As the US population shifts to warmer areas, the amount of energy required to keep people warm has gradually declined over the past five decades on a per capita basis.
The map and graph illustrate the pattern. The Energy Information Administration defines annual “heating degree days” as the cumulative number of degrees that the daily average temperature falls below 65 degrees and therefore requires indoor heating to be comfortable. The lowest of course is zero and the highest number encountered in cold regions can be in excess of 8500 degree-days. A state in which the temperature fell to 55 degrees on 100 days of the year, for instance, would have an annual total of 5500 degree-days.
The map divides the states into six degree-day categories. The hottest (0-2499 degree-days) are represented in red (Texas, Florida and Arizona). The very coldest (8500+ degree-days) are represented in dark blue (North Dakota and Minnesota). In between the gradations are, from hottest to coldest, yellow, light green, dark green and light blue.
The line graph below shows the percent of the US population living in these different regions from 1960 to the present. The horizontal axis, left to right, represents that time period. The vertical axis represents the portion of the population living in each region from zero to 50%.
In 1960, 51.8% of the population lived in the temperate dark green region that includes Utah, Idaho, Washington, Nebraska and the Industrial Belt of the east. That number has since declined to 31.6%. At the same time, population in the hottest region (red) has climbed from 10.7% to 17.9% and population in the second-hottest region (yellow) has risen from 20.5% to 25.5%. Meanwhile population in the moderately warm region (light green) and in the two coldest regions (the blue) has stayed pretty much the same.
This migration from the “Rust Belt” to the South and Far West has significantly reduced the amount of energy expended on residential and commercial heating. But what about air conditioning? Hasn’t the amount of energy required for indoor cooling more than compensated for the reductions in heating?
According to the EIA, the number of cooling degree-days – days when the temperature rises above 65 degrees – has indeed increased, both in terms of national average temperatures and in the per capita count of people experiencing these higher temperatures. This is both because of the migrations and because the average global temperature has been rising, possibly because of the effects of human activity on climate. Fortunately, the increase number of cooling-degree days has not matched the decrease in heating-degree days and the overall result has been less energy per capita expended in achieving moderate indoor temperatures.