By Paul Homewood
As WUWT points out, John Holdren is one of many who have tried to link the cold winter in the USA this year to global warming.
In his White House video in January, he had this to say:
“A growing body of evidence suggests that the kind of extreme cold being experienced by much of the United States as we speak is a pattern that we can expect to see with increasing frequency as global warming continues….
We also know that this week’s cold spell is of a type there’s reason to believe may become more frequent in a world that’s getting warmer, on average, because of greenhouse-gas pollution.”
But is there any evidence that extreme cold winters are becoming more common, or, for that matter, more extreme?
First, let’s check the temperature trends for the CONUS in winter.
Clearly, on a national basis, recent winters have not been unusually cold. In the last 10 years, only three winters have been colder than the 1901-2000 mean. Moreover, no winters in recent years have come anywhere near to being as cold as some of the winters in the 1970’s, for instance, or earlier.
But this graph only tells half the story. As it covers the whole country, it could cover up regional extremes. As we know, this winter has seen particularly cold weather in Mid West and East, but warmer conditions out West. The result is that, to some extent, they cancel each other out.
So, is there a way we can isolate the warm from the cold, and see whether cold winters are becoming more extreme in just parts of the country?
There is actually a very simple method, and that is to use NOAA’s own Climate Extremes Index. This provides the percentage of the country which have had extreme temperatures (or precipitation, drought etc) during the year. As both above average and below average temperatures are shown separately, we can look at extreme cold weather on its own.
The graphs below cover the Winter months (Dec to Feb) only, with the first using mean monthly maximum temperatures, and the second minimums. The results seem pretty similar.
It is abundantly clear that much less of the country has been affected by extreme cold this winter, and indeed other recent ones, when compared with the 20thC. There is also no trend towards cold winters becoming more common.
What is also interesting is that there does not seem to be much of a trend towards milder winters taking over. Only the winter of 2011/12 stands out in this respect, and there have been plenty of similar years previously.
There has been nothing unusual or unprecedented about this winter. And, as cold winters have become less frequent in the last couple of decades, there is absolutely no evidence to support Holdren’s claim that “this week’s cold spell is of a type there’s reason to believe may become more frequent in a world that’s getting warmer”.
NOAA give this definition for the (maximum temperature) index:
The U.S. CEI is the arithmetic average of the following five or six# indicators of the percentage of the conterminous U.S. area:
- The sum of (a) percentage of the United States with maximum temperatures much below normal and (b) percentage of the United States with maximum temperatures much above normal.
And their definition for “much above normal”:
In each case, we define much above (below) normal or extreme conditions as those falling in the upper (lower) tenth percentile of the local, period of record. In any given year, each of the five indicators has an expected value of 20%, in that 10% of all observed values should fall, in the long-term average, in each tenth percentile, and there are two such sets in each indicator
The Climate Extremes Index can be accessed at the link below. It covers temperatures, drought, rainfall and hurricanes, and can used on a seasonal or annual basis. There is also a regional section.