Abnormal Climate Normals

I've never been a fan of the phrase "climate normals" to describe averages of past climate.  It perpetuates the myth that climate is stationary and it suggests that an average is normal when weather normally fluctuates in a range.  I grimace whenever I hear someone say "normally the high today is 64˚F."  

I always wondered who started using this phrase and why.  I don't know the answer to that question, but according to the National Centers for Environmental Information (NCEI), about 100 years ago the World Meteorological Organization (WMO) instructed member nations to calculate climate normals using 30 year periods, beginning with 1901-1930.  

So, near as I can tell, we call them climate normals and we average over a 30-year period because we've always done it that way.  That's simply not a good reason, especially now that we are living in a period of accelerating climate change and we know that for some regions, a 30-year average is likely insufficient for some variables like precipitation, which exhibit slowly evolving decadal or multidecadal variations.  

This issue has come to the forefront this week because NCEI issued the new 1991-2020 climate normals.  Below is a series of maps produced by Jared Rennie of the North Carolina Institute for Climate Studies showing how the 30-year temperature normals have evolved compared to the 20th century average.  The warming trend is apparent.   

Source: https://www.noaa.gov/news/new-us-climate-normals-are-here-what-do-they-tell-us-about-climate-change

Precipitation is more complicated, however, as there's more regional variability and variations that occur over decades.  For example, 1941-1970 was relatively dry over the southwest, 1971-2000 relatively wet, and 1991-2020 relatively dry again.  

Source: https://www.noaa.gov/news/new-us-climate-normals-are-here-what-do-they-tell-us-about-climate-change

For the southwest United States, annually averaged temperatures have increased at a rate of approximately 0.7˚F per decade since 1975.  Thus, the 1990-2010 temperature normals are probably already about 1˚F lower than they should be.  They reflect the past climate average, not the current climate average.  

Source: NCEI

For precipitation, the situation is more complicated.  Instead of there being a clear long-term trend, precipitation in the southwest has exhibited wet and dry periods, with the latter predominating in the past 20-25 years.  

Although there is strong evidence that drought will become more frequent, prolonged, and severe over the southwest US during the 21st century, it is less clear what long-term trends in annual precipitation will be (drought is affected not only by precipitation amount over long periods of time, but also temperature, precipitation type, precipitation intensity and other factors).  Thus, we should be cautious interpreting the recent spate of low precipitation years as a long term trend.  

The climate system is changing and while knowledge of the past is helpful, it is increasingly important that those of us in Utah recognize that these climate models are averages of the past climate and that the future will be hotter and drier, even if the average annual precipitation doesn't change that much.