Der er sand middelalderstemning i klimadebatten efter superorkanen Sandy ramte USA’s østkyst i går. Alarmisterne har travlt med at udlægge et vejrfænomen som straffen for vorers synder. “Weird, extreme weather makes people worry, makes them think the world is changing” skriver Chris Mooney for Mother Jones og tilslutter sig den antagelse. For selv om Mooney medgiver at Sandy er et sammenrend af naturlige omstændigheder der sker fra tid til anden så kan man med en eksperts udtalelse om menneskets bagvedliggende påvirkning spekulere “it might be” - og den teoretiske mulighed er vel så godt som et bevis. Og som John McCains datter Meghan ifølge The Hill langede ud efter sine partifællers bagstræberiskhed: “So are we still going to go with climate change not being real fellow republicans [sic]?” slog tidligere vicepræsident Al Gore fast at “Dirty energy makes dirty weather“ . I samme logik foreslår Huffington Posts Kevin Welner polemisk at man opkalder fremtidige orkaner efter klimabenægtere; det er jo dem der spreder denne ’dirty doubt’.
Men ”there remains far too much natural variability in the frequency and potency of rare and powerful storms — on time scales from decades to centuries – to go beyond pointing to this event being consistent with what’s projected on a human-heated planet.” minder Andrew Revkin os køligt om i New York Times og citerer et par uhysteriske eksperter
Great events can have little causes. In this case, the immediate cause is most likely little more that the coincidental alignment of a tropical storm with an extratropical storm. Both frequent the west Atlantic in October…nothing unusual with that. On rare occasions their timing is such as to result in an interaction which can lead to an extreme event along the eastern seaboard. As to underlying causes, neither the frequency of tropical or extratropical cyclones over the North Atlantic are projected to appreciably change due to climate change, nor have there been indications of a change in their statistical behavior over this region in recent decades (see IPCC 2012 SREX report).
So, while it will rain like “black cats and Frankenweenies” over the midatlantic, this is not some spell conjured upon us by great external forces….unless you believe in the monster flicks of Universal Stuidios fame!
Kevin Trenberth of the National Center for Atmospheric Research offered these thoughts (I’m adding links to explain some of the acronyms):
The sea surface temperatures along the coast are 5 degrees F. or more above average and 1 degree F. is from global warming. Stronger storm and more precipitation results.
But with respect to the Arctic connection, I don’t believe it. Yes the NAO and NAM have gone negative: the NAO since about the middle of October, and it is projected to go back to close to zero in a week or so: heading to more positive now. The NAO and NAM (or AO as some call it) are natural modes of variability. They occur in models with no external forcings and just climo SSTs. The SAM in the Southern Hemisphere is similar in that regard and the SAM has been affected by the ozone hole and perhaps CO2 to make for a more positive sign. This is clear. So the natural mode can be influenced by externalities. There are several possibilities in the Northern Hemisphere. One may be the Arctic sea ice melt, another might be ozone depletion and certainly events in the stratosphere (including solar effects). To the extent that cooling in the Southern Hemisphere makes for a more positive phase of SAM, one might argue that warming in the Northern Hemisphere works the other way, but it is far from clear. NAO and NAM can do this all by itself. How less sea ice does anything is not clear. It does mean air is apt to be warmer and moister and with prospects for more snow on nearby land in the Fall. But the actual heating of the atmosphere is very small to cause it to do anything.
The studies published on this report associations that, to me, do not tell us cause and effect.It is true that hurricanes normally recurve and head east, especially at this time of year. So we do have a negative NAO and some blocking anticyclone in place, but the null hypothesis has to be that this is just “weather” and natural variability. The more definitive study on effects is by Balmaseda et al in QJRMS last year. [*There's more from Trenberth here.]
Here’s a thought from Patrick J. Michaels, the climatologist best known for his work for the libertarian Cato Institute:
By any standard, this is an impressive cyclone for our latitude. You might want to check Ludlum’s “Early American Hurricanes” for the Snow Hurricane of 1804, which was earlier and a bit further north — but NYC showed a pretty similar barometric pressure. Are you familiar with his great series of books on pre-1900 weather?
Alluding to my line about variability excluding any global warming link beyond saying the storm is consistent with projections, Michaels wrote this:
It’s also consistent with a planet with colder temperatures as well as one with warmer ones. More important, events like this are inevitable on a planet that has an ocean with the geography of the Atlantic (meaning a Gulf Stream-like feature), a large north-south continent on its western margin without a transverse mountain range to inhibit the merger of tropical warmth with polar cold, and four seasons in the temperate latitudes. And I predict confidently that we will survive Sandy, which should not be a tropical cyclone at landfall.
Og Roger Pielke jr. har efter forvoldte skader opstillet en top 10 over lignende orkaner der har ramt USA i samme område som Sandy