Disrupting the Theory on PV and Global Warming
February 4, 2019
Guest post by Bob Vislocky

From the “where theory doesn’t agree with the numbers” camp comes this Washington Post article and many others like it that we’re all familiar with by now which claims that global warming will cause more polar vortex disruptions thereby leading to more frequent extreme cold outbreaks like the one observed last week:

https://www.washingtonpost.com/politics/science-says-get-used-to-polar-vortex-outbreaks/2019/01/28/6c3eba52-233f-11e9-b5b4-1d18dfb7b084_story.html?noredirect=on&utm_term=.e1cac7e64271

This note attempts to address is whether there is any data to back up such a claim. If global warming is causing more frequent or severe cold outbreaks in the mid-latitudes then this should manifest itself in the observational temperature history. Specifically, the warmest day in winter should increase over time as one would expect with a warming planet, but if the proposed theory is true then the coldest day in winter would not be expected to rise as much or remain steady for stations in the northern half of the US.

Therefore, if global warming is causing more potent cold outbreaks due to disruption of the polar vortex, then the temperature difference in winter between the warmest day and the coldest day should increase over time. Fortunately this is very easy to test using available NOWDATA from NOAA, and five stations from New York to Denver that have a long historical temperature record were selected for the analysis (see figures 1-5 below).

Figure 1. Difference between the warmest daily temperature and coldest daily temperature in winter (Dec-Feb) by year for New York City (in blue) and the least squares trend line (in black).

Figure 2. Same as Figure 1, except for Pittsburgh.

Figure 3. Same as Figure 1, except for Chicago.

Figure 4. Same as Figure 1, except for Minneapolis.

Figure 5. Same as Figure 1, except for Denver.

Results show that the temperature range (difference between the warmest day in winter and the coldest day) has actually decreased substantially over time at Minneapolis

(-0.065*/year) while Denver (-0.020*/year), New York (-0.017*/year) and Chicago (-0.010*/year) displayed more modest decreases in the trend. Only Pittsburgh (+0.001*/year) showed any increase in the temperature range over time but it was quite insignificant. Therefore, there does not appear to be any observational evidence to support the claim that global warming is causing greater or more frequent cold outbreaks like the one observed last week in the north-central US.