The Current Wisdom is a series of monthly articles in which Senior Fellow Patrick J. Michaels reviews interesting items on global warming in the scientific literature that may not have received the media attention that they deserved, or have been misinterpreted in the popular press.
The Current Wisdom only comments on science appearing in the refereed, peer-reviewed literature, or that has been peer-screened prior to presentation at a scientific congress.
Wasn't it just a few short years ago, during the summer of 2005, that headlines were screaming "Hurricanes Growing Fiercer with Global Warming" and "Increase in Major Hurricanes Linked to Warmer Seas"? Now, this hurricane season starts off with "Global Hurricane Activity at Historical Record Lows."
What gives? Where have all the hurricanes (Atlantic and Eastern Pacific) ,willy-willys (Australia), typhoons (Western Pacific), Big Winds (Philippines), blowhards (Michaels), or more generally, tropical cyclones, gone?
Gone to graveyards, every one?
More like they are just being subject to the waxing and waning of the natural forces which promote their development (and also to keep them in check).
The new paper that generated this summer's "Historical Record Low" headlines was written by Dr. Ryan Maue from the Center for Ocean and Atmosphere Studies at Florida State University and has just been accepted for publication in Geophysical Research Letters.
Maue (rhymes with zowie) analyzed the location and intensity of every tropical cyclone (a warm-cored low pressure system with sustained wind speed exceeding 39 mph) that has been observed during the period January 1970 through May 2011. For each storm, he calculates the Accumulated Cyclone Energy (or ACE) which is the square of the maximum observed wind speed (reported every six hours) summed over life time of the storm system. The ACE is a reasonable indicator of the cumulative strength of each storm system. The individual storm ACE values can be combined by basin, season, etc. to examine patterns and trends in tropical cyclone activity.
Figure 1 shows the annual ACE values for the Northern and Southern Hemispheres, as well as for the total globe, since 1970. Notice several things. There's much more activity in the Northern Hemisphere, and there is also more variability here. You can inkle a positive trend in the global ACE in the beginning of Maue's record from 1970 through the mid-2000s (the "oughties") — precisely when the headlines proclaimed that global warming was making hurricanes worse than ever. However, you can also see a declining trend from the early 1990s through the end of the record, and, perhaps, claim that global warming is reducing hurricane activity.
In either case, you'd probably largely be wrong — at least about causation.
Figure 1. The annual ACE index from the Southern Hemisphere (green), Northern Hemisphere (blue), and the entire globe (red), from 1970 through 2010 (from Maue, 2011).
Maue describes this situation in the abstract of his paper:
Tropical cyclone accumulated cyclone energy (ACE) has exhibited strikingly large global interannual variability during the past 40-years. In the pentad since 2006, Northern Hemisphere and global tropical cyclone ACE has decreased dramatically to the lowest levels since the late 1970s. Additionally, the global frequency of tropical cyclones has reached a historical low. Here evidence is presented demonstrating that considerable variability in tropical cyclone ACE is associated with the evolution of the character of observed large-scale climate mechanisms including the El Ni&ntidle;o Southern Oscillation and Pacific Decadal Oscillation. In contrast to record quiet North Pacific tropical cyclone activity in 2010, the North Atlantic basin remained very active by contributing almost one-third of the overall calendar year global ACE.
So, while the Atlantic basin as been quite active since about 1995, tropical cyclone energy in the Pacific has been relatively low, primarily as a result of the natural variability in large-scale atmosphere/ocean circulation patterns (i.e., "El Nino Southern Oscillation" and "Pacific Decadal Oscillation") operating in the Pacific basin.
Further, during the most recent few years, the global ACE has plummeted to the lowest level since the late 1970s, driven by a decline in the Southern Hemisphere activity and record inactivity across the entire North Pacific Ocean, even though the ACE in the North Atlantic basin has remained above average.
This is another lesson — which our friends of all climate stripes seem to ignore — that what takes place locally may very well be a poor indication of what is happening on the globally.
Another twist: Despite the elevated level of ACE in the North Atlantic, we are experiencing anomalously few direct hurricane landfalls along the coast of the U.S. mainland. The last major hurricane (category 3 or higher) to strike the U.S. was hurricane Wilma in October of 2005, making the 2006 though 2010 period only the third time in the 160 years of records that five consecutive hurricane seasons passed without a major one striking the U.S. There have never been 6 consecutive such seasons. The last hurricane of any strength to strike the U.S. was hurricane Ike back in September of 2008. Only five times since 1851 have two complete hurricane seasons passed with no U.S. landfalls (as was the case in 2009 and 2010). The longest period between hurricane strikes on the U.S. mainland was from November 1861 to September 1865, which encompassed three full seasons and a partial season — although the coast was a lot less settled back then, and that record is a bit questionable. (The citizenries of the U.S.A. and the C.S.A may have had bigger fish to fry). If somehow we make it through this September without a direct hurricane landfall, we'll break the modern (post-1900) record of misses. So much the stranger that we are threatening all these records during a time of elevated activity in the Atlantic.
I hope that it seems obvious by now that there is no need to bring anthropogenic global warming into this conversation — as it is, we are pretty clueless about what is going on. As Maue puts it:
It is still a fundamental research question as to what are the atmosphere and ocean mechanisms responsible for the observed annual global [tropical cyclone] frequency of ~87 storms (Frank and Young, 2007)... . [I]t is critical to have the best possible diagnosis of periods of global TC inactivity and incorporate the recent pentad of historical lows into the context of natural and anthropogenically forced climate variability (Knutson et al., 2010). Furthermore, research must better explain the role of tropical cyclones in the climate system especially during this current period of record inactivity.
Maue, R.N., 2011. Recent historically low global tropical cyclone activity. Geophysical Research Letters, doi:2011GL047711R, in press.