A Hot Topic: Graphing Temperature Trends

Statistically, it's growing warmer.

April 9, 2019
Field with Hay Bales
Pixabay

Recently, as I was working with a spreadsheet that has all of my weather forecasts for The Old Farmer’s Almanac for the 23 years that I have been making them, it occurred to me that here was a database that contains a great deal of information that might be useful in looking at trends.

To do this, I moved the data around within the spreadsheet to be able to create graphs showing how temperatures have changed over the past 23 years. These graphs show some interesting trends that seem to fit, at least in a general sense, with the idea that it’s growing warmer. 

Above-Normal Temperature “Normals”

Let me explain these graphs. As you may realize, each year we forecast the coming year’s weather for 18 United States regions and seven Canadian ones. The maps of these regions that appear in The Old Farmer’s Almanac print editions and on Almanac.com show several cities within each; it is these cities for which I have compiled data on actual temperatures and how they have departed from normal. 

For example, when I say that the temperature departure for December 2018 was 2.61 degrees F (see Figure 1, below), this means that the average of all of the cities shown on all of these regional maps for the United States and Canada was 2.61 degrees F warmer than their most recent official 30-year normals, which cover the period of 1981–2010. The “normal temperatures” that I use are the official ones updated once each decade by the United States and Canadian government meteorological services, based upon smoothed averages of daily temperatures for the 30-year period covered.

Figure 1: Monthly temperature departures from normal, 1996–2018
Figure 1. Monthly temperature departures from normal, 1996–2018.

Thus, if I say that the 2019 average temperature is 1 degree above normal, this temperature is going to be different from what it would be if I were referring to the 2009 average temperature being 1 degree above normal, because over the ensuing decade the “normal” temperatures would have been recalculated and changed. 

Since temperatures generally have been rising, this means that each decade, when the new “normal” temperatures are computed, they are higher than the previous “normal” temperatures. The effect of this is that a graph that shows how temperatures have departed from normal over the years actually understates the real increase in temperatures, since the normals are rising along with the actual temperatures.

When we examine the graph showing the monthly departure of temperature from normal (Figure 1, above), we see that most—but not all—months in the past 23 years have had temperatures above normal. However, there does not seem to be any clear trend in these temperature departures, or at least not one that I can readily see. The greatest positive departure was 8.67 degrees F back in January 2006, while the greatest negative departure was 4.78 degrees F in December 2000.

12-month_moving_average_departures_from_normal_large_full_width.jpg
Figure 2. 12-month moving average of monthly temperature departures from normal, 1996–2018.

When we look at the 12-month moving average of these departures (Figure 2, above), their pattern becomes more apparent, as there does seem to be a general rising trend in the temperature departures, with nearly all above normal. The only times when this 12-month moving average of temperature departures for the United States and Canada were below normal were in 2001 and 2014.

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Figure 3. 36-month moving average of monthly temperature departures from normal, 1996–2018.

Examining the 36-month moving average of temperature departures (Figure 3, above) seems to show a trend emerging. Although there are still peaks and valleys, a general upward trend seems more apparent, with each peak higher than the previous one.

What Causes these Changes?

Meteorologists and climatologists have proposed various explanations for the month-to-month and annual variations, including teleconnections such as El Niño, changes in ocean currents, and changes in solar output. 

As for the clear trend of rising temperatures over the past decades, I have seen only three viable proposed explanations: (1) natural variability (perhaps combined with changes in land use and population), (2) an expected general rise in average temperatures as we are still coming out of the last ice age, and (3) warming temperatures due to human activities that increase the levels of carbon dioxide and other atmospheric greenhouse gases.

Find predicted temperatures for the upcoming year in The 2019 Old Farmer’s Almanac.

About This Blog

Mike Steinberg is Senior Vice President for Special Initiatives at AccuWeather Inc in State College, Pennsylvania. He is also a member of the National Weather Association and the Canadian Meteorological and Oceanographic Society.

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