The Blackboard:

NASA’s Earth Observatory News announced the PDO has entered a cool phase, thought to enhance the effects of La Ninos and diminish those of El Nino. I snagged the larger NASA image of sea surface temperatures for April 14–21, 2008and highlighted the cool spot caused by the PDO and the cool spot caused by La Nina:

Josh Willis, JPL oceanographer and climate scientist, says of the PDO,

“These natural climate phenomena can sometimes hide global warming caused by human activities. Or they can have the opposite effect of accentuating it.”

Yes. Indeed they can accentuate it or mask it. This might suggest that if the PDO is masking Global Warming now then it might have been accentuating during the 80s and 90s.

The empirically inclined might ask is: “How much masking or warming might exist in recent and future data?”

My answer is, “Beats me!”

I have not yet found anything suggesting the likely amplitude of peak-to-trough temperature variations associated with phase switches in the PDO. What I do know is PDO phases are thought to persist 20-30 years. What I don’t know is whether the the peak-to-trough effect thought to be 0.1K — as suggested for the solar cycle? Or 0.2K? Or a mega-whopping 0.5K? (That would be rather amazing! Needless to say, I’d also be surprised to hear such a number.)

Does it matter to the climate-blogging and/or voting public how large the magnitude might be? Well… I think it does.

I may be wrong, but I can’t help but believe that if climatologists believe the effect is large-ish, an estimate of that magnitude, and a comment on the current phase of the PDO, might have been welcomed by readers who downloaded “verifications” of IPCC projections against measured data. Possibly, readers of Rahmstorf 2007 or the IPCC AR4 would be curious to know whether the comparison between predictions and data occurred while the PDO was thought to amplify or mask global climate change; after all, both documents discussed comparison over partial cycles of the PDO. So, ideally, the amplitude might be discussed near those data comparisons.

On the other hand, if the effect of the PDO is (or was) thought small, then failing to mention that effect during verifications would be reasonable. (It follows that readers who assume climatologists make reasonable, non-tendentious choices when publishing their data comparison, might further assume that those authors must think the effect of the PDO is small.)

Still, as I have no idea what the effect might be, but I knew the PDO existed, I long ago suggested the PDO switch might be responsible for the recent flatness in GMST trends. The PDO phase switches are infrequenct, and so might trigger statistical outliers. (In fact, that’s the definition of an outlier: something that doesn’t happen often.)

So, in that regard, let us suppose the “warm” phase is associated with “warm” GMST anomalies and the cool phase is associated with “cool” ones. Then, what might one make of this?

Hhhhmm… maybe we’ve started “masking”, rather than “enhancing?”

Would it be cynical of me to suggest that we may read explicit estimates of the magnitude of the effect on GMST now that it might be masking rather than accentuating global warming?

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