temperature changes is also based on incomplete historical records. But there are more data points for temperature — more recording stations in more places, documenting a thing that happens every day, rather than a rare event that happens a few times a year, said Robert Tuleya, a climate modeler at Old Dominion University’s Center for Coastal Physical Oceanography.
The physics that connects rising levels of greenhouse gasses in the atmosphere to rising global temperatures is also a lot less complex, with a clearer path from cause to result, than the physics that connects greenhouse gases to hurricanes. For instance, although rising sea surface temperatures would tend to increase the ferocity of hurricanes — and, indeed, the historic hurricane seasons of 2005 and 2010 were associated with higher water temperatures — rising atmospheric temperatures would tend to make storms milder, Knutson said. So the effects of climate change can have contradictory impacts on hurricane formation. Moreover, Vecchi said, although measuring temperature in a specific spot can tell you something about temperatures nearby, the same isn’t true about hurricanes. The absence of a storm in Louisiana is not evidence of a storm’s absence in Mississippi.
Finally, there’s evidence that Atlantic hurricanes go through multidecadal cycles when their intensity and frequency waxes and wanes. The early 1970s to mid-1990s appear to contain something of a hurricane drought, compared to what came before and after, Vecchi said. But, because of the problems with the historical data, we don’t really know whether these cycles are a product of natural variability, or, say, the result of increases in dust and factory pollution after World War II, or some combination of both. Which also means that, when we see a trend in increasing hurricane number or ferocity since the 1960s or ‘70s, we don’t know whether that’s a product of climate change, a natural cyclical change, other kinds of human-caused changes to the climate system — or all three at once.
The result of all this is that it’s much easier to spot climate change-related patterns in temperature than it is in hurricanes. The trick is to spot the signal in the noise, Knutson said. And the noise is louder for hurricanes.
Our uncertainty surrounding hurricanes and climate change is built on a data gap that can’t be filled. Barring Vecchi’s magical time-traveling satellite and an abundance of localized information gleaned from paleotempestology, there’s no real way to get what we don’t already have. The only way we’re going to see a clear signal of climate change in real-world hurricane data is to collect more of it, as we go forward through time. We’ll know it when we see it, basically.
But, Vecchi said, we don’t actually need to fill the historical data hole to know whether climate change is increasing the risks hurricanes pose to humans. All we need to know is whether sea level is rising — an effect of climate change that Vecchi’s first-year students replicate in the lab every year, and for which there is a clear statistical signal. Temperatures rise, molecules of water expand, sea levels rise. It’s relatively simple, and it means that even a workaday hurricane of today can produce a higher storm surge than its historical counterparts. The water is already higher to begin with. That means the risks of hurricanes are getting bigger, Vecchi said, regardless of whether climate change altered the storm.