Is Climate Change Causing Wild Weather?

Almost a year ago, Amy Calder of the National Journal asked:

Are extreme weather events, when considered collectively, evidence that climate change is occurring? If not, what are the missing links scientists still need to study in order to make a more conclusive find? Could these weather events revive congressional efforts to pass comprehensive climate legislation?

My answer is still the same, even though I grapple with what my resonsibilities and moral obligations are as a scientist learning, a tenured professor teaching, and a citizen voting. Here it is:

No, climate does not cause weather, the balances of forces, masses, and energies in the atmosphere do. Furthermore, the atmosphere interacts with oceans, ice sheets, lands, and livings things. Ask an equally ill-posed question “Is climate change contributing to wild weather?” and my answer becomes yes, but with the caveat that butterflies flapping their wings in Tokyo contribute as well. There is more to the question than meets the eye.

Globally averaged air temperatures have increased by about 0.6 degrees Celsius per decade over the last 50 years. This warming is not uniform as it varies in both space and time. Some places cool, some places warm, some places cool or warm more than expected. Floods, droughts, mudslides, and calving glaciers always have and always will occur. Some weather events separated in space and time are physically linked via large-scale tele-connections such as Rossby waves in the atmospheric jet stream or the El Nino-Southern Oscillations.

So, how much of the currently observed extreme weather events are due to globally increasing air temperatures that also coincide with globally increasing ocean temperatures? Does global warming increase, say, the intensity of hurricane by 1% or 10% or 50%? These much tougher questions are at the forefront of both observational and computational work on environmental physics. The IPCC numerical models and new understanding of key physical processes, I feel, are the only way to attribute global warming effects on extreme weather events. Ice-ocean interactions around Greenland are one such physical process poorly incorporated in IPCC models. Another such process is the way that hurricanes may dominate the ocean heat flux towards Greenland.

Three weeks before Katrina hit New Orleans in 2005 MIT professor Dr. Kerry Emanuel published that the power dissipation by hurricanes has increased by about 60% over the last 30 years and that this increase correlates with increasing sea surface temperatures in the tropical North Atlantic. Nevertheless, Dr. Emanuel himself stressed that nothing could be more absurd than stating that Katrina was caused by global warming. Furthermore, refining his methodology in 2008, he finds that “… global warming should reduce the global frequency of hurricanes, though their intensity may increase in some locations.” [Emanuel et al., 2008: Hurricanes and global warming, Bull. Amer. Meteor. Soc. 89, 347-367.]

Just because a pattern of extreme weather events feels like evidence of global warming, it does not make it so. This scientific uncertainty, however, should not distract from the potential costs that a potentially man-made climate change will cause. Climate zones may shift, sea level may rise, volatile weather events may become more volatile, etc. All of this may cause additional political instabilities in marginally stable nation states ill-equipped to deal with either natural or man-made disasters.

4 responses to “Is Climate Change Causing Wild Weather?

  1. But see Kerry’s more recent work (Nature cover article ~Feb. 2010) postulating much-enhanced TC numbers during the Pliocene. I’m not sure how he reconciles the two results, although perhaps there’s a reason to expect a short-term reduction followed by a long-term increase.

  2. I had a look for more recent material from Kerry and found this, which seems to stand in somewhat direct contradiction to the BAMS 2008 article.

    See also this RC discussion I instigated, although I don’t know if there are as yet any more results on the stratospheric cooling influence. I notice that Kerry resubmitted the same paper (unchanged) to the AMS general meeting earlier this year.

    Shifting gears, I see that I’m your first commenter. Welcome to the blogosphere!.

    • Thank you for your rich additional insights and links. The devil of all science is in the evolving details … my main point here, however, was not to review a static fact, but the emphasize how good science evolves. I am pretty sure that neither Dr. Emanuel’s 2008 nor his 2010 papers or talks will be the last word on this subject. To be continued … [bed time for me].

  3. Just to add that Kerry’s view of Katrina’s causation is probably unchanged by all of this.

    I expect, though, that he would probably agree with Kevin Trenberth’s observation that all weather events now have natural and anthropogenic components. Given the inherent difficulty of disentangling these influences, all that can be done is to either do statistical attribution of a certain class of event or show that an event could not have happened as it did absent an anthropogenic influence (e.g. the 2003 European heat wave). It’s also possible to attribute large-scale changes that in turn influence weather, e.g. the expansion of the tropics. As Trenberth also points out, it’s inevitable that the increase in atmospheric water vapor content (~4% in the last 30 years) will affect lots of things.

    (IANAS BTW, although I follow the science closely.)

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