On Sunday I posted about the USHCN climate station of record in Tucumcari, NM highlighting its positive points since it has all the hallmarks of a well sited station with a long and uninterrupted record. But something was odd with the temperature record that didn’t quite make sense at first glance.
I also cross posted my report on Climate Audit since I always value additional input from the community there.
I noted that while this station is in fact well sited, and rates a CRN2, it has some oddities with it’s temperature record around the year 2000, something that looked like a step function to me.
Click for larger graph from NASA GISTEMP
Of course, even though this a truly rural station, 3 miles from the outskirts of town, Hansen and GISS apply adjustments to it anyway, which is part of the flawed “nightlights” algorithm incorrectly flagging this station for adjustment. Even though the adjustment makes the present cooler, it still seems misplaced given the station quality and history. Steve McIntyre said it best:
Here we have a rural station where there doesn’t seem to be any reason to adjust the temperature for population growth/UHI. But in this case, Hansen adjusts Tucumcari as though it were a city. Why is he even adjusting Tucumcari at all? (The “reason” is that its lights value removes it from the rural classification and it goes into the adjustment pool.) While Hansen sometimes seemingly cools rural stations in the past, for the GISS dset2 version here, he warms the past of the station (cools the present).
It’s more that this is a case of another unjustified adjustment by the “adjuster in chief” showing once again that the Hansen adjustments do not do what they are supposed to do – and the best that can be hoped from the Hansen adjustment program by users of this dataset is that the adjustments overall end up being pointless and random, rather than pointless and biased.
The adjustment by GISS looks like this:
Ok adjustments aside, the fact that there is a step at 2000 that remained unexplained until commenters on CA started looking at the data themselves. DaleC provided this graph:
Click for original image
Note the arrow that I place at the year 2000. Notice anything?
The annual average minimum temperature has exceeded 45°F and maintained the rise since 2000. For the first time in the station history going back to 1905, the minimum temperature has gone above that 45°F mark and stayed there. Yes there have been some previous brief excursions above 45°F, but none appear to have lasted more than 2 years. Note that average annual maximum temperatures did not increase during the same period.
What could cause that? We can rule out adjustments, since this is GHCN data before Hansen gets his adjuster mitts on it. We can rule out location change or equipment change, since according to NCDC metadata the station has been in the same place since at least 1946 and possibly longer. It still uses mercury max/min thermometers, so there’s no MMTS next to a building or parking lot to blame.
So what is left? Something around the station in the measurement environment that affects the nighttime readings. I recalled seeing this before. And back in early 2007, I had posted a story about a paper from Dr. John Christy of UAH where he studied a number of stations in the San Joaquin Valley in California because they had exhibited this same symptom:
Christy remarks: “Another factor is the dry air, something common to all deserts. Water vapor is a powerful greenhouse gas. Desert air lacks water vapor. The air turns cold at night because it doesn’t retain much warmth from the daytime and it can’t trap what little heat might rise from the ground at night.”
Evaporation from irrigated fields adds water vapor to the air — a process that cools summer days but traps heat rising from the damp soil at night.
“If there is anything I’ve learned in Alabama, it is that humidity can make summer nights very warm,” said Christy, a Fresno, Calif., native who has lived in Alabama since 1987.
Once I mentioned this as a possibility to explain the increase in nighttime temperatures, it didn’t take Steve McIntyre long to find some anecdotal evidence that correlated: