Tropical Cyclone Intensity and Climate Change

5 10 2017

Dale C. S. Destin|

This is a continuation of our series – Tropical Cyclones and Climate Change – TCs (tropical depressions, tropical storms and hurricanes). In this blog, we will look at whether climate change is having an impact on TC intensity/strength, especially with respect to wind speeds and provide you scientifically based answers.

As many Caribbean islands rebuild after the havoc caused by Hurricanes Irma and Maria, which killed over 185 persons and caused over US$150 billion in damage, many – including political leaders, have declared that these hurricanes were caused by climate change.

Even the United Nations (UN) Secretary-General seems to be peddling the notion that climate change caused Irma, Maria and the hyperactive 2017 Atlantic hurricane season.

But is climate change really to be blamed? Are these statements in harmony with the findings of the Intergovernmental Panel on Climate Change (IPCC)? What is the consensus of TC researchers?

The 2017 Atlantic hurricane season

Thus far, the season has produced 14 named storms, 8 of which became hurricanes and 5 major hurricanes – Category 3 and over. Of the 5 major hurricanes, 2 (Irma and Maria) became Category 5 – the highest category on the Saffir-Simpson Hurricane Wind Scale.2017 Atl Hurricane Season TracksRecord shows that Hurricane Irma was no ordinary Category 5 Hurricane. Far from it – it was more like a Super Category 5 Hurricane. If there were a Category 6 – it would have easily been so categorised.

Hurricanes Irma and Maria are among the strongest hurricanes to ever form over the Atlantic Basin. Irma had peak sustained winds of 295 km/h (185 mph), which makes her joint holder with three other hurricanes for the second strongest Atlantic hurricane on record, dating back to 1851. Only Hurricane Allen of 1980 was stronger with 305 km/h (190 mph) winds. As for Maria – her peak sustained winds of 280 km/h (175 mph) tied her with seven other hurricanes for the eighth strongest on record.

Super Category 5 Hurricane Irma on our doorsteps

Super Category 5 Hurricane Irma on our doorsteps – Sep 5, 2017

Have TCs become more intense?

According to the IPCC, “unlikely“. The IPCC is the UN international body designated to assess the science of climate change. It was set up in 1988 by World Meteorological Organization (WMO) and UN Environment Programme (UNEP) to provide policymakers with rigorous and balanced scientific information on climate change.

This is what the IPCC Assessment Report Five (AR5) – the latest report, says about the impact of climate change on TCs: “In summary…recent assessments indicate that it is unlikely that annual numbers of tropical storms, hurricanes and major hurricanes counts have increased over the past 100 years in the North Atlantic basin…”

So, based on the examination of many peer-reviewed scientific research papers, the IPCC concluded that there is no trend in the intensity of tropical cyclones – there is no robust data to support the notion that climate change has caused TCs to be “stronger and bigger” “with each passing hurricane season”.

There is, however, a line in the IPCC report that some may point to as evidence of climate change causing TCs to be stronger. It says: “Evidence, however, is for a virtually certain increase in the…intensity of the strongest tropical cyclones since the 1970s in that [North Atlantic] region.” Is this an endorsement of the view that climate chance is causing TCs to be stronger? Certainly not.

TC activity in the North Atlantic, like most places, go through phases – inactive and active periods that last for multiple of decades at a time, which are closely linked to the Atlantic Multidecadal Oscillation (AMO). Active phases are marked by above normal number and strength of TCs and the opposite for inactive phases.

3-Year-Average ACE

A plot of three-year-averaged Accumulated Cyclone Energy (ACE). It is a measure of the activity of a hurricane season based on strength, duration and the number of TCs. It shows the active and inactive phases of the Atlantic hurricane season. These phases are natural – NOT caused by human-induced climate change.

So, from the 1970s to the mid-1990s, the Atlantic went through an inactive phase, where there was a decline in the frequency and intensity of the number of TCs from the mid-1920s to 1960s – the previous active period before our current one.

The Atlantic is currently in an active phase which began around the mid-1990s. So, certainly, the record shows an increase in the frequency and intensity of TCs since the 1970s. But this does not mark an overall increase – it is just a part of the decadal cycle. Further, according to the IPCC’s latest report, there is low confidence that this increase is due to human-induced climate change. In other words, there is an 80% chance that this increase is not due to climate change.

So, the IPCC, and by extension, the UN is clear about the impact of climate change on TCs – it is currently “unlikely” having any effect on TC intensity. The same IPCC indicates that it is unequivocal that man is changing the climate. This is a position shared by over 99% of climate scientists and people in general. If we believe this part of the report, we must believe the part that speaks about the impact or lack thereof, at this time, of climate change on TCs.

This research position also shared by WMO and the TC research community.

Will TCs become more intense?

According the IPCC AR5, “more likely than not”. Going forward, IPCC AR5 says that climate change will “more likely than not” cause changes in TC intensities late in this current century – near year 2100.

This conclusion by the IPCC is consistent with one of the most authoritative scientific papers on the subject written by Knutson TR, McBride JL, Chan J, et al.. It says that late in this century, not now, based on models, there will be a 2 to 11% increase in wind speeds of TCs.

Empirical Data and the impact of climate change on TCs

Let’s look at the empirical data to see what they say regarding the impact of climate change on TCs. Based on the https://coast.noaa.gov/hurricanes/ database, the following is true:

CountOfHurricanes1924-1969VS1970-2016

A count of the number of hurricanes – 1924 to 1969 vs. 1970 to 2016. The bracketed numbers are the yearly averages.

From the above, clearly, there is no significant difference between numbers for the period 1924-1969, when climate change was not an issue, and the period 1970-2016, when climate change became an issue. What is interesting is that the numbers for 1924-1969 would have been higher, if not for a number of TCs being missed, due to a lack of satellite technology prior to the mid-1960s.

Further, some of the most powerful hurricanes to form across the Atlantic Basin occurred when climate change was not an issue – before the mid-1980s. Hurricane Allen which still holds the record for the highest maximum sustained wind speed ever on record over the Atlantic occurred in 1980. Of the top 12 strongest hurricanes, in terms of maximum sustained winds, six occurred before the mid-1980s.

Based on the IPCC and the vast majority of the TC researchers, TCs are an unworthy poster child for climate change. By the preponderance of research papers, climate change has NOT caused TCs to be stronger. Climate change had nothing to do with the strength of Hurricanes Irma and Maria or the activity of the 2017 Atlantic hurricane season. It is all due to natural variability.

However, by late this century “more likely than not” climate change WILL cause an intensification of TCs. But this is not yet evident. This is the conclusion of IPCC AR5 – we either accept the full report or none at all. And if we accept all of it, we would not be blaming hurricane intensities on climate change, at this time.

Our next blog in this series will look at the impact, if any, of climate change on tropical cyclone frequencies.

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Tropical Cyclones and Climate Change

28 09 2017

Dale C. S. Destin |

With the current Atlantic hurricane season being hyperactive, thus far, and several countries being severely impacted by hurricanes, tropical cyclones (tropical depressions, tropical storms and hurricanes) continue to be made the “poster child” for the potential dangerous impacts of climate change.

Hurricane Irma - Sep 5-6, 2017. Travelling Across Barbuda and Anguilla.

Cat. 5 Hurricane Irma With Winds of 185 mph – Sep 5-6, 2017. The Eye travelled Across Barbuda and the Northern Leeward Islands.

This notion was crystallized by Former U.S. Vice President Al Gore on the cover of his book – “an inconvenient truth”, when he depicted a tropical cyclone (TC) spinning out of smoke stacks. Since then, several world leaders have supported this view, including former U.S. President Barak Obama, who said that “storms [are] growing stronger with each passing hurricane season”.

Over the years, many Caribbean leaders, have also joined in asserting that TCs have become stronger and more frequent due to manmade, greenhouse gas caused climate change. According to Prime Minister Gaston Browne of Antigua and Barbuda, “hurricanes are stronger and bigger because they are absorbing moisture from increasingly warmer seas, caused by global warming.” Browne’s colleague, Prime Minister Roosevelt Skerrit of Dominica supports this view and is convinced that Hurricane Maria is the product of climate change. He told the recent United Nations General Assembly that Maria was a climate change “truth we have just lived”.

Cat. 5 Hurricane Maria - 10:30, Sep 18, 2017- Eye Over Dominica.

Cat. 5 Hurricane Maria With Winds of 160 mph – 10:30 pm, Sep 18, 2017 – Eye Over Dominica.

There is no doubt that climate change is real and is happening. There is much evidence to point to. Contrary to the current U.S. President’s assertion, it is NOT a hoax created by the Chinese. However, do TCs deserve this poster-child position? How have TCs responded to manmade climate change? How will TCs respond to future climate change? Are there any doubts as to cause of Hurricanes Irma and Maria?

How have TCs responded, and how will they respond to human-induce climate change are topics of intense interest and public and scientific debates. Over the next few blogs, I will use the latest peer-reviewed scientific papers to answer these and related questions.

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Is Hurricane Alex a Sign?

18 01 2016

Dale C. S. Destin |

By now you may have heard of Hurricane Alex, the first Atlantic hurricane to form in January since the only other one in 1938, according to HURDAT2. Alex is also only the third hurricane to exist over the Atlantic in January. There was Alice2 which formed late December 1954 and continued over into January 1955.

Hurricane Alex, as seen via a NASA satellite image on Jan. 14, 2016.

Hurricane Alex, as seen by a NASA satellite on January 14, 2016.

I suppose a few of the questions raised in some persons minds by Alex are: what does this mean for the upcoming hurricane season; does a January storm equate to an active or inactive season and was Alex caused by climate change?

Looking back at the three other times tropical cyclones – the generic name for depressions, tropical storms and hurricanes – formed in January for clues, I see none.

In 1978, there were 12 named storms (NS), 5 hurricanes and 2 major hurricanes (MH). In 1951, there were also 12 NS, with 8 becoming hurricanes and 3 MH. In 1938, the record shows that after Hurricane One in January, the eventual totals for the year were 9 NS, 4 hurricanes and 2 MH. Nothing unusual!

Based on El Nino Southern Oscillation data, which go back to 1950, Tropical Storm One of 1951 occurred during a moderate La Nina, whereas the 1978 system occurred during a weak El Nino. Of course, Alex formed in one of the strongest, if not the strongest El Nino on record.

Looking at the preceding hurricane seasons to the four January tropical cyclones, I also see nothing interesting.

In 1977, there were 6 NS, 5 hurricanes and a MH; 1950 had 16 NS, 11 hurricanes and 6 MH and 1937 had 11 NS, 4 hurricanes and a MH.  And of course, in 2015, we had 11 NS, four hurricanes and 2 MH. Except for 1950, the other prior years had near normal number of storms.

Of the three previous years with January tropical cyclones, only one had a second preseason tropical cyclone formed before the official hurricane season – June 1 to Nov 30. This was 1951 with Hurricane Able in May. So, one cannot really say that January storms are a precursor to other preseason ones.

Looking a bit more broadly, there have been, at least, six years with preseason hurricanes (May 1970, May 1951, Jan 1938, March and May 1908, May 1889, May 1863) on record since 1851 prior to Alex. For such small sample sizes, no definite conclusions can be made. However, it may be instructive that these six seasons had near normal number of NS and hurricanes.

Here is what Dr. Rick Knabb – Director of the U.S. National Hurricane Center (NHC) – had to say on the meaning of Alex and preseason tropical cyclone activity:

The activity of a hurricane season is more closely linked to the existing climate during the given season. This climate is very difficult to predict in advance of the season, which is evident by the fact that the best performing hurricane season forecasts are those issued around early August.

In speaking with Phil Klotzbach of Colorado State University via email, he shared the view that, “any tropical cyclone forming in January to be more responding to last year’s climate forcings than anything that would be present in 2016.”

So, Alex can be considered a super late hurricane that may be best credited to the 2015 hurricane season as opposed to being a super early system, which will be credited to the 2016 season.

Then there is the question of climate change, which has nothing to do with Alex. The previous three tropical cyclones forming in January occurred before 1979, pretty much predating climate change as an issue. This shows that January cyclones were more frequent before climate change was a concern.

Although HURDAT2 only has three tropical cyclones on record in January, it is a widely known fact that a number of such cyclones were missed prior to the satellite era and possibly shortly thereafter. This is especially so for those short-lived (lasting less than 48 hours) and forming in a similar area to that of Alex.

Given the advanced weather monitoring enterprise that exists today, we in the meteorological fraternity are pretty confident that no such system has formed in the past 20 years, at least. Thus, it would stand to reason that such January tropical cyclones like Alex are becoming fewer with climate change.

Further, according the IPCC, climate change is likely to cause a reduction in the number of tropical cyclones. Thus, based on our understanding of the science, it would be reasonable to expect that out-of-season tropical cyclones, such as Alex, will become even rarer as the more ideal conditions of the summer months would be required for their formation.

One of the things that climate change will alter, that will lead to fewer storms, is the lapse – the change of temperature with increasing height. It will cause it to decrease, leading to a less unstable, hurricane unfriendly atmosphere. In Alex’s case, the environmental lapse rate was very large contrary to the decreasing trend being caused by climate change.

Tropical cyclones in January and preseason hurricanes in general over the Atlantic are very rare; hence, the sample size is very small. Thus, one cannot be dogmatic about what Hurricane Alex means for the upcoming season. Notwithstanding, I am very confident that Alex is an aberration, virtually a freak hurricane, and means nothing for the upcoming hurricane season. It is also clearly not the fault of climate change, which is making it harder for hurricanes to form. Rather, it is simple natural climate variability.

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