It seems that a number of folks have missed one of the more important conclusions from our first paper on the surfacestations project. Co-author Dr. John Nielsen-Gammon has some things to say about it the finding that is the title.
For layman readers that don’t know what diurnal variation is, it is the daily variation of temperature due to the variation of incoming solar radiation from rotation of the earth on its axis.
It looks like this:
Fall, S., A. Watts, J. Nielsen-Gammon, E. Jones, D. Niyogi, J. Christy, and R.A. Pielke Sr., 2011: Analysis of the impacts of station exposure on the U.S. Historical Climatology Network temperatures and temperature trends. J. Geophys. Res., in press. Copyright (2011) American Geophysical Union.
Fall et al. 2011: What We Learned About the Climate
By Dr. John Nielsen-Gammon
Our paper has a lot of info and analysis about temperatures and temperature trends and their correspondence to siting class. Perhaps the most important question, “Is the mean temperature trend different from previous estimates?” is answered in the negative, albeit with an asterisk associated with the limited scope of the study. While negative results are useful, they’re also boring. So in this post I’ll talk about something we did learn about the climate that’s new and different, and why I think it matters.
This new finding is stated succinctly in the abstract as: “According to the best-sited stations, the diurnal temperature range in the lower 48 states has no century-scale trend.”
The diurnal temperature range is simply the difference between the daily maximum temperature and daily minimum temperature, “diurnal” being a more impressive way of saying “daily”. It’s conventionally abbreviated DTR.
The change in global DTR trends roughly coincides with the change in phase of the Pacific Decadal Oscillation, for example, so it’s hard to tell whether the DTR trends were natural or forced.
That’s where Fall et al. 2011 comes in. The figure below shows the change in DTR anomalies over time over the United States, as estimated using data from each of the four classes of station siting. The data goes all the way back to 1895, making this (as far as we know) the longest evaluation of regional DTR trends done anywhere.
Dotted lines represent average values and are plotted 1.5 C apart from each other
All four classes of stations show the decline from the 1950s through the 1970s. But if you take a broader view, you see that the black line, representing the estimate from the best-sited stations, has a long-term positive (!) trend using unadjusted data or time-of-observation adjusted data, and has no trend at all for fully adjusted data (top set of curves). The lower-quality siting classes all show a negative long-term trend, and the worse the siting, the larger the trend.
These results suggest that the DTR in the United States has not decreased due to global warming, and that analyses to the contrary were at least partly contaminated by station siting problems. Indeed, DTR tended to increase when temperatures were fairly stable and tended to decrease when temperatures rose. Maybe DTR really isn’t a robust signal of global warming, and maybe the discrepancy between models and observations is primarily a problem with the observations rather than the models!
I’ve used the words “suggest” and “maybe” here. That’s because I regard our results as tentative. The zero trend estimate is based on only 80 stations, which might be only marginally adequate. The systematic change in trend with station siting quality makes me more confident, but the fact that the closest poorly-sited stations have a weak but positive DTR trend suggests that DTR may be strongly site-specific and makes me less confident. Maybe the best-sited stations have actually improved their siting over time, and maybe the adjustments haven’t fully corrected for this. Because of all this, I think these results need to be confirmed through other means or in other parts of the world before I will wholeheartedly believe that the real DTR has not been decreasing.
Nonetheless, all the ongoing work to understand the consequences of a faster rise in minimum than maximum temperatures for ecosystems and human health might, just might, be misguided.
Read the full post here