November 13, 2009

U.S. Record Temperatures—A Closer Look

Filed under: Surface, Temperature History

A new paper that is soon to appear in the journal Geophysical Research Letters finds that across the U.S. daily record high temperatures are being set at about twice the frequency of daily record low temperatures and that this ratio—number of record highs to the number of record lows, has been growing larger over the past 50 years.

The popular press seems to be particularly taken with this finding, although headline proclamations fail to disclose important details of the actual findings reported by the National Center for Atmospheric Research’s (NCAR) Gerald Meehl and colleagues.

Although you can hardly blame the press, because the NCAR press release did much to lead them down this muddy path.

Meehl et al. find that the reason more daily maximum temperature records are being set than daily minimum temperatures records is because there are fewer than expected daily lows records being set, not because there are more daily high records than expected.

In other words, our days are not becoming extremely hotter, but our nights are becoming less extremely cold. This fact is buried in the press release and consequently in most of the coverage—likely, because this finding has pretty benign, if not beneficial, implications.

Instead of highlighting this, the NCAR press release not only tries to confuse it with all sorts of graphs and numbers (presented without a proper reference frame of the expectations), but by also suggesting that this observation—nights warming more than days—is what is expected because of a rising greenhouse effect, and they have a model to prove it.

While it is true that an enhanced greenhouse effect should warm nights more than days, so too does the processes of urbanization—something which has not been accounted for in the results of Meehl et al. (because it is virtually impossible to do so at a daily level), but something that is widely known to be occurring.

A report just published earlier this week estimates that urbanization and other land use changes are responsible for half of the observed temperature rise in recent decades in the U.S. This is similar to what Ross McKitrick and WCR editor Pat Michaels found for global land-based temperatures in a paper published two years ago.

Further, despite the contention that climate models show the nights-warming-more-than-days expectation from an enhancing greenhouse effect, the model results depicted in Meehl et al.’s paper don’t show this at all. In fact, the model in the paper—the NCAR climate model—shows that it expects daily high records should be being increasing (above expectations of no climate change) at about the same frequency that daily minimum temperature records should be decreasing. This model expectation is shown by observations to be wrong.

This model error can be seen by comparing Figures 1 and 2 below. They show the expected progression of the number of record daily temperatures with time (black curves) (note the number of expected daily records in any given year decreases with time). The red dots on Figures 1 and 2 represent the number of daily maximum temperature records set each year and the blue dots represent the number of daily minimum records. Figure 1 shows the observations, and Figure 2 shows the expectations from the NCAR climate model.

First let’s focus on the top graph in Figure 1 and see what the observations say is going on.

• with the exception of the 1960s and 1970s when fewer daily high temperature records were set than expected (the red dots fall below the black curve), for the most part, the number of daily high temperatures set each year falls off as the theory (which assumes no climate change) predicts it should (i.e. the red dots are pretty evenly distributed about the black curve).

• the observed data show that the number of daily minimum records being set (blue dots) is progressively falling further below the theoretical expectations. This confirms what we said earlier—that it is the decrease in minimum temperature records that is dominating the increase in the maximum/minimum temperature ratio.

Figure 1. Observed number of daily maximum (red dots) and daily minimum (blue dots) temperature records set each year across the United States, compared to expectations of assuming no temperature change (black curve). (Source: Meehl et al., 2009).

Now let’s have a look at what the NCAR model thinks should have been going on over the same time period. Notice in Figure 2 that the number of daily high temperatures progressively decrease less than expected under the assumption of no temperature change (red dots getting progressively higher than the black curve), while the number of expected minimum records decrease more than expected (blue dots progressively fall further below the black curve).

Figure 2. Projected number of daily maximum (red dots) and daily minimum (blue dots) temperature records set each year by the NCAR climate model, compared to expectations of assuming no temperature change (black curve). (Source: Meehl et al., 2009).

So while the observations suggest that our nights are warming faster than our days, this is not so in the NCAR climate model which suggests that the days and nights are warming up at the same rate. Such a model error leads to the model grossly overestimating the frequency and intensity of future heat waves.

Hopefully the powers-that-be in the EPA take note of this, because the specter of expanding heat waves in the future is something that the EPA highlights as it tries to justify regulating greenhouse gases under the Clean Air Act. Maybe they should go back and rethink that section as the climate models—or at least the NCAR climate model—fail to capture the behavior of the observations in this regard.

Repeat after us—if the models can’t replicate reality (for the right reasons) they can’t reliably predict the future.

So, the bottom line here is this—climate change in the U.S. during the past 50 years has resulted in fewer extreme nighttime low temperatures, while the daily extreme high temperatures have been little affected. And, at least one leading climate model fails to correctly capture this behavior.

Ask yourself this, is this the impression that you got from the coverage of this in the popular press?


McKitrick, R. R., and P. J. Michaels, 2007. Quantifying the influence of anthropogenic surface processes inhomogeneities on gridded global climate data. Journal of Geophysical Research, 112, D24S09, doi:10.1029/2007JD008465.

Meehl, G. A., et al., 2009. The relative increase of record high maximum temperatures compared to record low minimum temperatures in the U.S., Geophysical Research Letters, doi:10.1029/2009GL040736, in press.

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