Over the last two days I've been sickened by some I have seen on the cable news outlets attempting to use the tragedy in Louisiana, Mississippi, and Alabama to further this or that environmental cause. Mostly they claim that global warming caused or contributed to this storm. Unfortunately, for them, the facts do not back them up, although you are hardly likely to hear that on CNN these days. Using the latest research from NOAA (which even they admit probably still UNDER-estimates 19th century hurricane frequency and intensity) I will show how the claims that global warming is changing hurricane patterns in the Atlantic basin is simply wrong.
Myth One: "We have more hurricanes today than in years past."
In many ways this is the easiest to demolish. From the 1850's to the 1940's we had four different decades (1880's, 1890's, 1910's & 1940's) when 20 or more hurricanes hit the United States. Since 1950's no more than 18 storms have hit the U.S. coastline in any decade. From 1851 to 1939 an average of 1.87 hurricanes hit the U.S. every year. Since 1940 the average is down to 1.66 per year.
Average of Hurricanes Per Year, Per Decade:
1850's 1.8
1860's 1.5
1870's 1.9
1880's 2.6
1890's 2.0
1900's 1.7
1910's 2.1
1920's 1.5
1930's 1.7
1940's 2.3
1950's 1.8
1960's 1.6
1970's 1.2
1980's 1.6
1990's 1.4
2000's 1.8
Myth Two: "More hurricanes are hitting closer together."
It is true that 2004 was a very active years for hurricanes with 6 hitting the U.S. coast. However it is far from unprecedented. In 1893, 1909 & 1933 five storms hit; in 1916 & 1985 six storms struck; and in 1886 seven hurricanes walloped the United States. It is hard to see that 2004's 6 storms make it anything all that unusual statistically speaking.
Myth Three: "We are getting more early hurricanes because of global warming."
This is also easy to demolish because we are not in fact getting more hurricanes early in the storm season. Since 1851 we average 2.62 storms per decade that strike the U.S. before August. From the 1850's to the 1920's we averaged 3.12 early hurricanes a decade, and from the 1930's to 2000's we averaged 2.30 early hurricanes a decade. So, there are in reality fewer early hurricanes now, although I find it difficult to believe the decrease would be considered statistically significant.
It is interesting to note that if global warming was producing a noticeable effect on hurricane formation we might expect that the hurricane season would extend longer into the calendar year. This has not been the case. From 1851 to 1939 a grand total of four (4) late hurricanes (after Oct. 31st) struck the U.S. shore. Since 1940 only one (1) has ("Kate" in 1985.)
Myth Four: "More large hurricanes are striking the U.S. than in the past."
On average 6.2 large hurricanes (i.e. category 3 or higher) hit the U.S. every decade. Only five decades have seen more than six major hurricanes, the 1890's, 1910's, 1930's, 1940's and 1950's. From 1960 to 1999 we averaged only 5.25 per decade. With half the decade gone (2000-2004) we are on pace to have 8 major hurricanes this decade, a lot but not a record or that unusual. It certainly doesn't look like an outlier (as does the 1860's with only two (2) major hurricanes.)
Major hurricanes as a percentage of all hurricanes looks like the following:
1850's 31.25%
1860's 13.33%
1870's 31.57%
1880's 23.08%
1890's 35.00%
1900's 29.41%
1910's 33.33%
1920's 33.33%
1930's 47.06%
1940's 34.78%
1950's 50.00%
1960's 37.50%
1970's 33.33%
1980's 37.50%
1990's 35.71%
2000's 44.44%
The overall average of major hurricanes to all hurricanes is 33.94%. Since the 1940's 7 out of 8 decades are above that average. Before 1940 only 2 decades are above the average. This is the only measure that supports the idea that stronger happen more frequently, not in an absolute sense, but as a ratio of all hurricanes. However, this is not unambiguous. Chances are the strength of pre-1930's hurricanes are under-estimated. There was very little equipment that could measure wind-speeds over 100+ miles an hour. Estimated wind speed (based upon barometric pressure readings) are dependent upon actual readings that may have been quite far from the lowest point of pressure for the storms involved. It isn't until the 1890's that an attempt at scientific monitoring of weather conditions nationwide is made. While researchers have an easier time identifying cyclonic storms from historical material, accurate portrayal of storm intensity is another matter.
All of this begs us to remember that we only have about 50 years of top quality data for these storm systems. That is not a lot of information to work with. So to claim that you already see a radically divergent trend emerging from the data would require a very drastic change indeed. No such drastic change is evident from the evidence. So the next time someone tries to bolster their environmental cause by exploiting the death and destruction wrought by Katrina, tell them to stick it where the sun don't shine.
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