The recent spate of hurricanes has inevitably attracted attention and spawned wildly inaccurate headlines, such as “a 1000 year event”, “the most powerful Atlantic storm on record”, “storm of the century”, and even “most deadly storm in history”.
Many climate scientists have also jumped on the bandwagon, to claim that these storms have been exacerbated by climate change.
With Hurricane Maria now weakening and heading north into cooler waters, it is perhaps time to take a rational look at what has actually been happening.
First, let’s look at the two most notable storms:
When Harvey made landfall at the Texas coast on Aug 25th, it ended the longest period without a major hurricane making landfall in the US on record. The previous one had been Hurricane Wilma in 2005.
With winds of 130mph, Harvey was not an unusual storm in any sense, and Texas is often on the receiving end of major hurricanes (defined as Cat 3 and over). There is usually at least one every decade, but prior to Harvey, the last one had been Gilbert in 1988.
However, what made Harvey so catastrophic was that the storm stalled close to Houston for five days, thus dragging in large amounts of moisture from the Gulf, and dumping it all in a relatively small area. In the end, the area around Houston received up to 51 inches of rain in six days.
This stalling of storms, caused by weather blocking patterns, is not unusual and can be explained by perfectly natural meteorological reasons. For instance, in 1978 Tropical Storm Amelia dropped 48 inches of rain in four days on Medina in Texas.
It has been claimed that warmer ocean temperatures had increased the intensity of rainfall from Harvey, but Texas has seen far worse in the past. During Tropical Storm Claudette in 1979, an incredible 43 inches of rain fell on Alvin in Texas, still a record for the US as a whole.
What made Harvey so damaging was that the rain fell on and around Houston, which has a history of notorious and catastrophic flooding, since it was founded in 1836. The reason is the topology, with the city established at the confluence of the Buffalo and White Oak Bayous, and in the middle of very flat, low-lying country.
Damage is worsened because, as the city expanded, much construction took place in flood-prone zones. Subsidence and concreting over of wide areas have served to make matters even worse.
It has been widely, but wrongly, reported that Irma was the “most powerful Atlantic storm on record”.
Irma had 1-minute sustained wind speeds of 185mph, but there have been three other Atlantic hurricanes since 1980 alone, which has been at least as strong – Allen, Gilbert, and Wilma in 1980, 1988 and 2005 respectively. Allen was even more powerful, with winds of 190mph.
None of these storms were anywhere near as damaging as the Labour Day Hurricane, which devastated Florida in 1985 with winds of 185mph. This was because the others reached full strength out over the ocean and well away from land.
And here lies the problem in trying to compare today’s hurricanes with those of the past. We have only had comprehensive monitoring of hurricanes in the Atlantic by satellites since around 1980.
Prior to that hurricane hunter aircraft began coverage in 1944, but in the early decades did not enter the eyewalls of the strongest storms, for understandable reasons! Consequently, the strength of storms in those times tended to be underestimated. Even then they only provided coverage over about half of the ocean.
It is therefore not possible to say whether there were other storms in the past as powerful as Irma.
2017 Atlantic Hurricane Season
So far this year, there have been four major hurricanes, Harvey, Irma, Jose, and Maria. How unusual is this?
Well, as it turns out, not unusual at all. According to NOAA’s Hurricane Research Division, there have been 26 other years with four or more.
The record year was 1950, which had as many as eight. Prior to the deployment of hurricane hunter aircraft, there have been numerous storms missed entirely.
As can be seen, there is no evidence that major hurricanes are becoming more common either. The dip around the 1970s and 80s corresponds with the cold phase of the Atlantic Multidecadal Oscillation (AMO). It is well established that the AMO affects the intensity of hurricanes in this way. According to NOAA, during warm phases of the AMO, the numbers of tropical storms that mature into severe hurricanes is much greater than during cool phases.
Again, when we look at Accumulated Energy (ACE), there are no obvious trends either. The record year was as long ago as 1933.
US Landfalling Hurricanes
Given the lack of coverage prior to 1980, the only reliable way to make long-term comparisons is to look at hurricanes that made landfall in the US where reliable records have been kept since the 19thC.
The Hurricane Research Division has carefully reviewed these records and classified the storms accordingly. They say that there have been 24 hurricanes as powerful or more as Irma.
The decade with the most major landfalling hurricanes was the 1940s, and, as already noted, we have just gone nearly 12 years without one at all, the longest spell on record.
By far the most powerful storm at landfall was the Labour Day Hurricane, followed by Camille in 1969, and Andrew in 1992.
There is an inference that two major landfalling hurricanes in one season, Harvey and Irma, is somehow unusual. This is nonsense, as it has happened on 14 other occasions since 1851, with the first such occasion being in 1879.
What Part Do Sea Temperatures Play?
Certain climate scientists have claimed that warmer sea surface temperatures have played a major role in intensifying this year’s hurricanes. While this is superficially an attractive idea, the historical record outlined above suggests that things are not as black and white.
Hurricane experts maintain that sea surface temperatures in the tropical Atlantic, Caribbean, and the Gulf of Mexico are high enough every year to potentially spawn catastrophic hurricanes.
There are, however, many and extremely complex, natural meteorological factors which determine the intensity and frequency of hurricanes. Things like wind shear, the presence of drying influences such as pockets of Saharan dust, the speed of hurricane tracks and their direction.
Given the recent dearth of major Atlantic hurricanes, it is extremely naive to single out one factor, sea surface temperatures, to put the blame on.
I have already mentioned the influence of the AMO. It is perhaps worth pointing out that hurricane droughts in the Atlantic during the cold phase of the AMO are directly linked with droughts in the Sahel and beyond to India.
Unfortunately the world’s weather is not perfect, and, as one part of the world suffers from hurricanes, another benefits from welcome rainfall.
Hurricanes In The Past
As already noted, it is difficult comparing storms of today with those of the past. Whereas we now have satellites and hurricane hunter aircraft to provide comprehensive monitoring, not very many years ago all we had was ground instruments.
When a very strong storm appeared, anemometers were usually destroyed before the strongest gusts could be recorded. The same often occurred with other equipment.
Moreover, the highest wind speeds in a hurricane only occur in a relatively small part of the eyewall. The odds of having reliable recording equipment at the exact part of the coast where this occurs, are actually pretty slim, particularly in earlier decades. As a consequence the top speeds of hurricanes were very rarely actually measured.
What we can compare though is the impact they have had.
You could write a book about the devastation brought about by hurricanes in the 19th and 20thC. Hurricanes such as the one which destroyed Galveston in 1900, leaving up to as many as 12,000 dead.
Or the Great Miami Hurricane which wiped Miami off the map in 1926. Florida was also on the receiving end of two other devastating storms around that time, Lake Okechobee in 1928 and the Labor Day Hurricane in 1935.
The sort of death tolls seen in those days would be beyond the imagination of people these days. And there is a very good reason – resilience.
Buildings and infrastructure are now better built to withstand storms, and warnings and evacuation procedures more effective.
Hurricanes always have been, and always will be, deadly and destructive. No number of wind farms or solar panels will alter that fact. We would be much better off investing money on protecting vulnerable regions, rather than on trying to make hurricanes go away.
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