Thursday, January 15, 2015

It's not that cold. Really.

Well, this may come as a true victory for all the climate-curmudgeons out there (you know, the ones who invalidate you by telling you they remember it stormier, hotter, colder, snowier), but the recent cold in the Midwest has not been unusual. Now, you can look at the actual temperatures, and their departures from normal (see below, courtesy of the Midwest Regional Climate Center), and tell me I'm cracked. Clearly it's been colder than normal. But I'm telling you, this isn't unusual.

Well, actually it has been unusual, but not because of how cold it's been. It's been unusual because of how not cold it's been.

It's mid-January, and the region is now in its climatologically-coldest time of the of season, which will peak (in terms of statistically-averaged coldness) between now and th 28th in most areas. The cold temperatures we have been experiencing are likely (though not at all guaranteed) to be the coldest of the season. At Minneapolis, that would mean a wintertime low of -11 (F). At International Falls, it would mean -28. People from outside the region may find either of those values unthinkable. But to those in the region, if those are indeed season-lows, they are unthinkably warm. And this is the way it's been going.

Below you will see a graph of the coldest temperature of winter at Minneapolis, from 1900, right through the 2013-14 season (in purple). I have also included the average of the 15 coldest days of winters (in blue), along with 10-year averages of both values. One thing is abundantly clear: the coldest temperatures of winter are warming rapidly.

The coldest temperature of the year has warmed from the -20s (F), into the -10s. For all the hype it garnered, the "Polar Vortex" episode from early January 2014, only brought the temperature down to -23 at Minneapolis. You can look at 2013, and then mouse over the purple trace on the image, and see just how many times the lowest value of winter was that cold or colder. It's kind of astonishing, how quickly we've lost this defining component of Upper Midwest winters. You can also see that -11 stands among the warmest, in a three-way tie for third place.

We can examine the issue a bit differently, as ask how frequently temperatures have fallen below certain thresholds, historically. In this example, I'll just use -10, again at Minneapolis.

For the period up until about 1990, temperatures at Minneapolis reached -10 (F) or colder an average of 11.8 times per winter. From 1991 onward, that average has fallen to 6.3, and the rapid decline from the peak in 1973 is pretty obvious.

And of course, it's not just the coldest nights that are getting warmer. You know those cold days, when the high temperature doesn't even come close to zero? Those are getting warmer too.

So, the coldest nights aren't as cold as they used to be, the coldest days are not as cold as they used to be, and we don't fall below certain cold-weather thresholds as much as we used to.

All of this, by the way, is true for the whole of Minnesota and the Upper Midwest. But maybe you're skeptical because Minneapolis is a big city with all sorts of urban heat contamination (which is true). So here is the annual frequency of lows at or below -25 at the Sandy Lake Dam, which is part of the US Historical Climate Network.

And where is the Sandy Lake Dam? Exactly. Hopefully you can accept that the pattern of increased temperatures (and decreased frequency of cold temperatures) is not unique to Minneapolis. No matter how you look at it, one thing is obvious region-wide: we do not get as cold as we used to.

And the temperatures we have been experiencing? They're not that cold. Really.

Note: this information was gathered in support of research conducted with and for Hennepin County Emergency Management

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