Cliff Mass shows a graph, taken from the Seattle Times, of the hottest temperature each July from 1945 through 2022 at Seattle/Tacoma airport (SEATAC). He then says “… and there is very little upward trend! How could this be?”
Then he goes further:
Just to check on the Seattle Times… I did the same thing for July and August over the past 50 years, plotting the warmest observed temperature at both SeaTac and Pasco (see below).
Hardly any change in the extreme high temperatures each year at either site. No long-term trend….and you would expect a trend if global warming was important for the extreme heat waves!
Unfortunately for Cliff Mass, the data contradict him.
I retrieved the data for SEATAC, although the source from which I acquired it (Climate Explorer) doesn’t include this year, it only extends from 1948 through 2021. I then noted the hottest temperature during each year’s July/August, which looks like this:
The red line is a trend estimate from least-squares regression, and indicates that the hottest July/August temperature is rising at about 6 °F/century, and has gone up about 4.5 °F during the period of record. And — the trend is definitely “statistically significant.”
So … why does Cliff Mass say “No long-term trend“? He does have an extra year’s data — which is not really quite right, because this year (2022) isn’t over yet, the entire month of August is yet to come. More important, he doesn’t start his graph until 1971. If I restrict my trend analysis to the period from 1971 onward, I get the trend line shown here in blue:
It shows nearly the same trend as the full analysis, rising at a rate of 5 °F/century. Over that period of record, the total rise is 2.3 °F. Really, it’s the same trend (within the error ranges) but based on a shorter time span of data. And because of that, the trend estimate using only the shortened data set is no longer “statistically significant.”
Which is no surprise at all. When you shorten the time span, even if the trend doesn’t change at all, the statistical significance will go down. Shorten the time span enough, you’re sure to eliminate statistical significance. It seems to me, that is exactly what Cliff Mass has done.
One other thing strikes me as odd. In climate, “summer” is usually defined as June/July/August (JJA), yet Cliff Mass quite ignores the month of June. Thing is, the hottest day of the year often occurs during June, which makes his reference to “extreme high temperatures each year” incorrect, at least for the 20 years during the record when the “extreme high temperature” occured in the month of June.
If we look at the actual “extreme high temperature each year” at SEATAC, it looks like this:
Again, the red line is a trend estimate by least-squares regression. It indicates warming at a rate of 7.6 °F/century, and shows a total rise of 5.5 °F over the period of record. And it is most assuredly “statistically significant.”
Lest you think that’s only an accident because of last year’s super heat-wave, if I remove that year (2021) from the analysis, it’s still statistically significant.
Cliff Mass considers it a “dilemma” that there’s no trend in the hottest day of the year at SEATAC, and he promises to answer this dilemma in his podcast. I’d like him to answer a different question: How did you fail to see the rather obvious — and strongly statistically significant — trend in the hottest day of the year at SEATAC?
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