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Does global warming make tropical cyclones stronger?

Filed under: — stefan @ 30 May 2018

By Stefan Rahmstorf, Kerry Emanuel, Mike Mann and Jim Kossin

Friday marks the official start of the Atlantic hurricane season, which will be watched with interest after last year’s season broke a number of records and e.g. devastated Puerto Rico’s power grid, causing serious problems that persist today. One of us (Mike) is part of a team that has issued a seasonal forecast (see Kozar et al 2012) calling for a roughly average season in terms of overall activity (10 +/- 3 named storms), with tropical Atlantic warmth constituting a favorable factor, but predicted El Nino conditions an unfavorable factor.  Meanwhile, the first named storm, Alberto, has gone ahead without waiting for the official start of the season.

In the long term, whether we will see fewer or more tropical cyclones in the Atlantic or in other basins as a consequence of anthropogenic climate change is still much-debated. There is a mounting consensus, however, that we will see more intense hurricanes. So let us revisit the question of whether global warming is leading to more intense tropical storms. Let’s take a step back and look at this issue globally, not just for the Atlantic.

Tropical storms are powered by evaporation of seawater.  More than 30 years ago, one of us (Emanuel) developed a quantity called potential intensity that sets an upper bound on hurricane wind speeds. In general, as the climate warms, this speed limit goes up, permitting stronger storms than were possible in the past.

Of course there could be other changes in the climate system that counteract this – e.g. an increase in wind shear that tears the hurricanes apart, changes in the humidity of the atmosphere, or increases in natural or anthropogenic aerosols. This question has been investigated for many years with the help of model simulations. The results of numerous such studies can be summarized briefly as follows: due to global warming we do not necessarily expect more tropical storms overall, but an increasing number of particularly strong storms in categories 4 and 5, especially storms of previously unobserved strength. This assessment has been widely agreed on at least since the 4th IPCC Report of 2007 and reaffirmed several times since then. A review article in the leading journal Science (Sobel et al. 2016) concluded:

We thus expect tropical cyclone intensities to increase with warming, both on average and at the high end of the scale, so that the strongest future storms will exceed the strength of any in the past.

Models also suggest that atmospheric aerosol pollution may have weakened tropical storms and masked the effect of global warming for decades, making it more difficult to detect trends in measurement data.

What do the data show?

Nevertheless, observational data support the expectation from models that the strongest storms are getting stronger. We focus here on the period from 1979, because this is the period covered by geostationary satellite data (thus no cyclones went unobserved) and also the period over which three quarters of global warming has occurred. These data show an increase in the strongest tropical storms in most ocean basins (Kossin et al. 2013). However, these data are not homogeneous but are estimated from a variety of satellite, and air- and ground-based instruments whose capabilities have improved over time. The homogenization of these data by Kossin et al. (2013), which is generally recognized as very careful, reduces the trends, but does not eliminate them. The strongest increase can be found in the North Atlantic (which is more than 99% significant) where the trend has likely been boosted by the decrease in sulfate aerosols over this period.

One consequence of this increase is that in most major tropical cyclone regions, the storms with the highest wind speeds on record have been observed in recent years (see Fig. 1 based on reanalysis by Velden et al. 2017). The strongest globally was Patricia (2015), which topped the previous record holder Haiyan (2013).

Fig. 1 The strongest storms for the major storm regions Western and Eastern North Pacific, North Indian, South Indian and South Pacific, Caribbean/Gulf of Mexico and open North Atlantic. Of these seven regions, five had the strongest storm on record in the past five years, which would be extremely unlikely just by chance. Irma was added by personal communication from Chris Velden, and a tie of two storms with equally strong winds in the South Indian was resolved by selecting the storm with the lower central pressure (Fantala). (Graph by Stefan Rahmstorf, background image from Robert Rohde, Creative Commons License CC BY-SA 3.0.)

Other recent records are worth mentioning. Sandy (2012) was the largest hurricane ever observed in the Atlantic. Harvey (2017) dumped more rain than any hurricane in the United States. Ophelia (2017) formed further northeast than any other Category 3 Atlantic hurricane – fortunately it turned north before striking Portugal, against initial predictions, and then weakened over cool waters before it hit Ireland. September 2017 broke the record for cumulative hurricane energy in the Atlantic. Irma (2017) sustained wind speeds of 300 km/h longer than any storm on record (for 37 hours – the previous record was 24 hours by Haiyan in 2013). Cyclone Pam in March 2015 was  already beaten again by Winston in February 2016 according to the Southwest Pacific Enhanced Archive for Tropical Cyclones (though not in Velden’s data analysis). Donna in 2017 was the strongest May cyclone ever observed in the Southern Hemisphere. All coincidence?

One of us (Emanuel) performed an analysis of linear trends in historical tropical cyclone data from 1980 to 2016. These include some observations by aircraft, ships, buoys, and stations on land in addition to the satellite data, but these have not been treated for inhomogeneities.

Fig. 2 Percentage increase 1980 to 2016 (as a linear trend) in the number of tropical storms worldwide depending on their strength. Only 95% significant trends are shown. The strongest storms are also increasing the most. Red colors show the hurricane category on the Saffir-Simpson scale. Graph by Kerry Emanuel, MIT. Creative Commons License CC BY-SA 3.0.

A significant global increase (95% significance level) can be found in all storms with maximum wind speeds from 175 km/h. Storms of 200 km/h and more have doubled in number, and those of 250 km/h and more have tripled. Although some of the trend may be owing to improved observation techniques, this provides some evidence that a global increase in the most intense tropical storms due to global warming is not just predicted by models but already happening.

However, global warming does not only increase the wind speed or frequency of strong storms (which is actually two ways of looking at the same phenomenon, as frequency depends on wind speed).  The average location where the storms are reaching their peak intensity is also slowly migrating poleward (Kossin et al. 2014) and the area where storms occur expands (Benestad 2009, Lucas et al. 2014), which changes patterns of storm risk and increases risk in regions that are historically less threatened by these storms (Kossin et al. 2016).

Most damage caused by tropical storms is not directly caused by the wind, but by water: rain from above, storm surge from the sea. Harvey brought the largest amounts of rain in US history – the probability of such a rain event has increased several times over  recent decades due to global warming (Emanuel 2017; Risser and Wehner, 2017; van Oldenborgh et al., 2017). Not least due to global warming, sea levels are rising at an accelerating rate and storm surges are becoming more dangerous. A recent study (Garner et al. 2017), for example, shows that the return period of a certain storm surge height in New York City will be reduced from 25 years today to 5 years within the next three decades. Therefore, storm surge barriers are the subject of intensive discussion in New York (Rahmstorf 2017).

While there may not yet be a “smoking gun” – a single piece of evidence that removes all doubt – the weight of the evidence suggests that the thirty-year-old prediction of more intense and wetter tropical cyclones is coming to pass. This is a risk that we can no longer afford to ignore.

 


Kerry Emanuel
is professor of atmospheric science at MIT


Jim Kossin
is a NOAA climate scientist specializing in tropical cyclones

(And Mike and Stefan of course are co-founders and regular authors of Realclimate)

 

References

 

Benestad RE (2009) On tropical cyclone frequency and the warm pool area. Natural Hazards and Earth System Sciences 9(2):635-645.

Emanuel K (2017) Assessing the present and future probability of Hurricane Harvey’s rainfall. Proc Natl Acad Sci U S A.

Garner A, et al. (2017) The Impact of Climate Change on New York City’s Coastal Flood Hazard: Increasing Flood Heights from the Pre-Industrial to 2300 CE. Proc Natl Acad Sci U S A.

Kossin JP, Olander TL, & Knapp KR (2013) Trend Analysis with a New Global Record of Tropical Cyclone Intensity. J. Clim. 26(24):9960-9976.

Kossin, J. P., K. A. Emanuel, and G. A. Vecchi, 2014: The poleward migration of the location of tropical cyclone maximum intensity. Nature509, 349-352.

Kossin, J. P., K. A. Emanuel, and S. J. Camargo, 2016: Past and projected changes in western North Pacific tropical cyclone exposure. J. Climate29, 5725-5739.

Kozar, M.E., Mann, M.E., Camargo, S.J., Kossin, J.P., Evans, J.L. (2012)  Stratified statistical models of North Atlantic basin-wide and regional tropical cyclone counts, J. Geophys. Res., 117, D18103, doi:10.1029/2011JD017170.

Lucas, C., Timbal, B. & Nguyen, H. (2014) The expanding tropics: a critical assessment of the observational and modeling studies. WIREs Clim. Change, 5, 89–112.

Rahmstorf S (2017) Rising hazard of storm-surge flooding. Proc Natl Acad Sci U S A 114(45):11806-11808

Risser, M. D., & Wehner, M. F. (2017): Attributable human-induced changes in the likelihood and magnitude of the observed extreme precipitation during Hurricane Harvey. Geophy. Res. Lett., 44, 12,457–12,464.

Sobel A, et al. (2016) Human influence on tropical cyclone intensity. Science 353:242-246.

van Oldenborgh, G. J., and Coauthors, 2017: Attribution of extreme rainfall from Hurricane Harvey. Environ. Res. Lett., 12, doi: 10.1088/1748-9326/aaa343.

Velden C, Olander T, Herndon D, & Kossin JP (2017) Reprocessing the Most Intense Historical Tropical Cyclones in the Satellite Era Using the Advanced Dvorak Technique. Mon. Weather Rev. 145(3):971-983.

 

55 Responses to “Does global warming make tropical cyclones stronger?”

  1. 1
    Rasmus Benestad says:

    Another interesting aspect is the statistical distribution of wind speeds and its probability density function (PDF).

    It is commonly assumed that the probability associated with different wind speeds can be described through a Weibull distribution.

    The mathematical curve describing the probabilities allows for a very limited set of solutions with fewer and more intense tropical cyclones (defined as wind speed above a threshold value). However, such cases are a very small fraction of the total amount of solutions for the Weibull distribution.

    This is described in more details in a previous post: http://www.realclimate.org/index.php/archives/2011/11/the-ipcc-report-on-extreme-climate-and-weather-events/

  2. 2

    “Lessons not learned will be repeated”
    Thank you very much for this.
    Question about the max wind speed – graph seems to stop at 275mph… is that the top measured till now? What is the ultimate possible?

    It sent me looking for other sources “Grey Swan Predictions” in Nature Climate Change – ftp://texmex.mit.edu/ftp/ftp/pub/emanuel/PAPERS/nclimate2777.pdf

  3. 3

    In regards to the frequency of weaker hurricanes, in a previous paper of mine I found that African easterly wave activity was reduced in a warming scenario due to a systematic reduction in the effectiveness of the vortex stretching process. The mechanism relies on the projected increase in tropical static stability and “weak temperature gradient” balance (which is loosely satisfied). I’ve always thought this same mechanism might help explain why we don’t see an increase in the frequency of weaker hurricanes because they become harder to spin-up with as the troposphere warms. I haven’t been able to work on this topic in awhile, but I’d be curious to hear any thoughts on this idea if anyone has a chance to skim the paper.

    Hannah, W. M., and A. Aiyyer, 2017: Reduced African Easterly Wave Activity with Quadrupled CO2 in the Super-Parameterized CESM. J. Climate, 30, 8253–8274.

  4. 4

    “It is commonly assumed that the probability associated with different wind speeds can be described through a Weibull distribution.”

    Also a Rayleigh distribution, which can be described as a Maximum Entropy distribution of energy levels corresponding to a statistical mechanics view.

  5. 5
    Rohde A. Robert says:

    The background map of tropical cyclone tracks should be attributed to me, and not to the NASA Earth Observatory. I am the one who made it. NASA EO is one of the many places that has republished it. However, if you look at the NASA EO page where it appears, my name is credited at the bottom of their page.

    [Response: Sorry! I’ll fix it. – gavin]

    [Response: Fixed – Stefan]

  6. 6
    Steven Emmerson says:

    Is there some reason typhoon Tip (305 km/h, 870 hPa on 1979-10-12) isn’t mentioned?

  7. 7
    ab says:

    “Does global warming make tropical cyclones stronger?”

    Global warming, no.

    Increase in daily thermal amplitude (day-night) caused by historical global deforestation, yes.

    [Response: You like to throw in random nonsense hypotheses, right? But we are a science blog here so we like serious discussions which actually lead to some insights. If you do have something serious to say you are welcome, and please support it by evidence. -Stefan]

  8. 8
    Russell says:

    The observation that
    “The weight of the evidence suggests that the thirty-year-old prediction of more intense and wetter tropical cyclones is coming to pass. This is a risk that we can no longer afford to ignore.”

    raises the question of how to manage that risk .

    As Kerry and his colleagues were the first to examine the impact of hurricane track sea surface albedo on storm evolution and intensity, we look to them them to wrestle with the ramifications of mesoscale SRM

    Because storm damage is as local as greehouse forcing is global, this is a far cry from stratospheric geoengineering.

    https://www.nature.com/articles/d41586-018-05151-8

  9. 9
    nigelj says:

    I came across this research study a couple of days ago: “Intensification of landfalling typhoons over the northwest Pacific since the late 1970s, Wei Mei & Shang-Ping Xie, in Nature Geoscience”:

    https://www.nature.com/articles/ngeo2792

    It was linked in a Guardian article “Asian typhoons becoming more intense, study finds”

    https://www.theguardian.com/environment/2016/sep/05/asian-typhoons-becoming-more-intense-study-finds

    I’m more or less a lay person, but particularly interested as we get quite intense ex tropical cyclones in my country.

  10. 10
    Adam Lea says:

    I remember reading somewhere that there is an upper limit as to how strong the near surface winds can get in a tropical cyclone, and I’m not thinking about the maximum potential intensity. As wind speed increases, wind shear very close to the ground increases, and so does frictional dissipation. As a result, there comes a point where the frictional dissipation equals the conversion of heat to kinetic energy, and the upper limit of the strength of a tropical cyclone is around that of hurricane Patricia’s peak intensity (185 kts). Correct me if I’ve got something wrong here.

  11. 11
    Hank Roberts says:

    Writing from the US “Left coast” (California/Oregon/Washington) — is there reason to think we’ll be seeing Pacific storms coming onshore? I grew up in North Carolina and experienced quite a few Atlantic hurricanes, and I know the building and drainage infrastructure along the Pacific coast is quite unprepared for such. Same question for Europe, which has had some such arrive.

  12. 12

    Such a remarkable body of work over all these decades by Kerry and, increasingly, so many others. Glad I could be there at the beginning. Kerry’s initial results were included in my 1988 Discover cover story, which I scanned and posted here: https://www.slideshare.net/Revkin/global-warming-as-news-1988

    Excerpt:

    ~ Even as cities become more vulnerable to moderate storms, the intensity of hurricanes may increase dramatically, says Kerry Emanuel, a meteorologist at MIT. Hurricane intensity is linked to the temperature of the sea surface, Emanuel explains. According to his models, if the sea warms to predicted levels, the most intense hurricanes will be 40 to 50 percent more severe than the most intense hurricanes of the past SO years.

    James Titus, director of the Environmental Protection Agency’s Sea Leve! Rise Project, says communities will have two choices: build walls or get out of the way. ~

  13. 13
    David Appell says:

    How does the evaporation rate vary as SST increases?

  14. 14
    John Pollack says:

    Hank @11 See https://journals.ametsoc.org/doi/abs/10.1175/BAMS-85-11-1689
    for an account of the San Diego hurricane of 2 October 1858.
    No, they aren’t prepared for a repeat.

  15. 15
    Marco says:

    Oi, Gavin, the poor man’s name is Robert, not Richard Rohde!

  16. 16
    Greg Simpson says:

    Thanks for the update.

    I have previously felt that the argument for stronger cyclones was probably true, but not firm enough for me to include it in discussions with others. You’ve tied the evidence together nicely to make a compelling thesis.

  17. 17
    Pierre-Ernest says:

    Global warming decreases the différence in temperature between Tropics and Arctic which is the fuel of hurricanes. So, it should be logic, according to Richard Lindzen, to see the strength of cyclones decreasing and not increasing in the future.

    [Response: There you’re talking about the mid-latitudes, not tropical cyclones. -Stefan]

  18. 18
    nigelj says:

    New Zealand experienced an unusually intense ex tropical cyclone last summer that came unusually towards the south. Before it hit land, it increased in intensity dramatically, and according to our climate agency (NIWA) this was a result of unusually warm southern oceans. This warmth has been attributed to global warming, in combnation with a la nina affecting ocean overturning circulation, and the antarctic oscillation being in a positive phase, unusually so due to the effect of climate change.

    So it was quite a combination of things, and lead to an extremely destructive cyclone with winds beyond what our infrastructure is designed to withstand. But this combination may not be rare any longer, and is likely to become stronger. If these intense cyclones become more common, it will become a substantial economic cost in terms of repairs and lost productivity time. It is also only one facet of how these cyclones may change and affect my country.

    This is probably just too complicated to grasp for the average dim witted politician.

  19. 19
    ab says:

    Stefan @7,
    If you do have something serious to say you are welcome, and please support it by evidence.

    I am showing it to you: this article does not present any mechanism able to link global warming to stronger cyclones. And for cause: global warming can not explain stronger cyclones.

    What explains stronger cyclones is an increase in intensity of the atmospheric circulation.

    1) What does explain such an increase in intensity ?

    An increase in daily thermal amplitude at the surface: colder temperatures at night time and warmer temperatures at day time imply an increase in intensity of convective fluxes and heat fluxes, thus “boosting” the atmospheric circulation.

    2) What causes this increase in daily thermal amplitude ?

    Historical global deforestation, and particularly tropical deforestation, as showed by satellite observations and the comparison of forest covers with open lands by Li and al:

    https://www.nature.com/articles/ncomms7603/figures/3

    Most of tropical deforestation having took place in the XXth century, it explains the fact that tropical cyclones are getting stronger.

    [Response: Thanks, now we have something to discuss. However, the figure you link merely shows local differences in albedo and evapotranspiration between forested and non-forested areas. So far so well-known. What I would consider evidence in support of your idea would be (1) evidence that the diurnal temperature range over the oceans in hurricane formation regions has actually increased, and (2) that this has something to do with deforestation far away, over land. Model simulations (which could be wrong of course but at least provide some evidence) certainly suggest that land cover change affects the diurnal temperature range only over land and not over the oceans. -Stefan]

  20. 20
    Whachamacallit says:

    @ab That’s a strange claim, especially since the general land area occupied by forests have increased over the past century. One would think tropical storms would decrease in intensity if deforestation had anything to do with wind speeds.

  21. 21
    zebra says:

    @Richard Pauli #2,

    Excellent reference.

    Most current MC methods simulate synthetic TCs using (fairly limited) historical TC statistics.
    In contrast, a statistical–deterministic model, which is indepen-
    dent of the TC record, simulates TC environments statistically and
    generates TCs in the simulated environments deterministically. This
    statistical–deterministic approach may sometimes be more reliable,
    as observations of the large-scale TC environment are often better
    constrained than those of TC characteristics in areas with very
    limited TC history. It is also more likely to generate unexpected but realistic grey swan TCs, because, rather than extrapolating historical TCs, it applies physical knowledge of TCs and ample observations of the large-scale environment. Moreover, as the synthetic TC environments can be generated for any given climate state, this model can simulate grey swan TCs not only in the current and past climates but also in projected future climates.

    (My emphasis.)

    I made a comment in the previous thread that relates to this, and I’d like to pose a question for both the professionals here and the self-styled contrarians:

    At this point in time, does anyone actually think that if we say, double, CO2, there will not be significant changes in most if not all identifiable climate/weather phenomena? By significant I mean both readily measurable and, probably, consequential for at least some humans?

  22. 22
    Hank Roberts says:

    > San Diego hurricane of 1858
    Thank you John Pollack. Interesting storm track!

    Full article is at https://journals.ametsoc.org/doi/pdf/10.1175/BAMS-85-11-1689

  23. 23

    Work of buoyancy (CAPE) increases with the temperature and with the relative humidity (RH) of the raised air; both can be increased by spraying the air with warm water.
    There is a simpler way of calculating work of buoyancy than the usual CAPE.
    See: https://link.springer.com/article/10.1007%2Fs00703-012-0208-6
    Increasing 25°C 95% RH air temperature by 1°C increases CAPE by 2400 J/kg. Increasing 25°C 95% RH air relative humidity to 100% RH increases work by 1500 J/kg.
    See: https://twitter.com/VortexEngine/status/1002206326976602112
    Many more examples on Twitter timeline:
    https://twitter.com/VortexEngine
    A 1°C increase in sea surface temperature (SST) could increase work of buoyancy by 1200 J/kg corresponding to a velocity increase from 55 to 75 m/s and from hurricane category 3 to category 5.
    A decrease in zonal wind could further aggravate hurricane intensity. See: http://vortexengine.ca/Subside/Wind_Turbines.pdf

  24. 24
    Hank Roberts says:

    Global warming decreases the différence in temperature between Tropics and Arctic which is the fuel of hurricanes. So, it should be logic, according to Richard Lindzen

    I saw that reasoning promoted years ago, but as I recall, the Arctic has warmed much, much faster than the temperate zone, increasing the difference in temperature over time. Therefore, logically …. it would be wise to check.

    Have you checked for facts on which to base this logical thinking?

    Hm. http://time.com/4616573/climate-change-arctic-temperatures/

    Global temperatures were approximately 2.2°F higher than the pre-industrial average, likely to make 2016 the warmest year on record, but that temperature difference often hit the double digits in the Arctic.’

    “People have been talking about the Arctic for years now,” said Jeremy Mathis, director of NOAA’s Arctic Research Program, earlier this month. But “this year we have really seen the deepening of the change signal in the Arctic. The Arctic has shown a clear, stronger and more pronounced signal of persistent warming.”

    I would guess Lindzen would have retracted his old prediction given the new facts. Anyone seen mention of it?

  25. 25
    Hank Roberts says:

    https://journals.ametsoc.org/doi/abs/10.1175/WAF-D-17-0143.1

    On the Use of Ocean Dynamic Temperature for Hurricane Intensity Forecasting
    Karthik Balaguru
    Marine Sciences Laboratory, Pacific Northwest National Laboratory, Seattle, Washington

    https://doi.org/10.1175/WAF-D-17-0143.1
    Received: 25 September 2017
    Final Form: 29 January 2018
    Published Online: 28 February 2018

  26. 26
    Al Bundy says:

    AB and pe,

    Tropical cyclones don’t care much about what’s going on in the Arctic. The temperature differential they feed off is sea surface temp v upper atmospheric temp directly below and above the cyclone (corrections welcome, but only by those who understand climate science)

  27. 27
    nigelj says:

    ab @19

    “An increase in daily thermal amplitude at the surface: colder temperatures at night time and warmer temperatures at day time imply an increase in intensity of convective fluxes and heat fluxes, thus “boosting” the atmospheric circulation.”

    No, this doesn’t make any sense and is very vague. Increased convection like this would have a marginal effect on the hadley cell global circulation, because the global circulation is driven by huge differences in how the planet responds to incoming solar energy, and there is no plausible significant relationship between this type of global circulation change, and hurricane intensity or frequency.

    There is a simple and accepted physics based relationship in terms of the greenhouse effect, warming oceans and hurricanes.

    ” What causes this increase in daily thermal amplitude ?Historical global deforestation, and particularly tropical deforestation. Most of tropical deforestation having took place in the XXth century, it explains the fact that tropical cyclones are getting stronger.”

    No, most global deforestation, and particularly tropical forests, took place from 1960 – 1990 a period of generally mild global warming. There just isn’t a good correlation between deforestation and temperatures. Data on deforestation below:

    https://en.wikipedia.org/wiki/Deforestation#Rates_of_deforestation

    https://www.google.co.nz/search?q=graph+of+global+deforestation&tbm=isch&source=iu&ictx=1&fir=hWxYvEmrR8nSsM%253A%252CFhk423f2xeyd2M%252C_&usg=__RNh2RqoamANUhmpQ2ILkuiwDU1o%3D&sa=X&ved=0ahUKEwjy6NLs7bDbAhXLfrwKHQDMAqsQ9QEIKzAB&biw=1141&bih=588#imgrc=hWxYvEmrR8nSsM:

    Deforestation obviously contributes to climate change but can’t explain more than a small part of the changes. My comment is rushed, and I look forward to more detailed responses from experts. None of this means deforestation isn’t an ongoing problem in so many different ways.

  28. 28
    Hank Roberts says:

    Global temperatures from 2000–2009 were on average about 0.6°C higher than they were from 1951–1980. The Arctic, however, was about 2°C warmer. (NASA image by Robert Simmon, with GISS Surface Temperature Analysis (GISTEMP) data.)

    https://earthobservatory.nasa.gov/Features/ClimateStorms/page3.php

  29. 29
    Mal Adapted says:

    Hank Roberts:

    Writing from the US “Left coast” (California/Oregon/Washington) — is there reason to think we’ll be seeing Pacific storms coming onshore?

    Hank, this post is devoted to AGW’s effects on tropical cyclone intensity, but does mention geographic effects in passing, as it were:

    The average location where the storms are reaching their peak intensity is also slowly migrating poleward (Kossin et al. 2014) and the area where storms occur expands (Benestad 2009, Lucas et al. 2014), which changes patterns of storm risk and increases risk in regions that are historically less threatened by these storms (Kossin et al. 2016).

    Without knowing any more than that, one might suppose Pacific storms will be coming onshore farther north.

    Another easy prediction is that the recurrent “Pineapple Express” atmospheric river will bring more water ashore during the wet season. Coastal Washington, Oregon and California will feel even more like the Sock-It-to-Me Girl when that happens 8^(!

  30. 30
    Robert Damon says:

    Hank (#24): I believe it is well established that the arctic has warmed faster than the equatorial or temperate zones. However, typically the arctic is much colder than those zones. So the more rapid increase in temperature of the arctic relative to temperate or equatorial zones would DECREASE the temperature difference between those zones.
    As others have mentioned, this temperature difference (and its decrease) may be quite relevant to changes in various aspects of climate, but not necessarily to the formation of tropical storms. The driving force for those (as I have read — not a climate scientist) is, among other things, the actual water temperature where they are formed and exist, and not the difference between tropical and arctic temperatures.

  31. 31
    Iggie says:

    Are the higher cyclonic wind speeds due to satellite measurement (out to sea) rather than the traditional land monitoring systems? I have noticed that cyclones in Australia are classed higher than the land anemometer readings would suggest.

  32. 32

    #19 & #27–The former (ab) says:

    An increase in daily thermal amplitude at the surface: colder temperatures at night time and warmer temperatures at day time imply an increase in intensity of convective fluxes and heat fluxes, thus “boosting” the atmospheric circulation.

    The latter critiques alleged consequences and connections following the former’s quoted claim–however, without noting that:

    Diurnal temperature range (DTR) is a useful index of climatic change in addition to mean temperature changes. Observational records indicate that DTR has decreased over the last 50 yr because of differential changes in minimum and maximum temperatures.

    https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-13-00032.1

    And similarly, Stefan’s inline response to #19.

    But the fact is, ab’s claim is flatly wrong. DTR is not increasing, but decreasing, as nocturnal temperatures are rising faster than daytime ones. It’s what you’d expect from the greenhouse process (since during the night, the main driver of temperature is the magnitude of radiative heat loss), it’s what was long predicted, and now there’s a 50+ year observational record saying that, sure enough, that’s what’s happening. (Albeit models do not accurately reproduce this, consistently underestimating the trend–that’s actually the main thrust of the paper cited. But at least models get the sign right, unlike the claim in #19.)

  33. 33

    Are the higher cyclonic wind speeds due to satellite measurement (out to sea) rather than the traditional land monitoring systems? I have noticed that cyclones in Australia are classed higher than the land anemometer readings would suggest.

    There’s an obvious bias there, as a TC will start to weaken upon landfall. (I think that’s what you mean to suggest by the wording of your comment.)

    On the other hand, I’m pretty sure that’s well-known to researchers, and that they take steps to account for it as best as possible. Journalists, maybe not quite so much.

  34. 34
    Hank Roberts says:

    > Robert Damon
    Thanks, that makes sense of Lindzen’s prediction.

    And it’s warm water that drives the heat engine of a hurricane
    https://www.nasa.gov/vision/earth/environment/HURRICANE_RECIPE.html

  35. 35
    Hank Roberts says:

    https://www.yaleclimateconnections.org/2018/05/ocean-heat-as-fuel-for-hurricanes/

    Water vapor moves heat from ocean to atmosphere; precipitation releases that heat (that would be called “heat of condensation/vaporization”) into the air to drive the storm.

  36. 36
    Adam Lea says:

    31: The intensities of most tropical cyclones out at sea are devived from satellite data (e.g. the Dvorak technique), Atlantic storms near land are also sampled by reconnaissance aircraft, which can obtain more accurate estimates of intensity. Whatever the method of establishing intensity, the land based readings will nearly always be lower than the peak wind given in the storm advisory. This is because the maximum sustained winds affect only a very small area, and land based anemometers can be tens of miles apart, so the chance of a storms peak winds impacting any one anemometer is very small.

  37. 37
    Hank Roberts says:

    https://rmets.onlinelibrary.wiley.com/doi/full/10.1002/qj.2868
    Diurnal variations of the areas and temperatures in tropical cyclone clouds
    Qiaoyan Wu, Zhenxin Ruan
    First published: 27 June 2016
    https://doi.org/10.1002/qj.2868

    The disagreements among the results of previous studies may be caused by the relatively small number of storms for which observational databases exist and the different analytical methods used. Diurnal cycles in the areal extent of clouds and in the cloud‐top temperature in a TC may be caused by the presence of clouds with different properties. Satellite IR sensors only provide indirect estimates of the properties of deep convective clouds, and the properties of the interiors of such clouds cannot be determined. Cloud‐top temperatures measured using satellite IR sensors are generally similar for deep convective clouds and cirrus clouds (e.g. Liu et al., 1995; Sui et al., 1997). The average temperature within a fixed radius or annulus includes diurnal signals from different types of cloud. Different cycle parameters are found when the signals for different cloud conditions are combined, once the diurnal cycles of the areal cloud extent and temperature are not in phase for the different cloud conditions. Rather than studying diurnal variations in TC clouds with a fixed radius or annulus, we herein consider daily variability for whole convective clouds in TCs in terms of both the areal cloud extent and temperature, in order to allow the discrepancies between previous studies to be resolved.

  38. 38
    Al Bundy says:

    I have a question that could go in unforced variations, but it’s related enough (hopefully) to put here:

    Is a 1000 year storm based on a town, a county, a state, a nation, a set number of square kilometers, or what?

  39. 39
    Omega Centauri says:

    The worst inland flooding events are associated with slow moving or stalled tropical cyclones. Any data on the change in the probability/frequency of such storms stalling overland. Harvey was a real case in point, as it became a multi-day precipitation event.

  40. 40
    Hank Roberts says:

    Al B. — looks like the 1000-year events are based on the rainfall in the watershed and water flow in each river:

    https://www.usgs.gov/faqs/why-do-values-100-year-flood-seem-change-every-flood

    The amount of water corresponding to a 100-year flood, a 500-year flood, or a 1,000-year flood is known as a “flood quantile”. For instance, on a given river, the flood quantile corresponding to the 50-year flood might be 10,000 cubic feet per second (cfs) and the flood quantile corresponding to the 100-year flood might be 15,000 cfs. The estimates of the flood quantiles are calculated using actual data collected at a site.

    For a particular river, the USGS collects data over time, determines the largest flood in each year, and then calculate statistical data for that river. The more years of data available, the more accurate the estimates for the various flood quantiles.

  41. 41
    John Pollack says:

    Al @38 A 1000-year storm (or event) has a 0.1 percent chance of happening in a particular year,for the type of storm and area under consideration.

    Obviously, this is a statistical estimate, since we don’t have 1000 years of instrumental records. Even if we did, the climate is changing, so that this years’ 1000 year event might be different from one a decade or two ago. Furthermore, since it’s statistical, it depends on what kind of statistics you are using to make the estimate. The margin of error gets larger for rarer events.

  42. 42
    Al Bundy says:

    Hank,

    Thanks. You’re the Search God.

    Canada and the US have about 84 rivers according to Wiki’s list. (I counted quickly)

    So, North America should have one 1000-year flood every twelve years or so.

  43. 43
    Brian Dodge says:

    “The underlying premise of the Emanuel (1995b, 1986 theories is that the energy source for the mature hurricane is the air–sea disequilibrium and not the energy initially available in the moist tropical atmosphere. This would suggest that the ultimate intensity and structure of the vortex (one which achieves an optimal degree of disequilibrium with the underlying ocean) should be independent of any available energy that may be present in the initial atmosphere. We will demonstrate here that this is indeed the case.”
    https://journals.ametsoc.org/doi/full/10.1175/JAS-3370.1

  44. 44

    There is a reasonable suspicion that aerosols counter the creation of stronger hurricanes by reducing the warming caused by increased carbon dioxide in the atmosphere. Assuming for the sake of the argument that this is true, it will be interesting to see the effect of the decrease of aerosols. China and the far east will follow what America did decades ago as the population becomes more strident in their objection to air pollution and they will clean up their act. Most of the technology they need is ‘off the shelf’ (cleaning smoke stacks of sulfur and particulates and replacing fossil fuel vehicles with electrics) so the change could come rather rapidly. The effects will be interesting.

  45. 45
    John Smoot says:

    Help me understand, when I compare your data to Ryan Maue’s ACE and frequency data I do not see the same trends:

    https://pbs.twimg.com/media/DDgVakQUIAEgS8E.jpg:large – Frequency
    https://pbs.twimg.com/media/DDgXxOPV0AAn-IH.jpg:large – ACE

    Why the difference?

  46. 46
    Hank Roberts says:

    Are hurricanes moving more slowly, so dropping more rain and causing more flooding?

    https://www.npr.org/2018/06/06/616814022/hurricanes-are-moving-more-slowly-which-means-more-damage

  47. 47
    Piotr says:

    Re: 45 John Smoot: “Help me understand, when I compare your data to Ryan Maue’s ACE and frequency data I do not see the same trends”

    aren’t these apples and oranges? Ryan Maue throwing all hurricanes cat 2-5 into one bag (his lower line), while the graph above showing trends in frequency within cat 2, cat 3, cat 4, and cat 5, separately?

  48. 48
    nigelj says:

    John Smoot @45 “Help me understand, when I compare your data to Ryan Maue’s ACE and frequency data I do not see the same trends:”

    They are measuring different things. Maue’s data is frequency of all tropical cyclones while the realclimate article is talking about just category 4 and 5 cyclones.

    It’s not clearly documented where Maues data comes from, who complied it and reviewed it, and how reliable it is. He appears to be a meteorologist, who works for the cato institute, a right wing / libertarian think tank.

  49. 49

    #45, John Smoot–

    “Why the difference?”

    I’m not entirely sure. I looked at the question, starting with the fact that the Maue graph you link is nearly a year old. So I looked up the current version.

    But though there are certainly similarities, I couldn’t quite get the version you pointed to to match up to the latest update on Dr. Maue’s site, here:

    http://wx.graphics/tropical/

    On the current Maue plot, the lower track (major storms) does indeed seem to show an upward trend over time, just as discussed above, in contrast to what your link shows. Moreover, the frequency scales on each graph are completely different–something that should not change in less than a year, let alone from a max of 120 to a max of 70. Yet they are both titled similarly, and have a similar visual style.

    Any chance someone tampered with the image prior to posting on the site from which you linked? Or maybe I’m missing something big here?

    [Response: Maue has two different graphs: one of all tropical storms (with scale going up to 120) and one of hurricanes only (with scale up to 70). Hurricanes here are only the subset of tropical storms that reach hurricane force winds. The graph of hurricanes has two lines, the top one showing all hurricanes (categories 1-5), the lower one just “major hurricanes” which is those of categories 3, 4 and 5. As we state in our article, we don’t expect the number of hurricanes to increase, but rather the number of the strongest ones. For those of category 3 and higher, Kerry Emanuel’s graph above shows a moderate increase by maybe a third (just eyeballing his graph here – it’s the first nonzero point, marking the left edge of the cat 3 colour bar). Maue’s graph also shows an increase of all storms category 3 and higher, though the increase he gets is smaller; Kerry has contacted Maue to work out the reason for this difference. Will keep you posted. The key thing we are trying to illustrate here is the increase in the number of the strongest and potentially most damaging storms though, i.e. those of categories 4 and 5, and though we have asked him I have not seen a plot by Maue showing those yet. -Stefan]

  50. 50
    David says:

    #49 – Might it help if many of us ask Ryan Maue on Twitter to show what Kerry’s graph would look like with his own data? He claims Kerry’s graph is wrong but refuses to demonstrate that by presenting his version. Here’s the thread: https://twitter.com/rahmstorf/status/1003893539611267077

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