A lot of knee-jerk deniers (please don’t write in — I know that is redundant) misread “part 1,” as I knew they would. I was not wading into the issue of whether global warming has already made intense tropical storms more common. That remains a great subject of debate, mostly because of the inadequacy of historical hurricane records, before the satellite era, and especially before WWII. That said, the North Atlantic seems special because much of the hurricane-forming region in the summer is relatively close to the temperature threshold for hurricane generation — so a little warming goes a long way. I will return to this issue at the end of the post.

My point in this two-parter is completely different. All things being equal, if a storm took the same track of Gustav (or Katrina) in 2050, then, rather than weakening before making landfall, it would probably have strengthened considerably. Let’s look at the region in 2050, assuming BAU (business-as-usual) warming, or no effort to reverse current emissions trends:


Now that is bad news for New Orleans, the Gulf Coast, and the South Atlantic. The average warming in the Gulf, Caribbean, and coastal Atlantic is 1°C to 2°C, but this model has an enormous body of very warm water 2°C to 3°C over much of the typical storm path for a hurricane like Katrina or Gustav. There are two relevant points to recall:

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  1. The National Climatic Data Center 2006 report on Katrina notes 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.
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  3. In the case of both Katrina and Gustav, they hit colder water before hitting the coast — a key reason they were far weaker at landfall than they might have been, as their pictures make clear.

Here are some plots of the Gulf’s sea surface temperature as Gustav would have seen it (from Jeff Masters’ WunderBlog):


Figure 1. Tropical Cyclone Heat Potential for August 28, 2008. Values of TCHP greater than 80 are commonly associated with rapid intensification of hurricanes. The forecast points from the NHC 5 a.m. Saturday forecast are overlaid. Gustav is currently crossing over a portion of the Loop Current with extremely high value of TCHP of 120. However, Gustav will then cross over a cold eddy, and will miss crossing the warm Loop Current eddy that broke off in July.

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Figure 2. Forecast track and sea surface temperature response to the passage of Gustav, as simulated by the GFDL model at 8 a.m. EDT Saturday 8/30/08. Passage of Gustav over the relatively shallow depth of warm water near the coast will allow Gustav to upwell large amounts of cold water from the depths. This will chill the surface waters down by up to 5°C (9°F).

Now imagine we are in the year 2050 with the same storm track. We could easily be talking a Gustav2050 that is a Category 4 or even Category 5 at landfall, rather than just a strong Category 2.

What about Katrina? It did reach Category 5 status in the Gulf, but it made landfall only as a Category 3. Again, cold water played a role, as this University of Colorado graph reprinted in Wikipedia shows (where sea surface height is a proxy for SST, see here):

Image:Katrina vs sea surface height.JPG

Obviously, Katrina was able to ride the Gulf Loop Current and Eddy Vortex (!) closer to the coast than Gustav, but it still smashed into cold water. Again, in 2050, that weakening is going to be a lot less likely to occur.

What precisely would happen if a hurricane was ever able to ride warm Gulf water all the way to landfall? We have some idea because that appears to have happened once in the relatively recent past:

An example of how deep warm water, including the Loop Current, can allow a hurricane to strengthen, if other conditions are also favorable, is Hurricane Camille, which made landfall on the Mississippi Gulf Coast in August of 1969. Camille formed in the deep warm waters of the Caribbean, which enabled it to rapidly intensify into a Category 3 hurricane in one day. It rounded the western tip of Cuba, and its path took it directly over the Loop Current, all the way north towards the coast, during which time the rapid intensification continued. Camille became a Category 5 hurricane, with an intensity rarely seen, and extremely high winds that were maintained until landfall (190 mph / 305 km/h sustained winds were estimated to have occurred in a very small area to the right of the eye).

That of course was pure happenstance, pure bad luck. But by mid-century, the whole damn Gulf in the summer is going to be much, much warmer, thanks not to bad luck but to human emissions. So it seems a near certainty that the Gulf coast will see one or more Category 5 storms make landfall in the coming decades. Indeed, as I wrote in my book:

At the same time, the inland United States will heat up at an even faster rate, so the Mississippi River will not be such as cool a stream of water pouring into the Gulf. As the sea level rises, the protective outer delta of the Mississippi River will continue to disappear and storm surges will penetrate deeper inland. Hurricanes weaken rapidly over land. Even one foot of shallow delta water can dramatically reduce this weakening effect, allowing hurricanes to reach deeper inland with their destructive force.

So not only will we see increased Category 4 and Category 5 hurricanes, but sooner or later-probably sooner-one of the hurricanes that enters the Gulf will ride a wide and deep mass of warm water straight to the shore, and rather than weakening as it approached the shore, like Katrina did, it will maintain its strength. Then a Category 5 super-hurricane will bring havoc back to New Orleans and the Gulf Coast.

Only one major issue remains, I think. Clearly global warming means warmer surface water and, as I discussed in “part 1,” warmer deep water. All things being equal, that means future hurricanes that travel the same path are going to stay stronger longer and possibly even intensify where earlier hurricanes had weakened.

What we don’t know is if, in fact, all things will be equal. Perhaps global warming will create other conditions that might serve to weaken hurricanes or change their storm path. Some have suggested that climate change could lead to a permanent El Niño condition. That would be an unmitigated catastrophe for the planet, but would probably lead to fewer Atlantic hurricanes. Global warming could also lead to more wind shear, which tends to break up hurricanes.

That said, serious global warming has been going on for a few decades now, and just this decade we’ve had two major hurricanes ride straight up into the the New Orleans area, three if you count Rita. So I don’t think it makes much sense to hope or expect that future global warming means significantly fewer major hurricanes that end up on a path towards the Louisiana coast.

We are stuck with a fair amount of warming over the next few decades no matter what we do. But if we don’t reverse emissions trends soon, then Category 4 and 5 storms smashing into the Gulf coast seem likely to become a rather common in the second half of this century. And that will be a doubly untenable situation because by then we will be probably also be facing sea-level rise of a few inches a decade or more).

Preserving the habitability of the Gulf and South Atlantic Coast post-2050 means the time to act on climate change is now.

This post was created for ClimateProgress.org, a project of the Center for American Progress Action Fund.