Essay by Eric Worrall
Aussie researchers believe cyclone intensity may be rising, though their study was “too coarse” to address this question.
Global decrease in tropical cyclones identified by Australian scientists
Tropical cyclones are occurring less frequently around the world due to climate change, Australian scientists have found.
- Researchers constructed a history of tropical cyclones around the world back to the 1850s
- They say cyclones are becoming more intense due to climate change
- Australia is experiencing about 11 per cent fewer cyclones compared to the 19th century
The team led by Savin Chand from Federation University discovered tropical cyclones were happening about 13 per cent less frequently than in the pre-industrial period.
“We have consistently found that cyclone numbers are going down around the world,” Dr Chand said.
However, the authors said their study only looked at the frequency of tropical cyclones, not their intensity, which they said was increasing due to climate change.
“As the atmosphere warms, tropical cyclones forming have more fuel for their severity,” Dr Chand said.
“Even though cyclones will get fewer, they will get more intense.”
The abstract of the study;
Declining tropical cyclone frequency under global warming
Savin S. Chand, Kevin J. E. Walsh, Suzana J. Camargo, James P. Kossin, Kevin J. Tory, Michael F. Wehner, Johnny C. L. Chan, Philip J. Klotzbach, Andrew J. Dowdy, Samuel S. Bell, Hamish A. Ramsay & Hiroyuki Murakami
Assessing the role of anthropogenic warming from temporally inhomogeneous historical data in the presence of large natural variability is difficult and has caused conflicting conclusions on detection and attribution of tropical cyclone (TC) trends. Here, using a reconstructed long-term proxy of annual TC numbers together with high-resolution climate model experiments, we show robust declining trends in the annual number of TCs at global and regional scales during the twentieth century. The Twentieth Century Reanalysis (20CR) dataset is used for reconstruction because, compared with other reanalyses, it assimilates only sea-level pressure fields rather than utilize all available observations in the troposphere, making it less sensitive to temporal inhomogeneities in the observations. It can also capture TC signatures from the pre-satellite era reasonably well. The declining trends found are consistent with the twentieth century weakening of the Hadley and Walker circulations, which make conditions for TC formation less favourable.Read more: https://www.nature.com/articles/s41558-022-01388-4
On cyclone intensity;
… The 20CR results and the high-resolution climate model results presented here show clear downward trends in global and regional TC numbers between the pre-industrial and the more recent climate period. The downward trend remains robust after accounting for the effects of natural climate variability and aerosol effects for the North Atlantic and basin-specific biases in the 20CR data. It is hypothesized that these changes are probably due to the twentieth century weakening of the major tropical circulations, which creates more hostile conditions for TC formation. These findings provide new insights that can inform our confidence in future projections of fewer TC numbers associated with greenhouse warming12. While the resolutions of the current reanalysis products are too coarse to make conclusions about TC intensity, general consensus from observationally based records suggest an increase in the proportion of severe storms with anthropogenic-induced warming2. Going forward, it is anticipated that continued improvement in reanalysis and climate model products and in observational datasets can help identify attributable anthropogenic climate change signals on metrics such as TC intensity and landfalling activities. …Read more: Same link as above
The abstract of the reference 2 study on rising storm intensity;
Tropical Cyclones and Climate Change Assessment: Part I: Detection and Attribution
An assessment was made of whether detectable changes in tropical cyclone (TC) activity are identifiable in observations and whether any changes can be attributed to anthropogenic climate change. Overall, historical data suggest detectable TC activity changes in some regions associated with TC track changes, while data quality and quantity issues create greater challenges for analyses based on TC intensity and frequency. A number of specific published conclusions (case studies) about possible detectable anthropogenic influence on TCs were assessed using the conventional approach of preferentially avoiding type I errors (i.e., overstating anthropogenic influence or detection). We conclude there is at least low to medium confidence that the observed poleward migration of the latitude of maximum intensity in the western North Pacific is detectable, or highly unusual compared to expected natural variability. Opinion on the author team was divided on whether any observed TC changes demonstrate discernible anthropogenic influence, or whether any other observed changes represent detectable changes. The issue was then reframed by assessing evidence for detectable anthropogenic influence while seeking to reduce the chance of type II errors (i.e., missing or understating anthropogenic influence or detection). For this purpose, we used a much weaker “balance of evidence” criterion for assessment. This leads to a number of more speculative TC detection and/or attribution statements, which we recognize have substantial potential for being false alarms (i.e., overstating anthropogenic influence or detection) but which may be useful for risk assessment. Several examples of these alternative statements, derived using this approach, are presented in the report.Read more: https://journals.ametsoc.org/view/journals/bams/100/10/bams-d-18-0189.1.xml
“Medium confidence” of increased cyclone intensity might not mean what you think it means. The IPCC vaguely defines medium confidence to mean 33-66% probability, as likely as not, though the wording of the definition is a little ambiguous. A much clearer definition of the term “medium confidence” was the subject of climategate email 0967041809.txt, in which Climate Scientist Stephen Schneider advanced the term “Medium Confidence” as a direct replacement for the term “Inconclusive”.
From: Stephen H Schneider <redacted> To: redacted Subject: Re: THC collapse Date: Wed, 23 Aug 2000 10:43:29 -0700 (PDT) Cc: Thomas Stocker <redacted>, Jerry Meehl <redacted>, Timothy Carter <redacted>, redacted, redacted, redacted, redacted, redacted, redacted, redacted, redacted, "Stouffer, Ron" <redacted>, redacted Great Tom, I think we are converging to much clearer meanings across various cultures here. Please get the inconclusive out! By the way, "possible" still has some logical issues as it is true for very large or very small probabilities in principle, but if you define it clearly it is probably OK--but "quite possible" conveys medium confidence better--but then why not use medium confidence, as the 3 rounds of review over the guidance paper concluded after going through exactly the kinds of disucssions were having now. Thanks, Steve On Wed, 23 Aug 2000 redacted wrote: > > > Steve, I agree with your assesement of inconclusive --- quite possible is > much better and we use 'possible' in the US National Assessment. Surveys > has shown that the term 'possible' is interpreted in this range by the > public. > > Tom > > > > > Stephen H Schneider <redacted> on 08/23/2000 03:02:33 AM > > > > To: Thomas Stocker <redacted> > > cc: Jerry Meehl <redacted>, Timothy Carter > <redacted>, redacted, > redacted, redacted, > redacted, redacted, > Tom Karl/NCDC, redacted, > redacted, redacted, > "Stouffer, Ron" <redacted> > > > > Subject: Re: THC collapse > > > > > > > > > > Hello all. I appreciate the improvement in the table from WG 1, > particularly the inclusion of symmetrical confidence levels--but please > get rid of the ridiculous "inconclusive" for the .34 to .66 subjective > probability range. It will convey a completely differnt meaning to lay > persons--read decisionmakers--since that probability range represents > medium levels of confidence, not rare events. A phrase like "quite > possible" is closer to popular lexicon, but inconclusive applies as well > to very likely or very unlikely events and is undoubtedly going to be > misinterpreted on the outside. I also appreciate the addition of > increasing huricane intensities with warming moving out of the catch all > less than .66 category it was in the SOD. > I do have some concerns with the THC issue as dealt with here--echoing > the comments of Tim Carter and Thomas Stocker. I fully agree that the > likelihood of a complete collapse in the THC by 2100 is very remote, but > to leave it at that is very misleading to policymakers given than there is > both empirical and modeling evidence that such events can be triggered by > phenomena in one century, but the occurrence of the event may be delayed > a century or two more. Given also that the likelihood of a collapse > depends on several uncertain parameters--CO2 stabilization level, CO2 > buildup rate, climate sensitivity, hydrological sensitivity and initial > THC overturning rates, it is inconceivable to me that we could be 99% sure > of anything--implied by the "exceptionally unlikely" label--given the > plausibility of an unhappy combo of climate sensitivity, slower than > current A/OGCMs initial THC strength and more rapid CO2 increase > scenarios. Also, if 21st century actions could trigger 22nd century > irreversible consequences, it would be irresponsible of us to not mention > this possibility in a footnote at least, and not to simply let the matter > rest with a very low likelihood of a collapse wholly within the 21st > century. So my view is to add a footnote to this effect and be sure to > convey the many paramenters that are uncertain which determine the > likelihood of this event. > Thanks again for the good work on this improtant table. Cheers, Steve > > > On Wed, 23 Aug 2000, Thomas Stocker wrote: > > > DEar Jerry, Tim and Ron et al > > > > I agree that an abrupt collapse - abrupt meaning within less than a > decade, say > > - has not been simulated by any climate model (3D and intermediate > complexity) > > in response to increasing CO2. Some models do show for longer > integrations a > > complete collapse that occurs within about 100-150 years. If you put that > into > > context of the apparent stability of THC during the last 10,000 years or > so, > > this is pretty "abrupt". > > > > Following up on the discussion regarding THC collapse, I think the > statement Ron > > apparently added to Ch9 needs to be made more specific. In order to keep > Ch7 and > > Ch9 consistent, I propose to Ron the following revision: > > > > "It seems that the likelihood of a collapse of the THC by year 2100 is > less > > than previously thought in the SAR based on the AOGCM results to date." > > > > There is really no model basis to extend this statement beyond 2100 as > evidenced > > by the figures that we show in TAR. There are many models that now run up > to > > 2060, some up to 2100, but very few longer. > > > > Also I should add for your information, that we add to Ch7 a sentence: > > > > "Models with reduced THC appear to be more susceptible for a > > shutdown." > > > > Models indicate that the THC becomes more susceptible to collapse if > previously > > reduced (GFDL results by Tziperman, Science 97 and JPO 99). This is > important as > > "collapse unlikely by 2100" should not tempt people to conclude that THC > > collapse is hence not an issue. The contrary is true: reduction means > > destabilisation. > > > > Best regards > > > > thomas > > -- > > ------------------------------------------------------------------ > > Thomas Stocker > > Climate and Environmental Physics redacted > > Physics Institute, University of Bern phone: redacted > > Sidlerstrasse 5 NEW fax: redacted > > 3012 Bern, Switzerland http://www.climate.unibe.ch/~stocker > > ------------------------------------------------------------------ > > > > ------ > Stephen H. Schneider > Dept. of Biological Sciences > Stanford University > Stanford, CA 94305-5020 U.S.A. > > Tel: redacted > Fax: redacted > redacted > > > > > ------ Stephen H. Schneider Dept. of Biological Sciences Stanford University Stanford, CA 94305-5020 U.S.A. Tel: redacted Fax: redacted redacted
And of course, there is the obvious question; If conditions are less favourable to cyclone formation, why do the storms which do form have to be more intense? Wouldn’t climatically unfavourable conditions tend to retard the formation and peak intensity of those cyclones which do appear? Why not a rise in crop friendly low to moderate intensity rainfall events?
The thermodynamic and atmospheric water cycle books must balance, but there seems no compelling reason to assume that balance has to come in the form of a rise in world wrecking extreme weather events.