What is a Record Heat Wave, or a Record in Any Climate Metric?

The discussion on the significance of the recent heat wave in eastern Colorado continues (see the Rocky Mountain News article “Hot streak has experts divided“). This article illustrates an important issue in climate science: What measures do we use to identify a heat wave (or other climate extreme) as an all-time record? Can we make such claims from a record at an individual location?

The use of a single station, of course, is fraught with problems. The Denver site, which is referred to in the news article, is located at Denver International Airport (its location is given in this figure). Its position close to runways and buildings raises the issue of its exposure (we have requested photographs of the site). The site has also been moved twice since it was installed. Klaus Wolter and I (who are both referred to in the Rocky Mountain News article) will be completing a co-authored scientific paper over the next several weeks in order to place the heat wave in context, and we will report on our conclusions then. This is the proper procedure to determine if the heat wave in Colorado was really “unprecedented”, or is just an artifact of one observation site.

The highlighting of data from single sites has been misused in the past. For example, Alward et al. titled their paper “Grassland vegetation changes and nocturnal global warming“. This very comprehensive title was based on a reported increase in growing season at a single observation site in eastern Colorado. When we investigated other sites in eastern Colorado, we found that the site used in the Alward et al study was clearly not representative of the trends in growing season for other locations in eastern Colorado.

In our paper Spatial Representativeness of Temperature Measurements from a Single Site, we concluded that “It is unlikely that one or a few weather stations are representative of regional climate trends…”


“the assessment of a group of stations for …more qualitative trends….provides a reasonably robust procedure to evaluate climate trends and variability”.

We used temperature thresholds such as number of days with the temperatures above or below thresholds (such as the number of days above 90°F and 100°F).

Melillo used the results of the Alward et al paper as further evidence that the central grasslands and the Earth were warming. The title of his paper was “Perspectives: climate change – warm,warm on the range“. Escalating the issue further, the Associated Press report on this work sensationalized that “Global warming could mean trouble for ranchers on the plains of Colorado and New Mexico.” All of this based on data from one observation site!

Clearly, the interpretation of the recent heat wave in Colorado needs to be investigated more rigorously, than was reported in the Rocky Mountain News article. Also, other states would benefit by placing their reports of records in context using data from the set of observations available and not just from individual sites.

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