23 February 2010

Updated WMO Consensus Perspective on Tropical Cyclones

A team of researchers under the auspices of the World Meteorological Organization has published a new review paper in Nature Geoscience (PDF) updating consensus perspectives published in 1998 and 2006. The author team includes prominent scientists from either side of the "hurricane wars" of 2005-2006: Thomas R. Knutson, John L. McBride, Johnny Chan, Kerry Emanuel, Greg Holland, Chris Landsea, Isaac Held, James P. Kossin, A. K. Srivastava and Masato Sugi.

The paper reaches a number of interesting (but for those paying attention, ultimately unsurprising) conclusions. On North Atlantic hurricanes the paper states (emphasis added):
Hurricane counts (with no adjustments for possible missing cases) show a significant increase from the late 1800s to present, but do not have a significant trend from the 1850s or 1860s to present3. Other studies23 infer a substantial low-bias in early Atlantic tropical cyclone intensities (1851–1920), which, if corrected, would further reduce or possibly eliminate long-term increasing trends in basin-wide hurricane counts. Landfalling tropical storm and hurricane activity in the US shows no long-term increase (Fig. 2, orange series)20. Basin-wide major hurricane counts show a significant rising trend, but we judge these basin-wide data as unreliable for climate-trend estimation before aircraft reconnaissance in 1944.
The paper's conclusions about global trends might raise a few eyebrows.
In terms of global tropical cyclone frequency, it was concluded25 that there was no significant change in global tropical storm or hurricane numbers from 1970 to 2004, nor any significant change in hurricane numbers for any individual basin over that period, except for the Atlantic (discussed above). Landfall in various regions of East Asia26 during the past 60 years, and those in the Philippines27 during the past century, also do not show significant trends.
The paper acknowledges that the detection of a change in tropical cyclone frequency has yet to be achieved:
Thus, considering available observational studies, and after accounting for potential errors arising from past changes in observing capabilities, it remains uncertain whether past changes in tropical cyclone frequency have exceeded the variability expected through natural causes.
The paper states that projections of future activity favor a reduction in storm frequency coupled with and increase in average storm intensity, with large uncertainties:
These include our assessment that tropical cyclone frequency is likely to either decrease or remain essentially the same. Despite this lack of an increase in total storm count, we project that a future increase in the globally averaged frequency of the strongest tropical cyclones is more likely than not — a higher confidence level than possible at our previous assessment6.
Does the science allow detection of such expected changes in tropical cyclone intensity based on historical trends? The authors say no:
The short time period of the data does not allow any definitive statements regarding separation of anthropogenic changes from natural decadal variability or the existence of longer-term trends and possible links to greenhouse warming. Furthermore, intensity changes may result from a systematic change in storm duration, which is another route by which the storm environment can affect intensity that has not been studied extensively.

The intensity changes projected by various modelling studies of the effects of greenhouse-gas-induced warming (Supplementary Table S2) are small in the sense that detection of an intensity change of a magnitude consistent with model projections should be very unlikely at this time37,38, given data limitations and the large interannual variability relative to the projected changes. Uncertain relationships between tropical cyclones and internal climate variability, including factors related to the SST distribution, such as vertical wind shear, also reduce our ability to confidently attribute observed intensity changes to greenhouse warming. The most significant cyclone intensity increases are found for the Atlantic Ocean basin43, but the relative contributions to this increase from multidecadal variability44 (whether internal or aerosol forced) versus greenhouse-forced warming cannot yet be confidently determined.
What about more intense rainfall?
. . . a detectable change in tropical-cyclone-related rainfall has not been established by existing studies.
What about changes in location of storm formation, storm motion, lifetime and surge?
There is no conclusive evidence that any observed changes in tropical cyclone genesis, tracks, duration and surge flooding exceed the variability expected from natural causes.
Bottom line (emphasis added)?
. . . we cannot at this time conclusively identify anthropogenic signals in past tropical cyclone data.
The latest WMO statement should indicate definitively (and once again) that it is scientifically untenable to associate trends (i.e., in the past) in hurricane activity or damage to anthropogenic causes.