Are hurricanes linked to global warming? - Mother Nature Network (blog)

Hurricane Katrina approaches the U.S. Gulf Coast on Aug. 28, 2005. (Photo: NOAA)

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 â€" caused by greenhouse gases in the atmosphere, including those emitted by human activities â€" 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 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.)

Image: NOAA Geophysical Fluid Dynamics Laboratory

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 â€" 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 that temporary reduction."

That leads some skeptics to argue the past decade's hurricane flurry is just a rebound from this lull, but Knutson says there's little evidence of that, either. Rebounding may play a role, but it's hard to ignore the effect SSTs can have on hurricane intensity. And while it's premature to blame observed PDI increases on manmade climate change, such a relationship is still widely forecast for the near future.

"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 fact sheet, which adds this "would be substantially larger in percentage terms than the 2-11% increase in the average storm intensity." The following two graphs project this relationship through 2100, with the first one 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 that Knutson co-authored, this model not only foresees twice as many major hurricanes in 90 years, but also leads researchers to estimate that "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."

The chart below, which was used in the 2010 study, illustrates these projections by showing the modeled frequency of Atlantic cyclones under two scenarios: current conditions and a warmer climate in the late 21st century.

Image: NOAA GFDL

Despite scientists' general agreement that warming seas will boost the number of major Atlantic hurricanes, 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 in a summary of his research for NOAA. "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 major hurricanes, for example, ignoring major storms that hit other countries or remain at sea. Others point to a single year like 2012, which has had relatively few major hurricanes so far, and argue it proves such storms are becoming rarer. 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 recent PBS NewsHour interview with MIT's Kerry Emanuel on the subject:

Related severe weather stories on MNN:

• NASA satellites, drones study tropical cyclones
• How human activity is warming the oceans
• Global warming to drive weather disasters
• Climate link to hurricanes eludes insurers