Hurricane Bluster

Global Science Report is a weekly feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”

When it comes down to scaring people into accepting onerous reductions in carbon dioxide emissions, it’s always a good idea to trot out the specter of increased hurricanes, despite the lack of backing for this in the science literature.

“Bluster” isn’t the name of an Atlantic hurricane (although it would be a good one*), but rather our description of the stories about new research out of the Massachusetts Institute of Technology projecting an increase in the frequency and magnitude of hurricanes as a result of anthropogenic climate change.

Publishing in the Proceedings of the National Academy of Science, M.I.T.’s Kerry Emanuel projects a rather large increase in the global frequency of tropical cyclones as well as their intensity over the course of the 21st century.

Emanuel is the first to admit that the changes he found were largely of a different character to those in the generally accepted literature, which projects little change in the frequency of tropical systems (with perhaps even a slight decline) and only a slight increase in the future intensity.

The difference between Emanuel’s results and those from the bulk of other studies arises primarily for two reasons; 1) the future emissions scenario used to drive the global climate models; and, 2) the method of downscaling coarse climate model output to the finer scale necessary to model tropical cyclones.

When it comes to emission scenarios, Emanuel chooses to use the most extreme scenario, which more than triples the effective atmospheric carbon dioxide concentration by the end of the century, while most other studies have used a more modest scenario which leads only to about a doubling. With new technologies opening up vast abundances of lower CO2-emitting natural gas available for power generation, the extreme emissions scenario used by Emanuel seems unlikely.

(This might also prompt the question as to why it was necessary to use the extreme scenarios in the draft “National Assessment” of climate change released in January by federal climatologists. See our voluminous comments here).

With regard to the downscaling methodology, another paper, to be published later this year, uses a different procedure and arrives at nearly the opposite result. In the North Atlantic basin—the area in which the bulk of tropical cyclones which effect the United States occur—Thomas Knutson (of the Geophysical Fluid Dynamics Laboratory) and colleagues find a decline of hurricane frequency of nearly 25 percent with an intensity increase of only about 6 percent.

It’s worth noting that, according to research by Wang et al. (2011) and Murakami et al. (2012), future storms are actually more likely to remain at sea rather than striking the U.S.

Emanuel, on the other hand, finds increases in both frequency and intensity of storms in the North Atlantic. Emanuel notes that further research into the difference produced with his model compared with Knutson’s “may prove enlightening.”

We concur.

The bottom line is that the significance of the results of Emanuel’s new study depends on an extreme emissions/warming scenario and a methodology which needs further evaluation. And even so, the changes reported by Emanuel in the North Atlantic, the area or primary concern for U.S. interests, do not rise above the noise of natural variability for many decades into the future.

Contrary to the media bluster being generated by the new study, the true bluster of future hurricanes impacting the U.S. will likely be little different in the coming century than it was during the last—with any impact of anthropogenic climate change lost in the noise of the natural system.

As with most global warming scare stories, cocksure stories about increased hurricane activity are a bit blustery.

References:

Emanuel, K., 2013. Downscaling CMIP5 climate models shows increased tropical cyclone activity over the 21st century. Proceedings of the National Academy of Sciences, do:10.1073/pnas.1301293110

Knutson, T., et al., 2013. Dynamical Downscaling Projections of Twenty-First-Century Atlantic Hurricane Activity: CMIP3 and CMIP5 Model-Based Scenarios. Journal of Climate, doi:10.1175/JCLI-D-12-00539.1, in press.

Murakami, H., et al., 2012. Future Changes in Tropical Cyclone Activity Projected by the New High-Resolution MRI-AGCM. Journal of Climate, 25, 3237–3260. doi: 10.1175/JCLI-D-11-00415.1.

Wang, C., L. Hailong, S-K. Lee, and R. Atlas, 2011. Impact of the Atlantic warm pool on United States landfalling hurricanes. Geophysical Research Letters, 38, L19702, doi:10.1029/2011GL049265.


* Back when it was OK to be funny about politicians, Australia named its tropical cyclones for sitting senators.  Those were the days.