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 NOAA Climate Services 

July 6, 2009 

Talking Points related to concerns about whether the U.S. temperature record is reliable

Q. Over the course of time have U.S. weather stations been exposed to local environmental 

conditions that could unduly influence temperature readings e.g., located close to growing 

trees, buildings, parking lots, etc.? 

A. Yes. That is one reason why NOAA created the Climate Reference Network. These stations 

adhere to all of the Global Climate Monitoring Principles and are located are located in areas free 

local human influences and have excellent site location characteristics. They are closely 

monitored and are subject to rigorous calibration procedures. It is a network designed 

specifically for assessing climate change. http://www.ncdc.noaa.gov/oa/climate/uscrn/. 

Additionally, an effort is underway to modernize the Historical Climatology Network (a network 

of over 1000 long-term weather and climate stations), though funds are currently available only 

to modernize and maintain stations in the Southwest. Managers of both of these networks work 

diligently to locate stations in pristine areas where the site characteristics are unlikely to change 

very much over the coming decades. 

Q. How has the poor exposure biased local temperatures trends? 

A. A peer-reviewed study specifically quantified the potential bias in trends caused by poor 

station exposure (Peterson, 2006). The analysis examined only a small subset of stations – all 

that had their exposure checked at that time – and found no bias in long-term trends. 

Q. Does a station with good exposure read warmer than a station with poor exposure? 
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Not necessarily. Many local factors influence the observed temperature: whether a station is in a 

valley with cold air drainage, whether the station is a liquid-in-glass thermometer in a standard 

wooden shelter or an electronic thermometer in the new smaller and more open plastic shelters, 

whether the station reads and resets its maximum and minimum thermometers in the coolest time 

of the day in early morning or in the warmest time of the day in the afternoon, etc. But for 

detecting climate change, the concern is not the absolute temperature – whether a station is 

reading warmer or cooler than a nearby station over grass – but how that temperature changes 

over time. 

Q. Over the years, stations move in location for a variety of reasons and the local environment 

changes. If the local environment around the station changes could this cause a bias in the 

temperature record? Can that bias be adjusted out of the record? 

A. A great deal of work has gone into efforts to account for a wide variety of biases in the 

climate record, both in NOAA and at sister agencies around the world. Since the 1980s, 

scientists at NOAA’s National Climatic Data Center have been at the forefront of this effort 

developing techniques to detect and quantify biases in station time series. When a bias 

associated with any change is detected, it is removed so that the time series is homogeneous with 

respect to its current instrumentation and exposure. The latest peer-reviewed paper which 

provides an overview the sources of bias and their removal (Menne et al., 2009 in press), 

including urbanization and nonstandard exposures. They evaluated urban bias and found that 

once the data were fully adjusted the most urban stations had about the same trend as the 

remaining more rural stations. 

Q. What can we say about poor station exposure and its impact on national temperature 

trends? 

A. Surfacestations.org has examined about 70% of the 1221 stations in NOAA’s Historical 

Climatology Network (USHCN) (Watts, 2009). According to their web site of early June 2009, 

they classified 70 USHCN version 2 stations as good or best (class 1 or 2). The criteria used to 

make that classification is based on NOAA’s Climate Reference Network Site Handbook so the 

criteria are clear. But, as many different individuals participated in the site evaluations, with 

varying levels of expertise, the degree of standardization and reproducibility of this process is 

unknown. However, at the present time this is the only large scale site evaluation information 

available so we conducted a preliminary analysis. 

Two national time series were made using the same homogeneity adjusted data set and the same 

gridding and area averaging technique used by NOAA’s National Climatic Data Center for its 
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annual climate monitoring. One analysis was for the full USHCN version 2 data set. The other 

used only USHCN version 2 data from the 70 stations that surfacestations.org classified as good 

or best. We would expect some differences simply due to the different area covered: the 70 

stations only covered 43% of the country with no stations in, for example, New Mexico, Kansas, 

Nebraska, Iowa, Illinois, Ohio, West Virginia, Kentucky, Tennessee or North Carolina. Yet the 

two time series, shown below as both annual data and smooth data, are remarkably similar. 

Clearly there is no indication from this analysis that poor station exposure has imparted a bias in 

the U.S. temperature trends. 

Q. Is there any question that surface temperatures in the United States have been rising 

rapidly during the last 50 years? 

A. None at all. Even if NOAA did not have weather observing stations across the length and 

breadth of the United States the impacts of the warming are unmistakable. For example, lake and 

river ice is melting earlier in the spring and forming later in the fall. Plants are blooming earlier 
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in the spring. Mountain glaciers are melting. Coastal temperatures are rising. And a multitude of 

species of birds, fish, mammals and plants are extending their ranges northward and, in 

mountainous areas, upward as well. 

References 

Menne, Matthew J., Claude N. Williams, Jr. and Russell S. Vose, 2009: The United States 

Historical Climatology Network Monthly Temperature Data – Version 2. Bulletin of the 

American Meteorological Society, in press. 

Peterson, Thomas C., 2006: Examination of Potential Biases in Air Temperature Caused by Poor 

Station Locations. Bulletin of the American Meteorological Society, 87, 1073-1080. 

Watts, Anthony, 2009: Is the U.S. Surface Temperature Record Reliable? The Heartland 

Institute, Chicago. 29 pp.