Hurricanes are often seen as heralds of global warming, which makes sense. They're fueled by warm seawater, and the warming of Earth's oceans coincides with a recent spike in hurricane activity. Plus, scientists have grown increasingly confident that global warming — caused by greenhouse gases in the atmosphere, including those emitted by human activities — raises the risk of severe weather.
But has global warming already started affecting hurricane intensity? Can the planet's climbing temperatures help explain monster storms like Hurricane Katrina? Those are tricky questions, but the world's top climate experts are hard at work trying to answer them. One of those experts is Thomas R. Knutson, a research meteorologist with the U.S. National Oceanic and Atmospheric Administration (NOAA) who studies Atlantic hurricane activity and the impacts of greenhouse gas-induced warming.
"It's premature to conclude there is a significant long-term trend present," Knutson tells MNN, although he adds that's simply due to a lack of long-term data. "Our most reliable intensity records go back to 1980 or so, but things are a little trickier if you try to figure out if intensities were greater in the 1950s versus recently, or if there's a rise over time. That's more difficult to answer because of limitations in the data sets."
That said, Knutson and many of his colleagues expect global warming to eventually boost hurricane intensity, based on their knowledge of how hurricanes work as well as the forecasts of advanced computer models. Thanks to those models, scientists can simulate storms under past, present and future conditions, helping them recreate the recent uptick in storms and project what might happen next.
"These models are indicating, at least the higher-resolution models, a greater intensity of hurricanes in the warmer climate, even though some models have fewer hurricanes overall," Knutson says. "So the picture that's emerging is fewer tropical storms and hurricanes globally, but the ones we have would be a little more intense than the ones we have today, and the rainfall amounts would also be greater."
One measure of hurricane intensity is the power dissipation index (PDI), developed by MIT atmospheric scientist Kerry Emanuel to measure how much power a cyclone releases during its life span. Below is a time series, produced by Emanuel, that shows tropical Atlantic sea-surface temperatures (SSTs) each September compared with the annual PDI of hurricanes. (Note: The yearly data are smoothed to emphasize fluctuations on time scales of at least three years.)
The graph shows a strong correlation between SSTs and how much power a hurricane releases, and also reveals that the overall PDI of Atlantic storms has doubled since the 1970s. But it's worth noting this isn't due to rising SSTs alone, Knutson says. That's because other natural and manmade factors are also at work — namely the multidecadal variation in Atlantic hurricane intensity, some of which may be due to a different kind of anthropogenic emissions: aerosols.
"It's possible that aerosols over the Atlantic have caused some changes in hurricane activity over time, and I'm thinking specifically of the relative lull in activity in the 1970s and '80s," Knutson tells MNN. "That's an example of a possible anthropogenic effect on hurricane climate activity, but not strictly a long-term trend like you'd expect from the effect of greenhouse gases. There are some preliminary indications that aerosol forcing may have caused at least part of that temporary reduction."
That leads some skeptics to argue the past decade's hurricane flurry is only a rebound from this lull, but Knutson says there's increasing evidence it's not that simple. And while it would be premature to blame observed PDI increases entirely on manmade climate change, the latter is still widely forecast to affect the former at some point this century, even if its influence isn't clearly detectable in the data for several decades.
"There are better than even odds that anthropogenic warming over the next century will lead to an increase in the numbers of very intense hurricanes in some basins," according to a NOAA overview written by Knutson, who adds this "would be substantially larger in percentage terms than the 2-11% increase in the average storm intensity." These two graphs project this through 2100, with the first modeling hurricane activity based on local tropical Atlantic SST, and the second modeling it based on tropical Atlantic SST relative to average SST from the rest of the tropics:
Image: NOAA GFDL
There may be fewer tropical storms overall in coming decades, but one high-res model predicts "a doubling of the frequency of very intense hurricanes in the Atlantic basin by the end of the 21st century," according to NOAA. Used in a 2010 study Knutson co-authored, this model not only foresees twice as many category 4s and 5s in 90 years, but also tells researchers "the effect of increasing category 4-5 storms outweighs the reduction in overall hurricane numbers such that we project (very roughly) a 30% increase in potential damage in the Atlantic basin by 2100."
Much of this damage would be wrought by wind, since category 4s and 5s are defined by wind speeds of at least 130 mph. Storm surges are another threat, and Knutson says warming could amplify these regardless of its effect on cyclones themselves.
"Even if hurricane activity overall were to remain unchanged in the coming century, I would still expect an increase in the risk of coastal flooding from storm surges just owing to the sea-level rise, because the hurricanes would occur on a higher baseline sea level." And compared with hurricane activity, he adds, "there is relatively more confidence in attributing past sea-level rise at least in part to human influence, and higher confidence that sea-level rise will continue in the coming century."
Aside from wind and seawater, scientists also point to another peril that many people overlook: freshwater floods from heavy rain. As seen with recent U.S. hurricanes like Irene in 2011 and Isaac in 2012, rain is sometimes the deadliest part of a tropical storm. And as oceans heat up, that risk is likely to rise, too. "Anthropogenic warming by the end of the 21st century will likely cause hurricanes to have substantially higher rainfall rates than present-day hurricanes," Knutson says, noting that models project a 20 percent average spike in rainfall within 60 miles of a storm's center.
To illustrate how warmer seawater might affect the frequency of category 4 and 5 hurricanes, the graphic below models their behavior under two scenarios: the current climate and a warmer climate in the late 21st century. It's virtually impossible to accurately predict hurricane tracks even a few days in advance, but this graph offers a general idea of how things could change over time:
Image: NOAA GFDL
Despite a general agreement that warmer seas will yield more intense cyclones, there is still widespread caution not only in blaming climate change for individual storms, but also in blaming it for any tropical cyclone activity to date.
"[W]e estimate that detection of this projected anthropogenic influence on hurricanes should not be expected for a number of decades," Knutson writes. "While there is a large rising trend since the mid-1940s in category 4-5 numbers in the Atlantic, our view is that these data are not reliable for trend calculations until they have been further assessed for data homogeneity problems, such as those due to changing observing practices."
Nonetheless, this caution shouldn't necessarily be seen as doubt. Some skeptics conflate the recent drop in U.S. landfalls with an overall drop in high-intensity hurricanes, for example, ignoring storms that hit other countries or remain at sea. Others point to a single year like 2012, which had relatively few major hurricanes, and argue it proves such storms are becoming rare. But scientists note that seasonal twists like wind shear or dry air can temporarily suppress long-term trends, making it unwise to tout any individual storm or season as proof of something broader.
We may have to wait decades to learn precisely how global warming affects hurricanes, but Knutson also warns against confusing this uncertainty with a lack of consensus about warming itself.
"The relatively conservative confidence levels attached to [hurricane] projections, and the lack of a claim of detectable anthropogenic influence at this time, contrasts with the situation for other climate metrics such as global mean temperature," he writes, adding that international research "presents a strong body of scientific evidence that most of the global warming observed over the past half century is very likely due to human-caused greenhouse gas emissions."
For more about the relationship between climate change and hurricanes, check out this PBS NewsHour interview with MIT's Kerry Emanuel on the subject: