The author of today's post, Bill Chameides, Ph.D., is Chief Scientist at Environmental Defense.
This morning, the Wall Street Journal published an Op-Ed by Dr. William Gray titled "Hurricanes and Hot Air". In it, Dr. Gray argues that there is no link between global warming and the recent "increase in major hurricanes".
Unfortunately, this piece has several inaccuracies and omissions. Let me clear them up.
Dr. Gray makes three fundamental errors. First he says that "warming theorists" think global warming has increased the number of hurricanes since 1995. Actually, the studies indicate a link between warming water and stronger storms, not number of storms. Moreover, the increase is apparent since the 1970s, not just since 1995. Gray is certainly free to critique the research, but he shouldn't misrepresent what the research indicates.
Second is his argument that hurricane frequency has increased because of a "the speed-up of water circulating in the Atlantic Ocean" (called the thermohaline circulation, or THC). The problem with this conclusion is that the THC has not accelerated. In fact, a recent study indicates that the THC may have decreased 30 percent over the past 50 years.
But because observations of this large-scale phenomenon are just beginning to come in, the conservative scientific consensus is that there is still "no coherent evidence for a trend" in the THC (see Box 5.1, page 397 in Chapter 5 of the IPCC report). Either way, it's clear that Gray's hypothesis just doesn't hold water.
Gray's third erroneous claim is that scientists studying the relationship between hurricanes and global warming "rely more on theory than on observation". In fact, most recent studies of the link between global warming and hurricanes are based on data. There's a good summary of all the recent papers, including those by Dr. Gray and his colleagues, on our Web site.
The first study to indicate a link between global warming and hurricane intensity was based on actual measurements of sea surface temperature and hurricane intensity in the North Atlantic and North Pacific. The relationship was clear: the warmer the water, the stronger the storm. Starting in about 1975, sea surface temperatures began to increase dramatically, and the destructive potential of hurricanes followed suit.
The solid line shows the destructive potential of North Atlantic and North Pacific hurricanes; the dotted line shows average sea surface temperature in the two hurricane regions. Source: Figure 3 in Emanuel, 2005 [PDF].
Subsequent studies looked at all six hurricane basins – not just the Atlantic – and also found that storms have gotten stronger as the water has gotten warmer. One study showed that the number of Category 4 and 5 hurricanes almost doubled over the past 30 years.
Source: Figure 4 in Webster et al, 2005 [PDF].
These measurement-based studies show a strong and real relationship between sea surface temperature and hurricane strength. They also indicate that warming water is producing stronger storms all over the world, not just in the Atlantic. That's an important point to remember, since global warming is, well, a global phenomenon.
Unfortunately for those affected by hurricanes (and despite Dr. Gray's claims to the contrary), the best available scientific evidence is that global warming due to human activities is contributing to the recent increase in storm strength.