USA Divisions

We’ve occasionally looked at temperature time series from NCDC (National Climate Data Center) for USA48 (the 48 states of the continental USA excluding Alaska). But in addition to data for individual states, NCDC also offers data for each individual climate division within each state. The temperature data, for example, can be found here.

Segmenting USA48 by division rather than state will give us a much more detailed geographical picture of climate change in the mainland U.S. Let’s take a look at some of the things the data have to say.

First let’s see how the trend rate (from linear regression) breaks down by division. Here’s the trend rate for the entire time span (from 1895 to 2012.5), with red circles indicating warming trends, blue circles cooling trends, larger circles larger trends (click the graph for a larger, clearer view):

The strongest warming is in New England, the upper midwest, and the desert southwest. Much more of the country shows overall warming than cooling, but there are still a good many divisions with negative overall trend, especially in the southeast. The blue color is easier to see if you click the graph for a larger view, because the blue circles that are present are all rather small.

The U.S., like the globe, has not shown uniform warming (or cooling) throughout the entire time span. Here’s the same information, but trend rates are for the time span 1895-1910 only:

Note that many of the 1895-1910 trends are huge compared to 1895-2012.5 trends. But they’re not really that indicative of trend because the time span is so short (a mere 15 years) and divisional temperature time series are noisier than global, so the trend uncertainty is quite large. Some regions (including New England and the desert southwest) cooled substantially, others warmed, but these can’t properly be called trends — I’d call them fluctuations, or natural variation.

From 1910 to 1940 the map looks a lot different:

With only a few exceptions, the nation as a whole warmed during this time period. Although warming is strongest in the great plains and a (very) few spots show cooling, it’s a bit surprising how uniform the trend is across the country. Then, from 1940 to 1975, the situation reversed:

With few exceptions, most of the country cooled off. The eastern half shows consistently stronger cooling than the western.

Since 1975 we’ve seen near-universal warming, with only 4 out of 344 climate divisions giving negative trend rates:

People sometimes wonder what the “more recent” trends are. Again, with short time spans we can’t really consider them trends as much as fluctuations, but here for your enjoyment are the linear regression trend rates from 1990 to the present:

Over this brief span there’s been cooling in the Pacific northwest and the southeast (especially Florida), but most of the country has warmed, especially New England and the upper midwest. If we limit “recent” to just the data since 2000 (very brief!) we get this:

Again, some of the estimates are very large because of the brevity of the time span (a mere 12.5 years). Some regions have shown large variation, warming in New England, and cooling in the west.

We can also create anomaly maps for any given month, showing how much each climate division deviates from its long-term mean. I computed anomalies based on the entire time span of data, so the baseline period is 1895 to 2012.5. We’ve just experienced the hottest 6-month episode on record, so here are anomaly maps for the last six months to show where the heat has been concentrated:

Hot times have moved around the country, but been strongest overall in the upper midwest/Great Lakes/New England, while cooler times have been consistent in the Pacific northwest. Although the heat was consistently extreme for the nation as a whole, the most scorchingly hot extreme wasn’t the heat wave we’re in right now (covering the latter part of June and into July), but the amazing record-smashing “summer in spring” we had during March.

Last but not least, let’s look for recurrent patterns of time changes among the climate divisions. A natural way to do so is with principal component analysis. The strongest PC (“principal component”) is represented by this combination of time series records (red indicates positive coefficients, blue negative, larger circles indicate larger coefficients):

Essentially, this PC represents (is roughly proportional to) the average temperature over the eastern half of the country. The time series pattern looks like this (black is annual averages centered on Jan 1 so the final data point has a full 12 months, red is a lowess smooth, blue is the linear regression trend line):

We can see the overall pattern of warming, with rapid and consistent rise since 1975, and monster heat during the last 12 months.

The 2nd PC represents a different pattern:

With red in the west, blue in the east, in part this represents the east-west difference (west minus east). But there’s more to it than just that, because the red (positive) coefficients are consistently larger than the blue (negative). Here’s the time series pattern:

PC3 is essentially the south-minus-north pattern:

The time series for PC3 shows that the south has cooled relative to the north, or to put it another way, the north has warmed relative to the south:

PC4 is “coasts minus great plains”:

It has this time series pattern:

Higher-order PCs represent more complex geographical patterns, for instance the interesting map of PC6:

Those of you who are familiar with “orthogonal functions” will recognize that PCs like this one are really “empirical orthogonal functions” for the geographical pattern of temperature changes.

There’s a lot of detail in the set of temperature changes by climate division, and many interesting conclusions to be drawn. Perhaps some day I’ll give this the attention it deserves, but for now I’ll just note a few things:

Overall warming, especially since 1975.

Strongest warming in New England and the upper midwest, weakest in the Pacific northwest.

March 2012 was a monster.

Expect further changes.