Why global warming means killer storms worse than Katrina and Gustav, Part 1

Hurricane season officially begins June 1 — though global warming will ultimately move that date up just as it is moving up the spring snowmelt.  Indeed, some evidence suggests the hurricane season has been getting longer for decades (see here and below).

As Jeff Master, our favorite meteorologist and hurricane blogger, noted in November, “This year is now the only hurricane season on record in the Atlantic that has featured major hurricanes in five separate months” (see “A new record for the hurricane season of 2008“).  Saturday, Masters explained that had “the large extratropical storm (90L) that has been pounding Florida” this week “spent another six hours over water, it very likely would have been declared a tropical/subtropical depression/storm” — that is, it would have been “the season’s first named storm.”  So I won’t wait until June 1 to revise and update some posts from last year on why global warming will lead to much worse killer storms.

Relative sizes of Typhoon Tip and Tropical Cyclone Tracy

Hurricanes can get much, much bigger and stronger than we have so far seen in the Atlantic. The most intense Pacific storm on record was Super Typhoon Tip in 1979, which reached maximum sustained winds of 190 mph near the center. On its wide rim, gale-force winds (39 mph) extended over a diameter of an astonishing 1350 miles. It would have covered nearly half the continental United States.

“More than half the total hurricane damage in the U.S. (normalized for inflation and populations trends) was caused by just five events,” explained MIT hurricane expert Kerry Emanuel in an email. Storms that are Category 4 and 5 at landfall (or just before) are what destroy major cities like New Orleans and Galveston with devastating winds, rains, and storm surges.

In Part 2, we’ll look a little more in detail at Katrina and Gustav, and why they weren’t as strong and hence as devastating at landfall as they could have been.  But let’s first ask — How did Katrina turn into a powerful Category 5 hurricane?

The National Climatic Data Center 2006 report on Katrina begins its explanation by noting that the surface temperatures (SSTs) in the Gulf of Mexico during the last week in August 2005 “were one to two degrees Celsius above normal, and the warm temperatures extended to a considerable depth through the upper ocean layer.” The report continues, “Also, Katrina crossed the ‘loop current‘ (belt of even warmer water), during which time explosive intensification occurred. The temperature of the ocean surface is a critical element in the formation and strength of hurricanes.”

An important factor was that the ocean warming had penetrated to a considerable depth. One of the ways that hurricanes are weakened is the upwelling of colder, deeper water due to the hurricane’s own violent action. But if the deeper water is also warm, it doesn’t weaken the hurricane. In fact, it may continue to intensify. Global warming heats both the sea surface and the deep water, thus creating ideal conditions for a hurricane to survive and thrive in its long journey from tropical depression to Category Four or Five superstorm.

A 2005 study, “Penetration of Human-Induced Warming into the World’s Oceans,” led by Scripps Institution of Oceanography compared actual ocean temperature data from the surface down to hundreds of meters (in the Atlantic, Pacific, and Indian oceans) with climate models and concluded:

A warming signal has penetrated into the world’s oceans over the past 40 years. The signal is complex, with a vertical structure that varies widely by ocean; it cannot be explained by natural internal climate variability or solar and volcanic forcing, but is well simulated by two anthropogenically forced climate models. We conclude that it is of human origin, a conclusion robust to observational sampling and model differences.

This figure shows what they found:


Figure: Anthropogenic forcing signal strength (green hatched region) compared to that obtained from the observations (red dots). There is excellent agreement at most depths in all oceans. The hatched region shows the range of the signal strength estimates from five different realizations of identically forced simulation with the Parallel Climate Model, whereas the smaller green dots within the region are the individual realizations. Click to enlarge.

And yes, the latest analysis shows “that ocean heat content has indeed been increasing in recent decades, just like the models said it should.”

Note to deniers:  You can spare me the links to Roger Pielke, Sr.’s “analysis” of how there supposedly hasn’t been measurable ocean warming from 2004 to 2008.  In the middle of a strong 50-year warming trend, any clever (but cynical) analyst can connect an El Ni±o-driven warm year to a La Ni±a-driven cool year a few years later to make it look like warming has stopped.  I will blog on this soon.

Tropical cyclones are threshold events

Tropical cyclones are threshold events–if sea surface temperatures are below 80°F (26.5°C), they do not form. Some analysis even suggests there is a sea surface temperature “threshold [close to 83°F] necessary for the development of major hurricanes.” Global warming may actually cause some hurricanes and some major hurricanes to develop that otherwise would not have (by raising sea surface temperatures above the necessary threshold at the right place or time).

And the more warm, deep water that gets generated by global warming, the more super-intense hurricanes we will see. No wonder ABC News reported in 2006 that hurricane scientists are considering adding a Category 6, for hurricanes above 175 miles per hour. Ultimately, they may become common.

If we don’t reverse our emissions paths quickly, global temperatures will rise faster and faster through 2100 and beyond. This will translate into warmer oceans in all three dimensions: Warmth will spread over wider swaths of the ocean as well as deeper below the surface-we’ve already seen that in the first known tropical cyclone in the South Atlantic (2004) and the first known tropical cyclone to strike Spain (2005). That means we will probably see stronger hurricanes farther north along the East Coast in the coming decades.

More intense storms will be seen earlier and later in the season. The 2005 hurricane season was the most striking example of that trend, with Emily “the earliest-forming Category 5 hurricane on record in the Atlantic,” in July, and Zeta, the longest-lived tropical cyclone to form in December and cross over into the next year, where it became the longest-lived January tropical cyclone.

We have already seen a statistically significant increase in the length of the average hurricane season over the last several decades, according to a recent analysis (see here).  The data from the past century indicates that a 1°F increase in sea surface temperatures leads to an extra five tropical storms a year in the Atlantic–an ominous statistic in a world taking no actions to stop a projected 3°F increase in average sea surface temperatures over the Atlantic hurrican-forming region by 2050, and more than double that by century’s end.

Part 2 will look a little more in detail at Katrina and Gustav, and why future Gulf storms are all but certain to be more devastating at landfall.

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23 Responses to Why global warming means killer storms worse than Katrina and Gustav, Part 1

  1. paulm says:

    Costing the Earth
    The Carteret Islands – Sharks In The Garden

  2. K L Reddington says:

    I suspect common sense lets us find no civilisation in prior centuries that had so many hundreds of thousands living on land below sea level and depending on levies and pumps to evacuate flooding. The years since Katrina have really been light in activity.

  3. Pat Richards says:

    It’s pretty obvious that, given the major influence of ocean surface temperature on hurricane/typhoon strength, that the reason the 2008 season was so strong in the eastern Pacific was because there was a La Nina in effect that year (which sloshes the warmer water into the eastern Pacific). Now that has reversed that and we have moved into an El Nino situation (which sloshes the warm water back into the western Pacific). This warmer water pooling along the South and Central American coasts warms the air over the Gulf, which creates meterological highs that end up warming the ocean surface temperature of the Gulf and, you guessed it, the Southeastern U.S. gets an active hurricane season.

    The El Nino/La Nina effects get stronger all the time because the human-accelerated warming of the atmosphere also warms the ocean water, although more slowly than the air. Which means the total amount of warm water sloshing back and forth across the Pacific during the El Nion/La Nina events gets bigger and so the effects keep getting stronger. Naturally there is some variation in the strength of the events from year to year since nature is not linear in the short term, but the long-term trend is obvious and deadly.

    Heat and cold are the fundamental dynamic factors driving our planet’s weather. When we increase the heat factor with the activities of 7 billion people going 24/7, we alter that dynamic. I don’t know why so many people still feel in their gut that humans cannot affect the weather. Not in any short period of time, of course, but over the last 140 years of ever-increasing CO2 production… absolutely.

  4. David B. Benson says:

    And also

    (1) killing the oceans, see the May/June 2009 issue of Sierra;
    (2) looming end of industrial agriculture, see the article on phosphorus in the June 2009 issue of Scietific American.

  5. David B. Benson says:

    This thread
    offers suggestions that dust from the Sahara desert, blown out to sea and even to the U.S., appears to inhibit tropical storm formation.

  6. lizardo says:

    Joe, this is an excellent explanation and so well written. This post should go into the book!!! With whatever of the others aren’t in the first book… suggested title, more hell, more high water, heh, heh. (It doesn’t seem quite right to call it Climate Progress since we aren’t making enuf progress, are we?

  7. Emerson says:

    Very good article, very usefull!!

  8. Great Stuff Joe. I’ve seen a lot before, but you put it together well.

    You can see the upper ocean heat content in real time. I remember well, tracking Katrina, and seeing the huge area of high heat content that it crossed before landfall. Katrina took out an old and much loved Oak in my yard-400 miles from the Big Easy.

    FYI: The Heartland Inst. sent me (and I imagine a ton of other TV weather folks) a packet full of junk on climate today. Apparently the global surface temp. rise is all wrong…

  9. Mike Strong says:

    Joe, where the heck do you get this tripe? Do you live in a bubble?
    Your statement: ” though global warming will ultimately move that date up just as it is moving up the spring snowmelt. Indeed, some evidence suggests the hurricane season has been getting longer for decades.”

    The past two hurricaine seasons were a bust with some really big storms that petered out in hours down in Mexico. …and MANY fewer storms than predicted. This year, the NOAA just released their new prediciton which shows the season as “average”. They have learned their lesson from the prior two seasons of fear mongering hwere prdictions failed miserably.

    Once again, global ice is average. The sun spots are not showing up…and Joe Romm gets 8 comments while Anthony Watts, Steve McIntyre and Icecap average hundreds based upon SCIENCE, not IPCC and Hansen emotion. And these skeptic sites keep spewing out more and more data which shows Hansen is a fake.

    When are you gonna give it up? NO ONE IS LISTENING TO YOU because you have no science behind you anymore. It’s not CO2. It is the SUN. Duh!

  10. paulm says:

    Tropical cyclones maybe threshold events based on SST, but the historical plot of named storms over time indicates that there is a direct linear link to SST. So there are other things at work here.

    Graph of NH SSTs and Named Storms Questioned

  11. jorleh says:

    Most of the AGW warmth has gone to warm up oceans. What about to calculate the energy to warm up the upper 500 metres of the oceans 0.2 – 0.3 degrees Celcius?

  12. donnyinalaska says:

    why would 600 of the worlds top climatologist show proof that this government employed idiot al gore has made up this hoax ?
    why in places like alaska , north africa do the goverments not spray the skies with aluminum powder and other chemicals ?
    people had better wake up to this scam and start questioning authority , caue if ya believe everything they tell ya , then you’ll have some major problems down the road of life .
    this is no different then questioning the offical story of 9/11 , and most people with a few marbles in their heads can see why it was another scam just to invade another country .

  13. Mossy says:

    Dan S.,
    It’s great to see that a few weather people get it. (Knowing that the Heartland Institute also sends junk to weather people helps to explain why so many of them avoid this topic.) Science is fact, not opinion, folks!

  14. Lou Grinzo says:

    I recently read Chris Mooney’s book Storm World (review), which I highly recommend, and he details something that I think should get a lot more attention: The correlation between SSTs and the total power of storms.

    Kerry Emanuel did a study in which he used regular readings of storms to calculate each one’s total power, taking into account its size, duration, and wind speed. When you add up the power of all the storms for each year from 1950 until roughly 2005, and then plot those totals, you get a line that’s amazingly similar to the SSTs for the same years. Mooney shows the graph on page 141 of Storm World.

    While I think it’s definitely of value to look at the count and maximum strength of storms, their total destructive power strikes me as being even more relevant (since that’s what hurts human beings and damages the things we build), and that value correlates very strongly with SSTs.

  15. Dear Joe,

    Thanks for this post. I think a lot of people do not realize just how small a land mass the contiguous U.S. states represent, nor how this small land mass is tucked between two very massive bodies of water.

    Earlier this month, my daughter went through the “inland hurricane” in Carbondale, Illinois, which had wind speeds of 106 mph.

    This caused me to recall that last year, while fanatically (I admit) watching the doppler radar images over the U.S., I noticed something very odd that I had never seen in all the many years I have been “weather watching,” and that was the occurrence of two incredibly large systems over the western Midwest, circular in form, like big polka dots, looking like they had a diameter of well over 100-200 miles, and they were colored red, which indicates that they went way up high, too.

    Now, I am wondering if what I saw were two inland hurricanes trying to form up.

    As temperatures continue to increase at the equator, and more and more water vapor is being held by the air, with much of this being transported from the eastern Pacific at the equator up into Mexico, then Texas, then the Midwest, it makes sense to assume that we will see more inland hurricanes.

    If you have looked at any photos or videos of the one that hit Carbondale, you will see that they are just as bad as hurricanes that form over water.

  16. paulm says:


    Rare Cyclonic Storm over Nothern Argentina

    A low-pressure weather system over the heart of South America developed into an unusual cyclonic storm in mid-May 2009. According to MetSul meteorologist Luiz Fernando Nachtigall (who tipped The Earth Observatory with this story),

    the storm “remained quasi-stationary for 48 hours and was incredibly symmetric, resembling a tropical cyclone in the middle of the continent.”

  17. Just in case no one has noticed it, for the past 6 weeks, very excessive and out of season rains have fallen over northern and northeastern Brazil, flooding more than 400,000 out of their homes. The Amazon River is not expected to crest until the end of June. Yesterday, in Maranhão state, where 300,000 are flooded out, in a town that had been spared, there was a sudden 4-hour rain that left the entire city flooded. The weather announcers keep mentioning La Nina, but as far as I know, after the last El Nino, the La Nina never returned and it has be ENSO neutral ever since.

    There have also been occurrences of extra-tropical cyclones hitting southern Brazil for the past 5 years or so. I believe that the first one ever recorded was in 2004. They seem to be more and more regular now. And there was a cluster of 3 tornadoes in the south a few months back. Tornadoes are so rare that the weather announcer had to explain what they were to the TV audience.

    In the coastal city of Salvador, Bahia, the constant rain has caused thousands of mud slides and more than 200 homes have collapsed. Cities on the coast of north Bahia are beginning to flood. There is no end to all of this rain in sight.

    The so-called dry season should have begun by the end of March.

  18. David B. Benson says:

    Tenney Naumer — Saddening.

    Yes, my understanding is ENSO neutral just now.

    Several years ago Hadley Centre pointed out that global warming implies more extreme weather events. Maybe at least the Brazilians will now understand.

    Terrible way to learn it, tho’.

  19. A big part of the problem is that the public has no real understanding of scientific method and peer review. They cannot separate out the political spin from real Science.

    Our worsening science ranking/education standards are likely the source of this.

  20. David B. Benson says:

    “Brazil floods displace thousands”:

  21. I might add that the paper in Science that Joe showed an image from is a fantastic paper. Well worth paying for, even if you do not have a subscr.

    If you are in any doubt about anthropogenic warming vs natural cycles, an open minded reading of that paper will convince you.

  22. David B. Benson says:

    Tenney Naumer — Thank you!