We’ve been looking at patterns of climate change in the conterminous USA (the “lower 48 states”) recently. Let’s see what’s been happening in the different seasons of the year.
Specifically, let’s find out how temperature has been changing since 1985, the last 33 years. I’ll take the data for each of the 344 climate divisions in the USA and estimate the trend since 1985 by least-squares regression. Then I can make maps showing how fast they’ve been changing, by putting red dots where it’s getting hotter, blue dots where it’s getting colder, with larger dots indicating faster change, smaller dots slower change.
It probably won’t surprise you that there are a lot more red dots than blue dots — the nation as a whole has been heating up. First let’s see how things have changed on an annual basis, averaged over the whole year rather than restricted to a single season. In that case, there aren’t any blue dots because there’s no USA climate division with a negative temperature trend since 1985:
Two regions stand out with rapid warming: New England, and the west, especially the southwest. Warming has been notably less rapid in the southeast, and along the Canadian border and the Pacific northwest.
But what are the seasonal patterns? I’ll define the seasons in the usual climatological way, so that winter is Dec-Jan-Feb, spring is Mar-Apr-May, summer is Jun-Jul-Aug, and autumn is Sep-Oct-Nov. Here are the patterns for the different seasons of the year [Note: on some browsers, you can right-click on the graph and select “View Image” for a larger, clearer view]:
Winter warming has been strongest in New England, and also strong in western states. In the middle of the country, especially the norther part, winter warming has been much less — in fact some of those states, like North and South Dakota, Nebrasa, and Kansas have very small blue dots because their trend-since-1985 is ever so slightly downward.
Strongest summer warming has been in the west, especially the desert southwest, with New Mexico leading the pack. Much of New England also shows strong summer warming, but not as strong as western states, and it doesn’t extend as far as Maine.
The biggest seasonal differences are during spring and autumn. Spring has warmed less than other seasons overall, with some big blue dots showing cooling in the northernmost midsection of the country, especially North Dakota. Autumn has shown the year’s strongest warming, with the temperature increase in New England and the northern mid-country outstripping all other seasons and other regions.
Those graphs are temperature trends since 1985 for daily mean temperature, which is the average of the day’s high and low temperatures. What about the high and low temperatures themselves? Here are the year-round trends for high temperature:
Here are the seasonal patterns:
Here is the year-round trend pattern for low temperature:
And here it is for the different seasons of the year:
I could muse about the various patterns, but I’m guessing that you’re smart enough to draw your own conclusions with fair accuracy.
The interesting part of all this is the different trends we’ve seen over the last 33 years. Both different regions of the country, and different seasons, illustrate that regional and seasonal warming aren’t necessarily the same as global warming year-round. When it comes to climate change, age cannot wither, nor custom stale, her infinite variety.
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