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Why aren’t there more hurricanes?

Dr David Whitehouse, Science editor

Why haven’t hurricanes increased in frequency or intensity? We live in a warming world, so why haven’t they?

When it comes to hurricanes so many perform what I term the climate two-step. It’s a not an uncommon series of moves when it comes to the climate debate, but the particular dance over the increasing incidence of hurricanes due to global warming has become a rather farcical pirouette. Part of the problem is that many expect the incidence of hurricanes to have increased already and are clearly annoyed with the lack of cooperation they get from the available data.The confusion is between what has, or has not happened, and what is expected to happen. In the real world everyone knows they are not the same thing.

Last year in his book “Unsettled: What Climate Science tells us, what it doesn’t and why it matters,” and in his lecture to the GWPF, Steven Koonin said that hurricanes were not increasing in frequency. It was not a new statement. Ryan Maue has some very good data showing that over the past 40 years hurricanes have not increased in frequency or intensity. The argument looks good, so why the controversy?

The response to Koonin’s book was instructive. The counterarguments, stated forcefully, fall apart when looked at closely. Take Scientific American’s response to Koonin. They, and I mean 12 authors, pointed to this research, to knock down Koonin’s conclusion. But their research actually says; “Detection and attribution of past changes in tropical cyclone (TC) behavior remain a challenge due to the nature of the historical data, which are highly heterogeneous in both time and among the various regions that collect and analyze the data.While there are ongoing efforts to reanalyze and homogenize the data there is still low confidence that any reported long-term (multidecadal to centennial) increases in TC activity are robust, after accounting for past changes in observing capabilities [which is unchanged from the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5) assessment statement ].”

Game set and match then to Koonin and Maue one would have thought. But no, there is a two-step introduced into the logic, an injection of doublespeak into the reposte. It says, unscientifically, that just because we haven’t seen an increase in hurricane frequency doesn’t mean it hasn’t happened! It’s the two-step. It continues;

“This is not meant to imply that no such increases have occurred, but rather that the data are not of a high enough quality to determine this with much confidence. Furthermore, it has been argued that within the period of highest data quality (since around 1980), the globally observed changes in the environment would not necessarily support a detectable trend in tropical cyclone intensity. That is, the trend signal has not yet had time to rise above the background variability of natural processes.”

I repeat. Game set and match to Koonin and Maue.

So why haven’t hurricanes increased in frequency? Climate attribution studies have become more common in recent years consigning this or that extreme event into a statistical regime that could not exist on a world without climate change. Koonin has described such studies as being the scientific equivalent of being told you have won the lottery, after you have won the lottery!

But where are the attribution studies that explain why in a world that has warmed, warmth with so many predict will increase the incidence and severity of hurricanes in the future, there has been no increase in hurricanes? Surely this lack of an increase is an extreme event in itself? Why have hurricanes not increased as the global temperature has?

Let’s take another step in this climate dance.

Many people cite research that indicates that the incidence of hurricanes has in fact increased in line with global warming computer model predictions. The paper often referred to is by Kossin at al. which really is an exemplar when it comes to the study of the changing incidence of hurricanes. It starts off by saying that “theoretical understanding of the thermodynamic controls on tropical cyclone (TC) wind intensity, as well as numerical simulations, implies a positive trend in TC intensity in a warming world.” In other words, hurricanes should increase as the world warms.

So much for the theory but as is often the case in science the data is a little bit more messy, which the researchers admit is “generally unsuitable for global trend analysis.” One could say that they looked for a trend, but couldn’t find one.

To solve the problem Kossin et al reframe the data applying filters to it producing what they describe as a “homogenized data record based on satellite data” for the period 1982–2009. The analysis of those 28-years – the period of rapid increases in global temperature – “exhibited increasing global TC intensity trends.” Seems straightforward enough. Hurricanes have increased. One would have thought that it was a good result but it was actually scientific nonsense as they then say the trend was “not statistically significant at the 95% confidence level. In other words there never was a trend.

A logical next step would be to extend the data and see if it makes any difference, and they found an extra 8 years. It did! Well, barely, and of course it’s all “consistent with expectations based on theoretical understanding and trends identified in numerical simulations in warming scenarios.”

The paper didn’t find much of an increase in hurricane frequency, if any at all, and is clearly not robust. Look carefully at the bottom line of this paper and you will see that it finds increases in the incidence of hurricanes of between 2 to 15% per decade. This is a rather shaky conclusion. Yet this shaky conclusion has entered the debate as solid evidence. Look at how Carbon Brief reported it. Their headline was (parenthesis notwithstanding) that an increase in 15% had been found! It would not have been as good, or rather an acceptable story, if they had reported just a 2% increase with associated errors.?

The increase in hurricane frequency is now widely sloppily thrown in amongst other extreme events. In his book Hot Air (The Inside Story of the Battle Against Climate Change Denial) Peter Stott says that in 2013, “the increasing toll of death and destruction from heatwaves, floods, droughts and storms was progressing at a startling and terrifying scale.”

Hine’s Hole

Perhaps data obtained from a region called Hine’s Hole will give some pause for thought. It’s a blue hole on Cay Sal Bank – one of the largest submerged platforms in the Bahamas. Islands across the Bahamian Archipelago have been devastated by five major hurricanes from 2010 to 2020, including Category 5 Hurricane Dorian in 2019 that inundated parts of Abaco and Grand Bahama with up to 4 m of surge, killing 84 people and leaving more than 250 others missing.

Bahamian hurricane frequency is poorly understood. Writing in the journal Marine Geology Journal a team of researchers present a 530-year record of hurricane passage from Hine’s Blue Hole which it archives hurricanes in its disturbed sediments.

Hine’s Hole records 16 intense storms per century from 1850 to 2016, but there are three periods from 1505 to 1530, 1570 to 1620, and 1710 to 1875 with over twice as many intense storms per century.

These active periods are also found in other reconstructions from the Bahamian Archipelago and Florida Keys, where the effect seems more pronounced. Hine’s Hole provides data on weaker and more distal storms and provides unprecedented insight into changes in hurricane activity within the pre-industrial climate system. Its 170-year record shows many more hurricanes than seen in recent decades.

Hurricanes, like other extreme events, vary on timescales longer than the recent spell of global warming. While some contemplate the implications some are waiting for the increase in hurricane frequency. Others are sure it’s already happened. The debate can be described as unsettled.