August 31, 2007

More Hurricane News

Filed under: Climate Extremes, Hurricanes

Of all the global climate related news we cover at World Climate Report, we seem to spend more time on hurricanes (a.k.a., tropical cyclones) than any other subject. We could feature a prominent scientific article on hurricanes every month, and despite the evidence to the contrary, popular presentations on the consequences of increased greenhouse gas concentrations never fail to include something about the great threat we face from more and more hurricanes. As we have noted many times before, the United Nations Intergovernmental Panel on Climate Change (IPCC) clearly states in the Summary for Policymakers “There is no clear trend in the annual numbers of tropical cyclones.”

An article appears in a recent issue of the prestigious Nature magazine that at first might change the minds of the IPCC with the first sentence in the abstract stating “Hurricane activity in the North Atlantic Ocean has increased significantly since 1995.” The article is by a group of scientists from institutions in Sweden, Puerto Rico, Florida, Colorado, and Texas who focused their research of hurricane activity in the Atlantic Ocean. What starts out looking like a dream come true for the global warming crusade soon deteriorates into their nightmare.

Nyberg et al. begin their article stating “The years from 1995 to 2005 experienced an average of 4.1 major Atlantic hurricanes (category 3 to 5) per year, while the years 1971 to 1994 experienced an average of 1.5 major hurricanes per year. A major hurricane is defined as a tropical cyclone with maximum sustained (1 minute) surface (measured 10m above the surface) winds of ≥50 ms-1. This increase in major hurricane frequency is thought to be caused by weaker vertical wind shear |Vz| and warmer sea surface temperatures (SSTs) in the tropical and subtropical Atlantic.” If vertical wind shear seems familiar to you in the context of importance when it comes to hurricanes, you may recall reading our feature on the subject posted on April 19, 2007 titled “Hurricane/Global Warming Link Weakens Further (not much left).”

So Nyberg reveals to the world that hurricane activity picked up substantially in the most recent decade (this has been noted by many others) and to no one’s surprise, global warming advocates have trumpeted the trend as evidence of the undesirable anthropogenic influence on climate. The Gore film shamelessly blamed the Katrina disaster on the ongoing buildup of greenhouse gases and suggested that without some immediate policy action, more of the same could be lurking just over the horizon.

But if one looks only at the title of the Nyberg article, one would find a suggestion that hurricane activity was low in the 1970s and 1980s compared to the past 270 years, and maybe what we witnessed in the 1995 to 2005 period is something of a return to more normal conditions? Of course, the title suggests that the research team found some way to extend the Atlantic hurricane record back 270 years.

Here is their trick. Coral growing in the Caribbean Sea preserve a year-to-year luminescence intensity (something like color differences) and as noted by Nyberg et al. “luminescence intensity in corals reflects the degree of terrestrial runoff, which is controlled by the amount of precipitation. Decreased precipitation in the northeastern Caribbean during the hurricane season is associated with increased trade-wind speed and high |Vz|” and “Increased trade-wind speed corresponds to higher sea-level pressures, enhanced sinking motion and drying and a more stable lower atmosphere, which results in lower precipitation and a more sheared environment in the tropical Atlantic during the hurricane season.” Furthermore, they examined the skeletons of plankton (Globigerina bulloides) from a well-dated core take in the southern Caribbean Sea, and they note that more plankton “reflects more nutrient supply caused by enhanced upwelling due to increased trade-wind strength, which is related to high |Vz|” in the area of interest.

The team presents convincing evidence that the corals and plankton accurately reflect hurricane activity during the period of reliable records (Figure 1). However, the coral and plankton records allow a reconstruction of hurricane activity back to 1730. Nyberg et al. clearly note “The record indicates that the average frequency of major hurricanes decreased gradually from the 1760s until the early 1990s, reaching anomalously low values during the 1970s and 1980s. Furthermore, the phase of enhanced hurricane activity since 1995 is not unusual compared to other periods of high hurricane activity in the record and thus appears to represent a recovery to normal hurricane activity, rather than a direct response to increasing sea surface temperature.” More specifically, they note “Only the periods ~1730–1736, 1793–1799, 1827–1830, 1852–1866 and 1915–1926 appear to have been marked by similarly low major hurricane activity and high |Vz|.
Furthermore, the current active phase (1995–2005) is unexceptional compared to the other high-activity periods of ~1756–1774, 1780–1785, 1801–1812, 1840–1850, 1873–1890 and 1928–1933, and appears to represent a recovery to normal hurricane activity, despite the increase in SST.”


The reconstructed major hurricane activity and |Vz| series back to 1730. Also shown are the reliable observation records back to 1944 and 1949, respectively, the historical hurricane record back to 1851, the sea surface temperature data averaged over 10 to 26°N and 20 to 86°W during August–October back to 1854, zonal wind speed data centered at 11°N and 65°W back to 1890, luminescence intensities, and abundance of G. bulloides upon which the reconstructions were based. The dashed lines indicate 95% confidence intervals for estimated numbers of hurricanes and |Vz| values. All data are smoothed with a five-year running average. The ‘master’ luminescence curve is developed by averaging standardized luminescence intensity values during Aug–Oct in all of the coral cores for each year (from Nyberg et al., 2007).

Not much left to say about this. Instead of being unusually active, it looks like the current hurricane regime is simply a return to more normal conditions, following an unprecedented tranquil couple of decades.

Reference:

Nyberg, J., B.A. Malmgren, A.Winter, M.R. Jury, K.H. Kilbourne, and T.M. Quinn. 2007. Low Atlantic hurricane activity in the 1970s and 1980s compared to the past 270 years. Nature, 447, 698-702.




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