A look at climate change across eastern Utah and western Colorado over the past 100 Years...

How Has the Climatological Average Changed over the Last 100 Years
in Eastern Utah and Western Colorado?
Joe Ramey, Forecaster and Climate Services Focal Point
NWS Grand Junction

Abstract.

Since 1911, the climate in eastern Utah and western Colorado has become warmer, especially the minimum temperatures. There is also some indication that the region has seen increased precipitation. After a cooling trend from the 1940s through the 1960s, the trend towards warmer and wetter conditions has occurred since the 1970s. These general trends in regional temperature and precipitation are matched in surrounding sites. Large decade-to-decade and site-to-site variability was noted in the temperature and precipitation data.

Introduction. 

In 1911, the Wright brothers were flying in North Carolina, and the Ford Model T had been on the road for three years. In the new towns of eastern Utah and western Colorado, weather observers were beginning to record daily, monthly, and annual temperature and precipitation data. In eleven of those towns, those climate data continue to be recorded to this day.  This study is an analysis of the trends in those data.

 
Climate average for temperature, precipitation, and other weather variables is defined as the 30 year average. Climate average is updated every decade, with the current average defined as the 30 years between 1981 and 2010. Over the last 100 years there have been 10 updates to the climate average: 1911-1940, 1921-1950, 1931-1960, 1941-1970, 1951-1980, 1961-1990, 1971-2000, and 1981-2010.

Methods.

To explore how these averages have changed with time, all sites (called Study Sites) within the forecast area with strong climate records back to 1911 were analyzed, including maximum and minimum temperatures and precipitation.  There are eleven sites within eastern Utah and western Colorado that have mostly unbroken climate records back to 1911. In eastern Utah these sites include Vernal, Moab, Blanding, and Bluff. Western Colorado sites are Steamboat Springs, Grand Junction, Crested Butte, Gunnison, Montrose, Telluride, and Silverton. 

 Map of climate study sites within the NWS Grand Junction forecast area

Study Sites within the NWS Grand Junction Forecast Area

 

 

Climate data complications.

 

With the exception of Grand Junction and Montrose, all sites have some missing data. For example Silverton had significant missing data between 1977 and 1983. Telluride’s long and strong climate record ended in 2008. Blanding’s climate record started in late 1912.

 
There are other potential climate problems with these sites. Many or all of the sites have changed location through the years. Most sites have had significant changes to the instrumentation used, for instance from mercurial thermometers to thermocouples on automated towers. Many have had land use changes through the years, including urbanization in Grand Junction and Montrose. For instance, the Grand Junction observation site moved from downtown to the airport in 1946, a distance of 5.5 miles and approximately 230 feet higher in elevation.
 

There are climate sites in the forecast area that have minimized these potential problems, those sites located in National Parks and Monuments (Park Sites). On these protected lands, site, land use, and instrument changes have been minimized.  A disadvantage of these climate sites is that most of their observations began much later than 1911. There are nine Park Sites in the forecast area that extend back into the 1960s: Flaming Gorge NRA, Ouray NWR, Dinosaur NM, Colorado NM, Canyonlands NP The Neck and Needles, Natural Bridges NM, Hovenweep NM, and Mesa Verde NP. Arches NP climate data began in the 1980s so was not used.

 

Map showing National Parks and Monuments Sites

National Parks and Monuments Sites

 

One final climate data set was gathered, that is sites surrounding the forecast area (Surrounding Sites). Surrounding Sites and Study Sites can be compared to see if any local trends also occur within the wider geographic region. Ten Surrounding Sites were chosen: to the north Lander WY; to the east Ft Collins, Denver, and Rocky Ford CO; to the south Taos, Albuquerque, and Las Cruces NM (author’s previous hometown); to the southwest Prescott AZ; and to the northwest Salt Lake City, and Heber City UT.  

 

Map showing surrounding sites used in this climate study
Surrounding Sites 

 

These three data sets, Study, Park, and Surrounding Sites, were individually averaged and plotted together to compare climate trends over the last 100 years. The data sets were analyzed in 10 year and in 30 year periods, the latter shown here to examine how the climate average has changed. Note, the Park and Surrounding Sites have lower average elevations compared to the Study Sites. This shows in their offset towards warmer and drier conditions in the graphs.

 

Graph showing Maximum temperature increase over time

Map showing Minimum temperature changes over time

Graph showing change in precipitation over time at the study sites


Results.

Comparing the trends, the three data sets match very well. In both the Study and Surrounding Sites, there was a warming and drying trend from 1911 into the 1930s, then a cooling and wet trend from the 1940s into the 1970s, then a sharp warming trend to 2010. Since the 1970s, precipitation has generally increased, though 2001-2010 was a dry decade for all three data sets. 

1.      Comparing the Study Sites and the Parks and Monument data sets for the climate periods 1961-1990 through 1981-2010. These data sets show very similar temperature trends through the 30 year period. Both maximum temperatures have risen 0.4 degrees F and the minimum temperatures have risen 1.1 F. The exact same rate of change seen in these two data sets indicate that any site change problems with the Study Sites are not significant, at least at this resolution. Precipitation rates have also increased during this time frame, by a small 0.08 inch in the Parks Site’s data, by 0.59 inch in the Study Site’s data.

2.      Comparing the Study Sites to the Surrounding Sites data sets from 1911-1940 through 1981-2010. Both data sets indicate increasing temperatures with the largest rate of increase in the minimum temperatures, and both data sets show the rate of warming increasing since the 1961-1990 climate average. For maximum temperatures the Study Site increased 0.7 degree F while the Surrounding Sites increased 1.6 F. For minimum temperatures the Study Sites increased 1.5 F, the Surrounding Sites 2.4 F.
 
Precipitation data trends are more complicated.  Over the 100 years, the Study Sites averaged an increase of 0.45 inch, the Surrounding Sites have decreased precipitation by 0.33 inch mainly due to the dry period 2001-2010. It is important to note that the Surrounding Site data set are biased to the south, and the desert Southwest has suffered strong drought conditions over the last decade. 

Discussion.

Site Variability. The individual climate sites showed significant variation. Some notable variations: Crested Butte and Mesa Verde National Park (Mesa Verde’s climate record began in 1922) showed cooling in both maximum and minimum temperatures through their climate records.  Grand Junction and Colorado National Monument showed more warming in their maximum temperature than the minimums.

Temperature data from all three data sets Study, Park, and Surrounding Sites, show a strong correlation. All three data sets show an increasing warming trend over the last 30 years, strongest in the minimum temperatures. This fits well with NOAA’s national analysis of the current climate normal warming trend and also fits well with the climate model projections.

Precipitation has generally been on the increase since the 1970s for the Study Sites. This is the same time frame when temperature rate of change has also increased. The saw-tooth appearance (that is much more amplified in the 10 year plots) of the data is an indicator of the large year-to-year and decade-to-decade variation of precipitation for our region.
 
For the first 60 years of this study, the precipitation was inversely correlated to temperature. That is dry periods (non-stormy) are warm, and wet periods (stormy) are cool. This matches local forecaster observations of temperature and precipitation correlation on the daily to seasonal time scale. But since the 1970s, this correlation has switched from indirect to direct. That is, as temperatures have warmed, precipitation has been increasing. 
 
This study begs the question, what about the future? As of the end of May 2013, we are nearly a quarter of the way into the next climate normal period of 1991-2020. What are the temperature trends in these 2.5 years? Analyzing the 11 Study Sites and three of the Park Sites (Dinosaur NM, Colorado NM, Mesa Verde NP) show warmer than average temperatures. This is in spite of a very cold winter of 2012-13 and a cool spring of 2013. This period has been much drier than normal.
 
And what do climate models say about the coming years and decades? Climate models do very well with projections on the global scale but have less skill at the regional level. Here is the latest regional climate outlook from NOAA and other federal agencies.

Acknowledgments.

Thanks to co-workers Julie Malingowski and Aldis Strautins for map building, and Jeff Colton for web formatting and patiently listening to my analysis over several weeks. 



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