Home > Environment, Science > Is Katrina due to global warming?

Is Katrina due to global warming?

September 6, 2005 Leave a comment Go to comments

I noticed a letter in the paper the other day proclaiming that Katrina and other recent hurricanes in Florida were due to global warming. However, I also remember seeing an article over at Ectophensis proclaiming the opposite:

Subject: G4) Are we getting stronger and more frequent hurricanes, typhoons, and tropical cyclones in the last several years?

Globally, no. However, for the Atlantic basin we have seen an increase in the number of strong hurricanes since 1995. As can be seen in section E9, we have had a record 33 hurricanes in the four years of 1995 to 1999 (accurate records for the Atlantic are thought to begin around 1944). The extreme impacts from Hurricanes Marilyn (1995), Opal (1995), Fran (1996), Georges (1998) and Mitch (1998) in the United States and throughout the Caribbean attest to the high amounts of Atlantic hurricane activity lately.

As discussed in the previous section, it is highly unlikely that global warming has (or will) contribute to a drastic change in the number or intensity of hurricanes. We have not observed a long-term increase in the intensity or frequency of Atlantic hurricanes. Actually, 1991-1994 marked the four quietest years on record (back to the mid-1940s) with just less than 4 hurricanes per year. Instead of seeing a long-term trend up or down, we do see a quasi-cyclic multi-decade regime that alternates between active and quiet phases for major Atlantic hurricanes on the scale of 25-40 years each (Gray 1990; Landsea 1993; Landsea et al. 1996). The quiet decades of the 1970s to the early 1990s for major Atlantic hurricanes were likely due to changes in the Atlantic Ocean sea surface temperature structure with cooler than usual waters in the North Atlantic. The reverse situation of a warm North Atlantic was present during the active late-1920s through the 1960s (Gray et al. 1997). It is quite possible that the extreme activity since 1995 marks the start of another active period that may last a total of 25-40 years. More research is needed to better understand these hurricane “cycles”.

Interesting. So I went over to RealClimate to see what they say and they just happen to have a post about just that at the top of the page. After pointing out that we really can’t attribute one hurricane to global warming or a natural cycle, there’s some evidence to suggest that, while global warming isn’t causing more hurricanes (contrary to the letter in the Chronicle), it may be causing more intense hurricanes:

The key connection is that between sea surface temperatures (we abbreviate this as SST) and the power of hurricanes. Without going into technical details about the dynamics and thermodynamics involved in tropical storms and hurricanes (an excellent discussion of this can be found here), the basic connection between the two is actually fairly simple: warm water, and the instability in the lower atmosphere that is created by it, is the energy source of hurricanes. This is why they only arise in the tropics and during the season when SSTs are highest (June to November in the tropical North Atlantic).

But what about the past? What do the observations of the last century actually show? Some past studies (e.g. Goldenberg et al, 2001) assert that there is no evidence of any long-term increase in statistical measures of tropical Atlantic hurricane activity, despite the ongoing global warming. These studies, however, have focused on the frequency of all tropical storms and hurricanes (lumping the weak ones in with the strong ones) rather than a measure of changes in the intensity of the storms. As we have discussed elsewhere on this site, statistical measures that focus on trends in the strongest category storms, maximum hurricane winds, and changes in minimum central pressures, suggest a systematic increase in the intensities of those storms that form. This finding is consistent with the model simulations.

A recent study in Nature by Emanuel (2005) examined, for the first time, a statistical measure of the power dissipation associated with past hurricane activity (i.e., the “Power Dissipation Index” or “PDI”–Fig. 2). Emanuel found a close correlation between increases in this measure of hurricane activity (which is likely a better measure of the destructive potential of the storms than previously used measures) and rising tropical North Atlantic SST, consistent with basic theoretical expectations. As tropical SSTs have increased in past decades, so has the intrinsic destructive potential of hurricanes.

There’s a nice graph illustrating the point over there as well. So, what’s the verdict from RealClimate? It’s a bit of both:

Thus, we can conclude that both a natural cycle (the AMO) and anthropogenic forcing could have made roughly equally large contributions to the warming of the tropical Atlantic over the past decades, with an exact attribution impossible so far. The observed warming is likely the result of a combined effect: data strongly suggest that the AMO has been in a warming phase for the past two or three decades, and we also know that at the same time anthropogenic global warming is ongoing.

Finally, then, we come back to Katrina. This storm was a weak (category 1) hurricane when crossing Florida, and only gained force later over the warm waters of the Gulf of Mexico. So the question to ask here is: why is the Gulf of Mexico so hot at present – how much of this could be attributed to global warming, and how much to natural variability? More detailed analysis of the SST changes in the relevant regions, and comparisons with model predictions, will probably shed more light on this question in the future. At present, however, the available scientific evidence suggests that it would be premature to assert that the recent anomalous behavior can be attributed entirely to a natural cycle.

In a sense then, we don’t really know. It’s probably not a good idea to go around claiming Katrina is evidence that we need to work on reducing human induced climate change, but a greater proportion of Katrina-like hurricanes among all hurricanes is certainly something we can expect in the future.

Categories: Environment, Science
  1. sarah
    September 15, 2005 at 3:35 pm

    My science teacher is making us do a 5 page report on why there is an hurricanes and tropical storms in the gulf coast waters, and I have been looking everywhere for information on my topic and I keep on getting different answers! This is very confusing and I need some help on wether there is an increase due to global warming or natural variation! Thanks!

  2. sarah
    September 15, 2005 at 3:36 pm

    an increase* …left out the word increase!

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