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Why Global Warming Means Killer Storms Worse Than Katrina And Gustav, Part 1
Joe Romm, 26 May 09

Relative sizes of Typhoon Tip and Tropical Cyclone Tracy
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.

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 Niño-driven warm year to a La Niña-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.

Read these related posts in our archive:

Applying Climate Foresight

Foresight in the Age of the Storm

Pump up the Volume

New Orleans: Everything has Changed

This piece originally appeared in Climate Progress.

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we are learning about hurricans in school so this site helps! thanx!

Posted by: thera on 27 May 09

we are learning about hurricans in school so this site helps! thanx!

Posted by: thea on 27 May 09

A hurricane is a giant heat engine. Extra CO2 from humans burning 7+ fossil fuel gigatons per year reduces net radiative heat loss from the surface, allowing a higher sea surface temperature, which provides more energy and water vapor for the storm. Lots of info under "Extreme Events" at

Posted by: Bob Maginnis on 27 May 09

This is very interesting as speculation but you present it as established fact. As you well know these issues are the subject of intense research and debate in the hurricane science community. You start off carefully enough, using words like "may" and "suggest." But suddenly we are confronted with this bald claim: "And the more warm, deep water that gets generated by global warming, the more super-intense hurricanes we will see." From then on you abandon careful science in favor of reckless dicta. Speculation is the soul of science but false confidence is the mortal sin.

Posted by: David Wojick on 27 May 09

I note that many of your links that I followed lead to articles or blogs that purport to interpret scientific material, but do not themselves provide much in citations. I see that as very problematic. And while your commentary includes a vague comment about adjusting ("normalizing") storm damage values for inflation and population trends, that is an oft-forgotten consideration. More and more people keep moving businesses and residences into these dangerous areas, and we continue to damage and destroy natural buffer zones that protected communities for generations. I've never seen anyone yet adjust for those latter aspects of population migration and ecological change. And THAT IS NOT AGW, it's land use (and perhaps stupidity). And when those communities are damaged, what do we do? Rapidly "reinvest" and "rebuild". As a civil engineer, I'm appalled at the trust mankind puts into its own creations, that we think we can fix all natural hazards by building larger levees, straightening, smoothing and widening river channels, and other structural measures.

As for the length of the hurricane season becoming longer, the link you provide takes us to an article that says, in part: "The National Hurricane Center uses these dates because historically most storms occur within that span of six months and because having a definitive time frame helps to heighten the public's awareness of the dangers of hurricanes." So, the official "season" would appear to be defined only as a matter of convenience, not science.

One consideration often skipped over is the advances in technology for detection of hurricanes and other cyclonic storms. Prior to about the 1970s the world had no satellites in orbit to view the earth and notice such storms. Other than the U.S. Military "Hurricane Hunters" (going back to the mid-1940's), we apparently had to rely on if a storm was seen AND REPORTED by a ship or airplane. Those are big oceans out there. As for data on such hurricanes, I would expect that the military units, with weather as only part of their mission, was incapable of being routinely deployed on missions far from home. So the final analysis of an apparent upward trend in such events may be artificially biased by advances in technology, not by real storms.

I also understood that there remains considerable mainstream scientific debate about how hurricanes form, and if warmer temps will make the situation better or worse. That's not made clear in your article.

Finally, if we know so much about hurricanes and their reaction to global warming, why aren't they included in all Global Circulation Models (GCMs)? At last check, I understood the GCMs to be very poor predictors of cyclonic storms. You only reference the 2005 Scripps study that says the ocean temperature is "well simulated" by only TWO (2) GCMs. There are a lot more climate models in routine use than two!

Posted by: Gene L. on 27 May 09

Joe, I chuckled at the reference to Super Typhoon Tip in 1979. Back then, the hot potato issue was global cooling.

Maybe hurricanes and typhoons are monsters in their own right. Maybe the fraction of a degree in warming since 1979 doesn't amount to a hill of beans to a hurricane or typhoon. Maybe citations of natural calamity in all sizes, shapes and forms serves the purpose of speading alarmism which, today, is needed more than ever as our Congress debates cap and trade.

Maybe your prognostications depend too much on climate models, where assumptions fill-in gaps in knowledge and drive us toward financial ruin to "save the planet". In 1883, Mark Twain was attributed a comment on the state of science. He said, "There is something fascinating about science. One gets such wholesale returns of conjecture from such trifling investments of fact."

And that, Joe, is what you've given us: wholesale conjecture.

Posted by: Jeff on 27 May 09

Is the spring snowmelt really moving up? I thought some mountain pass in CO was blown open the LATEST ever in Summer 08. One spot doesn't make a trend but it's hard to reconcile if the trend is in the other direction. The Arctic sea ice seems to be rebounding a bit too. The Antarctic sea ice set a record high in 08 and is on a similar track in 09.

All this proves (or disproves) nothing but the opening line about spring snowmelt was unfortunate.

Posted by: Frederick Michael on 28 May 09

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