Population shifts in the United States, together with milder temperatures have resulted in an average annual decline in heating degree days of 0.4 percent between 1960 and 2010, according to the US Energy Information Administration.
The heating degree day (HDD) is a measure used for analyzing the relationship between temperatures and energy consumption.
Since the nation’s founding, the population of the United States has migrated westward. The advent of air conditioning nearly a century ago has contributed to population shifts to southern states as well, especially in the second half of the 20th century. This regional shifting of the population has affected the need for heating and cooling.
In 1960, 31.2 percent of the population lived in the warmest states, where annual HDD averaged less than 4,000, as indicated by the red and yellow lines in the figure. By 2010, this share of the population rose to 43.4 percent. In contrast, the share of the population living in cooler states, with HDD ranging from 4,000 to 6,999, declined from 59.7 percent of the population in 1960 to 48.3 percent in 2010, as indicated by the two green lines in the figure.
The share of the US population living in the coldest regions, where HDD average more than 7,000, has also declined slightly from 9.1 percent in 1960 to 8.2 percent in 2010, as shown by the two blue lines in the figure.
If heating degree days are dynamically weighted by each state’s annual population, instead of the population for a fixed year, HDD shows an average annual decline of 0.4 percent between 1960 and 2010.
This shift in the population toward warmer climates partially explains some of the flattening in per-capita residential energy consumption over the past few decades. Space heating accounts for more energy consumption than space cooling, so the decreased need for space heating as a result of population shifts to warmer areas of the country is only partially offset by increased energy consumed for space cooling.
In addition, observed winter temperatures have become milder over the past few decades. Estimates of average HDD based on static 2000 Census populations have declined an average of about 0.2 percent per year between 1960 and 2010.