Fewer but Stronger? What Scientists Knew in 2013 About Storms in a Warming World

Hurricane Sandy off the Atlantic coast before dawn on October 29, 2012, seen by the Suomi NPP satellite. NASA Earth Observatory image by Jesse Allen and Robert Simmon.

On October 29, 2012, a satellite caught Sandy — part hurricane, part nor'easter — churning off New Jersey at night, the city lights from Washington to Boston smothered by cloud. Within hours, parts of Manhattan were under water and seawater surged over the Jersey shore. It came after a run of extreme weather: the "Snowmageddon" blizzard of 2010, the deadliest month of tornadoes on record in April 2011, and record numbers of billion-dollar weather disasters in the U.S. in 2011 and 2012, most of them storms.

Was global warming to blame? In a 2013 feature, NASA's Earth Observatory asked the scientists. Their answer was more careful than the headlines: for several kinds of storm, a warmer world may bring fewer but stronger ones.

Why warmth feeds storms

Researchers group storms into three kinds: thunderstorms, the smallest; tropical cyclones (hurricanes and typhoons); and extra-tropical cyclones, the big low-pressure systems of the mid-latitudes. All need moisture, energy and the right winds.

A thunderstorm starts when warm, humid air is forced upward — by a cold front, colliding winds or rough terrain. As it rises it cools, and its water vapour condenses into droplets and ice, releasing latent heat that drives the cloud still higher. That is why scientists think warming strengthens storms: more heat and more water vapour in the air and ocean give them more fuel.

Rising global temperatures have raised the humidity of the atmosphere. Graph by Robert Simmon, NASA Earth Observatory, from NOAA National Climatic Data Center data.

The evidence gathered by 2013 pointed that way:

• Satellites measured a 4 percent rise in water vapour in the air over the previous 25 years.
• About 76 percent of U.S. weather stations had seen more extreme precipitation since 1948; one analysis found extreme downpours happening 30 percent more often.
• Hurricanes were reaching Category 3 wind speeds nearly nine hours faster than in the 1980s.
• MIT's Kerry Emanuel found in 2005 that Atlantic hurricanes were about 60 percent more powerful, over their lifetimes, than in the 1970s.
• NOAA found twice as many extreme regional snowstorms from 1961 to 2010 as from 1900 to 1960.

Forces pulling the other way

If it were only a matter of extra moisture, the answer would be simple. But warming also changes where the heat is. The Arctic has warmed about twice as fast as the mid-latitudes, largely because melting sea ice exposes dark ocean that absorbs more sunlight.

Temperatures in 2000–2009 compared with 1951–1980: about 0.6 °C warmer on average, and about 2 °C in the Arctic. NASA image by Robert Simmon, from GISS Surface Temperature Analysis data.

Extra-tropical cyclones draw their energy from the clash of cold polar and warm subtropical air. As the temperature gap between poles and tropics narrows, they may have less energy to draw on — though high in the atmosphere the tropics are warming faster than the poles, which could push the other way.

For hurricanes, wind shear matters: winds that change speed and direction with height tear tropical cyclones apart. Research suggested that Atlantic wind shear could increase as the planet warms. That is why many models concluded that the number of tropical cyclones will stay the same or fall even as the strongest get stronger. A 2010 assessment by leading storm experts projected that by 2100 their average intensity would likely rise by 2 to 11 percent, while their frequency would fall by 6 to 34 percent.

Warming may also change the jet streams that steer storms. Jennifer Francis of Rutgers University argued that the loss of Arctic sea ice had slowed the jet stream and made it wavier since 1979, favouring "blocking" patterns — like the one that helped push Sandy inland instead of out to sea.

Thunderstorms and tornadoes

For severe thunderstorms, two ingredients compete. Warming adds CAPE — convective available potential energy, the fuel of towering storm clouds — but the faster-warming Arctic should reduce the wind shear that turns weak storms into violent ones. NASA's Anthony Del Genio found in climate-model simulations that the number of severe U.S. storms changed little, but the strongest got stronger winds. A Purdue study led by Robert Trapp found that doubling greenhouse gases could double the days with conditions for severe thunderstorms in cities such as Atlanta and New York.

Where lightning strikes most often, May 1995 to 2011: a map of the strongest convection on Earth. NASA image by Robert Simmon, from Global Hydrology Resource Center data.

Tornadoes are harder still. Only about 1 percent of thunderstorms produce one, and meteorologists do not fully know why some do and others do not.

Can models tell?

Long, reliable storm records are scarce, so scientists turn to climate models, which divide the globe into boxes typically 150 to 200 km on a side. They handle large extra-tropical cyclones reasonably well, but hurricanes (about 150 km across) and thunderstorms (under 10 km) are smaller than a box. Modellers are "downscaling" — adding fine detail such as terrain and satellite data in key regions — and new high-resolution models can resolve much smaller features.

The GEOS-5 model, running at 3.5 km per grid cell, simulates the atmosphere on January 2, 2009. NASA image by Greg Shirah, GSFC Scientific Visualization Studio.

Patience

Sandy itself was a hybrid, drawing energy both from warm ocean water, like a hurricane, and from sharp temperature contrasts, like a winter storm — part of why it was so destructive. Warm Atlantic waters probably intensified it, and higher sea levels made its surge worse. But pinning all its fury on global warming was premature, said Marshall Shepherd of the University of Georgia, comparing it to steroids and home runs in baseball: some influence was surely there, but how much is hard to say.

And variability can mislead. After the record tornadoes of 2011, 2012 was unusually quiet. Understanding storms and climate, Del Genio said, is about trends and underlying processes, not about blaming individual events.

Sources

• NASA Earth Observatory, "In a Warming World, the Storms May Be Fewer But Stronger," 2013; rewritten in hubnx's own words. Images from NASA.