Volume 14, Number 12: 23 March 2011
Landsea et al. (2010) note that "records of Atlantic basin tropical cyclones (TCs) since the late nineteenth century indicate a very large upward trend in storm frequency," and they say that this increase in documented TCs "has been previously interpreted as resulting from anthropogenic climate change." However, they go on to state that "improvements in observing and recording practices provide an alternative interpretation for these changes," and they report that "recent studies suggest that the number of potentially missed TCs is sufficient to explain a large part of the recorded increase in TC counts."
In their study, the four researchers explored the influence of another factor -- TC duration -- on observed changes in TC frequency, working with the widely-used Atlantic hurricane database known as HURDAT; and in doing so, they found that the occurrence of short-lived storms of two days duration or less had increased dramatically, from less than one per year in the late 19th and early 20th centuries to about five per year since about AD 2000, while medium- to long-lived storms had increased "little, if at all." Hence, they concluded that the previously documented increase in total TC frequency since the late nineteenth century in the database was "primarily due to an increase in very short-lived TCs."
Shifting their focus to moderate and long-lived TCs, Landsea et al. next conducted a sampling study based on the distribution of ship observations, which provided quantitative estimates of the frequency of missed TCs with durations exceeding two days. And upon adding the estimated numbers of missed TCs to the time series of moderate and long-lived Atlantic TCs, they found that "neither time series exhibits a significant trend since the late nineteenth century." In fact, they report there was a nominal decrease in the adjusted time series.
In light of these several findings, Landsea et al. conclude that sub-sampling of TCs back in time will artificially introduce increases in a wide array of TC characteristics, including "frequency of hurricanes and major hurricanes, duration of TCs, length of season, peak intensity, and integrated TC measures [like Accumulated Cyclone Energy (ACE) and Power Dissipation Index (PDI)]," which they say "should not be used directly from HURDAT for climate variability and change studies without consideration of, or quantitatively accounting for, how observational network alterations are affecting these statistics."
As things stand currently, therefore, the search for global warming effects on Atlantic Ocean tropical cyclone characteristics is still rather barren of any clear-cut results, other than the yet-to-be-contradicted null hypothesis of no effect.
Sherwood, Keith and Craig Idso
Reference
Landsea, C.W., Vecchi, G.A., Bengtsson, L. and Knutson, T.R. 2010. Impact of duration thresholds on Atlantic tropical cyclone counts. Journal of Climate 23: 2508-2519.