ll provocatore estivo, cont.

David Archer ha mandato apprezzamenti e critiche al paper “molto importante ma faticoso” di Jim Hansen et al., e al loro modello per varie concentrazioni atmosferiche di CO2, conseguente fusione delle calotte polari e “tempo di raddoppio” dell’innalzamento del livello del mare; Andrew Revkin del New York Times segnala due articoli che dovrebbero contraddire l’evoluzione di tsunami e tempeste prodotta dal modello; Erik Stabenau, oceanologo del National Park Service per le Everglades, vorrebbe sapere “quali osservazioni attuali o entro un decennio verificherebbero gli assunti del modello”; Matthew Whipple obietta che non c’è consenso sul collasso del West Antarctic Ice Sheet durante l’Eemiano (stessa concentrazione d CO2 ma circa 1° C in più).

Il commento più bizzarro è di un certo Nabil Swedan che nega l’esistenza della back radiation:
Again, the work presented in this paper is based on a climate computer model that has incorrect physics. The energy balance of these climate models is based on the radiative forcing approach of the so called greenhouse gas effect. This effect is a fiction and the radiative approach violates the laws of thermodynamics. Colder atmospheric air “slabs” cannot radiate energy to warmer air “slabs” or surface.

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In un articolo per l’Huffington Post, Jim Hansen riassume il lavoro:

Our simulations were aimed to test my suspicion that ice sheet disintegration is a very nonlinear phenomena and that the IPCC studies were largely omitting what may be the most important forcing of the ocean: the effect of cold freshwater from melting ice. Rather than use an ice sheet model to estimate rates of freshwater release, we use observations for the present ice melt rate and specify several alternative rates of increase of ice melt. Our atmosphere-ocean model shows that the freshwater spurs amplifying feedbacks that would accelerate ice shelf and ice sheet mass loss, thus providing support for our assumption of a nonlinear ice sheet response.

Critica i tempi di fusione tra 200 e 900 anni previsti da modelli “primitivi” delle calotte polari:

If it is uncertain by an order of magnitude or more, why not 100-1000? Where does the 200-900 precision come from?
Why the peculiar 900 years instead of the logical 1000? Probably because nobody cares about matters 1000 years in the future (they may not care about 900, but 200-900 does not seem like infinity). A scientist knowing that sea level is a problem does not want the reader to dismiss it.
Why 200 years? For one thing, 100 years would require taking on the formidable IPCC, which estimates that even the huge climate forcing for a hypothetical 936 ppm CO2 in 2100 would yield less than one meter sea level rise. For another thing, incentives for scientists strongly favor conservative statements and militate against any “alarmist” conclusion; this is the “reticence” phenomenon that infects the sea level rise issue. .

Paragona il nuovo modello con quello “oceano-atmosfera” pubblicato nel 1988 (paragonato qui alle osservazioni fino al 2009) che aveva suscitato molto scetticismo tra i suoi colleghi, e il modo in cui ne aveva parlato al Congresso ancora di più:

I think there is an analogy of this paper to my congressional testimony in 1988-89. Then as now, conclusions are drawn from a combination of information from paleoclimate, modeling, ongoing observations, and theory.
Stakes in climate change are high, so conclusions about climate change are sure to draw fire. That’s as it should be; skepticism is the lifeblood of science, essential to success of an analysis. So criticisms of my testimony, as described well by Richard Kerr, were inevitable and useful.
Kerr’s article is instructive about scientific reticence, which can deprive policymakers of the gut feeling of experts. This is all important for sea level rise because of lags in the system (policies ? emissions ? climate change ? sea level rise). Information is needed as soon as possible.
The most perceptive comments in Kerr’s interviews may have been, as was often the case, from our good old friend Steve Schneider: “All that objective stuff rests on assumptions. The future is not based on statistics, it’s based on physics.” By “objective stuff” Steve referred to the arbitrary choices made to define probabilities of an outcome. The media accepts resulting probabilities as meaningful, yet entirely different results would be obtained from alternative initial choices.
Steve’s “objective stuff” defines IPCC’s sea level analysis precisely. They choose certain ‘process-based models’ as first choice to define future sea level. This gives sea level rise in 2100 (relative to 1986-2005 mean sea level) of 0.74 m with likely range 0.52-0.98 m for business-as-usual greenhouse gases (RCP8.5 scenario), where ‘likely’ is defined as >66 percent probability. Ugh.

In sostanza, è preoccupato per l’effetto domino nel caso di un collasso del West Antarctic Ice Sheet, come tanti e non da ieri (bello anche l’articolo di Greg Laden, e il post di Tamino).

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In tema di modelli su scala regionale basati su ricostruzioni paleoclimate, refrain su Nature Climate Science

Although models generally simulate changes in large-scale circulation sufficiently well to shift regional climates in the right direction, they often do not predict the correct magnitude of these changes. Differences in performance are only weakly related to modern-day biases or climate sensitivity, and more sophisticated models are not better at simulating climate changes. Although models correctly capture the broad patterns of climate change, improvements are required to produce reliable regional projections. 

Spiegazioni di James Annan, uno degli autori.

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Cooosa? L’esercito svizzero ha rubato l’acqua ai francesi per abbeverare le mucche confederali…