October 4, 2004

Not a Model World

Will future hurricanes will be stronger in a greenhouse-gas warmed world? A new climate model says yes, but (as usual) the observations suggest otherwise.

With all the hurricane activity as of late—both in the Atlantic Ocean and in the U.S. media—we’re starting to hear rumblings conflating hurricanes and global warming. Renowned hurricane experts say that notion is unfounded, given actual observations.

Yet according to climate models, the future will be one in which tropical cyclones may become slightly stronger if greenhouse gas buildup continues to heat things up. The most recent iteration was published September 15, 2004, in the Journal of Climate by Thomas Knutson and Robert Tuleya. New York Times science writer Andrew Revkin summarized those findings:

Global warming is likely to produce a significant increase in the intensity and rainfall of hurricanes in coming decades, according to the most comprehensive computer analysis done so far.

(As an aside without comment, it may be of interest to some to compare Revkin’s characterization to the authors’ own description of their findings: “CO2-induced tropical cyclone intensity changes are unlikely to be detectable in historical observations and will probably not be detectable for decades to come.”)

Global climate models are too coarse to actually include organized tropical systems. For that reason, Knutson and Tuleya began with model projections of future sea-surface temperatures, vertical temperature profiles, and vertical moisture profiles over regions where tropical cyclones form, using them to define a climate in which they used a finer-resolution hurricane model to spin up tropical cyclones. They then compared the characteristics of the model-derived storms in the model-derived future climate with the model-derived storms in the current observed climate. They found that in the model-derived future climate, model-derived hurricanes had a 14 percent increase in the central pressure fall, a 6 percent increase in the maximum surface wind, and an 18 percent increase in the average rate of precipitation with 60 miles of the storm center over the model-derived hurricanes in the current climate. All these changes were indications that the model-derived hurricanes of the model-derived future would be more intense than the model-derived hurricanes of today.

Perhaps our more astute readers can see where we are headed…the idealized setting of climate models does not come close to replicating the complexities of the real world.

Let’s examine the modeled world that Knutson and Tuleya created and compare it with its real-world counterpart.

Carbon dioxide levels in the modeled atmosphere were increased at a rate of 1 percent per year. That rate led to atmospheric carbon dioxide concentrations 80 years from now (the period Knutson and Tuleya chose to define the future climate conditions) that are more than double the levels of today.

In the real world, the concentration of carbon dioxide is growing at a rate of about 0.45 percent per year—a rate that is less than half of that presumed in the model.

At the current growth rate, the concentration 80 years from now will have changed by only one-third as much as the concentration used in the climate model. The current growth rate has been stable for three decades. It is not going to double tomorrow! Every indication is that trends in increasing energy efficiency and population will keep it pretty much where it has been for the foreseeable future.

At that rate, it will take nearly 180 years to reach the same levels that the modeled climate reaches in 80. By that time, it is highly likely that the energy structure of the world will be significantly different than it is today, with fossil fuels largely relegated to a curiosity of the past. The model-presumed concentrations will never be reached.

The modeled hurricanes grow in a climate that is ideal for growing storms—specifically, there is virtually no vertical wind shear. Vertical wind shear acts to interfere with the El Niño development of tropical systems by basically blowing the tops of the storms and preventing them from becoming well organized. One phenomenon that is responsible for increasing the vertical wind shear in the tropical Atlantic is El Niño.

A number of studies have demonstrated that the tropical activity in the Atlantic Ocean decreases in years with El Niños, as does the chance that the storms that do develop make landfall in the United States. Many climate models suggest increased El Niño-like conditions in the future. Many others don’t. Therefore, Knutson and Tuleya decided to sidestep the issue and assume that not only would there be no wind shear changes in the future, but that there would be virtually no wind shear at all in any of their models. This situation sets up an idealized climate for developing strong hurricanes—with the strength of the storms largely governed by the temperature of the underlying ocean surface.

The authors, in fact, note a strong correlation between sea surface temperatures (SSTs) and hurricane intensity—the warmer the sea surface, the stronger the storm. Figure 1 shows the relationship between SSTs and hurricane intensity used by Knutson and Tuleya. In their model, sea surface temperatures alone explain between 45 percent and 72 percent of the change in hurricane intensity. Since all the global climate models warm up the oceans when carbon dioxide levels are enhanced (even more so when they are unrealistically enhanced to levels that are more than double current levels in 80 years), higher CO2 leads to higher SSTs which lead to strong tropical cyclones.

SSTs and Hurricanes

Figure 1. Relationship between sea-surface temperatures and hurricane intensity as measured by minimum central pressure (the lower the pressure the stronger the storm) in the models used by Knutson and Tuleya.

But, the real world is not so welcoming to fledgling tropical disturbances. While certainly the temperature of the underlying ocean surface is a critical factor in tropical cyclone development (the SST must be at least 80ºF for storms even to develop at all), other factors, such as wind shear, effect the developing storm (see http://hurricane.atmos.colostate.edu/forecasts/ for more information on hurricane forecasting and the factors that determine hurricane strength and frequency). ). In Figure 2, we plot the relationship between sea surface temperatures in the region of the Atlantic used by Knutson and Tuleya vs. two measures of hurricane intensity—average peak wind speed in the 5 strongest storms each year and the total annual number of intense hurricanes (Category 3, 4, or 5 storms). Compare the results with Figure 1.

SSTs and Hurricanes

Figure 2. The observed relationship between sea-surface temperatures and two measures of hurricane intensity—the number of major hurricanes (Category 3, 4, and 5) each year (left), and the average peak wind speed in the five strongest storms in each year (right). (Data source for hurricane information: Landsea et al., 1996 and updates).

Put mildly, Figure 1 is an overstatement. The observations show that the relationship between hurricane intensity and SSTs is not nearly so well defined as the models imply. In fact, in the real world, SSTs explain less than one-tenth of the annual variation in hurricane intensity (compared to about two-thirds in the model).

If we use real world numbers in place of the modeled ones, here is how the Knutson and Tuleya results should be roughly adjusted: According to current trends, CO2 concentrations in 80 years will only be about one-third as great as those used by Knutson and Tuleya, since the relationship between warming and CO2 concentrations is generally linear, this means that the SSTs will rise only about one-third as much, thus we’ll adjust their numbers down to a 5 percent increase in the central pressure fall, a 2 percent increase in the maximum surface wind, and a 6 percent increase in the average precipitation rate. But we are not done yet. Instead of explaining about 66 percent of the variations in hurricane intensity (as indicated by Knutson and Tuleya’s model), SSTs explain only about 8 percent of the variation. This means that instead of SSTs playing the dominant role in determining hurricane intensity, SSTs play a very minor role, leaving other factors largely to determine the strength of the storms.

Putting all of this together means that no one will ever likely be able to detect a change in storm characteristics or impacts based simply on global warming raising SSTs. But such a proclamation would never make headlines in the New York Times.

Basically, this represents but another case where observational evidence does not support dire model-based conclusions of the atrocities that our coming climate holds in store. We must again ask, for what must be the umpteenth time, what on earth has happened to the peer-review process in climate science?

References:

Knutson, T.R., Tuleya, R.E., 2004. Impact of CO2-induced warming on simulated hurricane intensity and precipitations: sensitivity to the choice of climate model and convective parameterization. Journal of Climate, 17, 3477-3495.

Landsea, C.W., 1996. Downward trends in the frequency of intense Atlantic hurricanes during the past five decades. Geophysical Research Letters, 23, 1697-1700 (and updates).




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