Global Warming Makes Hurricanes Worse: No! Yes! No! Yes!

By Brandon Keim

Hurricane expert and anthropogenic global warming skeptic William Gray has an opinion piece in today's Wall Street Journal.

That hurricanes are becoming more frequent, he says, has nothing to do with climate change. That's a relatively uncontroversial statement. Not so his next proposition: that hurricane intensity is also independent of climate change.

The hypothesis that increasing carbon dioxide in the atmosphere increases the number of hurricanes fails by an even wider margin when we compare two other multi-decade periods: 1925-1965 and 1966-2006. In the 41 years from 1925-1965, there were 39 U.S. land-falling major hurricanes. In the 1966-2006 period there were 22 such storms -- only 56% as many. Even though global mean temperatures have risen by an estimated 0.4 Celsius and CO2 by 20%, the number of major hurricanes hitting the U.S. declined.

Gray has long held that hurricane severity is determined by oceanic surface temperatures that are themselves controlled by variations in water circulation.

Though most people don't realize it, the Atlantic Ocean is land-locked except on its far southern boundary. Due to significantly higher amounts of surface evaporation than precipitation, the Atlantic has the highest salinity of any of the global oceans. Saline water has a higher density than does fresh water. The Atlantic's higher salinity causes it to have a continuous northward flow of upper-ocean water that moves into the Atlantic's polar regions, where it cools and sinks due to its high density. After sinking to deep levels, the water then moves southward, and returns to the Atlantic's southern fringes, where it mixes again. This south-to-north upper-level water motion, and compensating north-to-south deep-level water motion, is called the thermohaline circulation (THC).

The strength of the Atlantic's THC shows distinct variations over time, due to naturally occurring salinity variations. When the THC is strong, the upper-ocean water becomes warmer than normal; atmospheric circulation changes occur; and more hurricanes form. The opposite occurs when the THC is weaker than average.

Since 1995, he explains, the Atlantic's THC has been unusually strong -- hence the stronger hurricanes. In the 25 years before that, THC and storms were correspondingly weak; for 20 years before that, they were strong; and in the first quarter of the 20th century, they were weak in tandem yet again.

Gray's among a tiny minority of climatologists who don't think that filling the atmosphere with CO2 affects the climate, but he has more company in the hurricane controversy -- and not just from industry flacks. Much more certainty exists about the general reality of climate change than its impact on storms. (That being said, many climate scientists do believe that the two are connected.)

So what do the experts say? Well, the Interngovernmental Panel on Climate Change says that "it is 'more likely than not' ... that there is a human contribution to the observed trend of hurricane intensification since the 1970s." (Paraphrasing by the Pew Center on Global Climate Change.) Hardly a ringing declaration one way or the other. The World Meteorological Association says that "Though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point." They add that the data is inconsistent and unreliable.

Just something to keep in mind when the next time you're arguing climate change with a skeptic: leave the hurricanes out of it.

Hurricanes and Hot Air [Wall Street Journal]

Image: NASA