Troy's Scratchpad

August 10, 2010

Specific Humidity, Temperature, and UHI

Filed under: Uncategorized — troyca @ 11:37 am

Over at Open Mind, Tamino finds something pretty cool: when you superimpose the scaled GISSTemp anamolies over the specific humidity anomlies, they track pretty well.  From his comment,  

It also raises a question for those who doubt the correctness of observed global temperature increase: if (as so many denialists claim) the globe isn’t warming because the global temperature estimates are wrong, then why does the specific humidity track it so well?

I get the impression that he’s implying (I could be reading too much into this) that there is no major issue with the temperature record — such as a UHI bias.  Maybe I’m just being obtuse here because I don’t want to think I’ve been on a fool’s errand trying to find a UHI signal , but I don’t see how the superimposition necessarily suggests that there’s no UHI bias in the record.  After all, he’s scaled the GISSTemp, so we could theoretically scale to a different value if we predicted a different degree of warming.

First, I’ll try to re-create his graph.  I’m using specific humidity data (B&K) from here, and GISSTemp data from here

I think that looks about right.  I scale based on the ratio of the specific humidity trend vs. GISSTemp trend (here it ends up being 0.003697), and use the 1980 value as my temperature baseline value (27). to calculate temperature anamolies.  The result is the graph above, with a correlation of 0.89.

Now, suppose we assumed that the 40% of the warming trend in GISSTemp over this time period is actually the result of a UHI bias.  I correct for this by subtracting SLOPE * 0.4 * YEARS_SINCE_1970 from all of the relevant GISSTemp points.  Let’s see if we can superimpose it again:

Here my scaling value is 0.006162, my baseline temp value is 19.17, and the correlation is 0.88.

Is it suprising to see that even if there was a large (40%)  UHI bias in the GISSTemp record, it would still track well?  That is, even if we assume that true global warming has been only a bit more than half of what’s shown in GISSTemp, we can still get a match to this specific humidity?  It shouldn’t be, since once again, we’re simply scaling the GISSTemp record. 

Now, it may be that a temperature increase of X predicts a specific rise in humidity of Y, in which case we can’t just change our scale willy-nilly.  However, I did not see those specific calculations at first glance. 

Furthermore, one may object that the UHI bias would not be quite as constant as we used it, where we basically assumed exactly an increase of 40% of the overall trend per year. 

Still, for the sake of completeness, I wanted to show that the tracking in itself does not necessarily vindicate the magnitude of warming in the record…not when we use scaling.

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