[1] This paper documents various unresolved issues in using surface temperature trends as a metric for assessing global and regional climate change. A series of examples ranging from errors caused by temperature measurements at a monitoring station to the undocumented biases in the regionally and globally averaged time series are provided. The issues are poorly understood or documented and relate to micrometeorological impacts due to warm bias in nighttime minimum temperatures, poor siting of the instrumentation, effect of winds as well as surface atmospheric water vapor content on temperature trends, the quantification of uncertainties in the homogenization of surface temperature data, and the influence of land use/land cover (LULC) change on surface temperature trends. Because of the issues presented in this paper related to the analysis of multidecadal surface temperature we recommend that greater, more complete documentation and quantification of these issues be required for all observation stations that are intended to be used in such assessments. This is necessary for confidence in the actual observations of surface temperature variability and long-term trends.Citation: Pielke, R. A., Sr., et al. (2007), Unresolved issues with the assessment of multidecadal global land surface temperature trends,

The use of temperature data from poorly sited stations can lead to a false sense of confidence in the robustness of multidecadal surface air temperature trend assessments.

Although climate change and variability involves all aspects of the climate system [Pielke, 1998],the assessment of anthropogenically‐forced climate change has focused on surface temperature as the primary metric [Mann and Jones, 2003; Soon et al., 2004]. Our contribution only addresses this very specific (and limited) metric of the climate system. The term “global warming” has been used to describe the observed surface air temperature increase in the 20th century. However, this concept of “global warming” requires assessments of units of heat (that is, Joules). Temperature, by itself, is an incomplete characterization of surface air heat content.

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New photographic metadata of eastern Colorado stations are examined, raising questions about the usefulness of current surface datasets for climate applications. The U.S. Historical Climate Network (USHCN; Karl et al. 1990) consists of a subset of stations from the National Weather Service (NWS) cooperative (COOP) station network. The USHCN stations are used in the construction of homogeneous climate data reference series and in the detection and monitoring of long-term climatic trends. Stations are selected for the USHCN dataset based on the length of period of record, the percentage of missing data in the station's record, the total number of station moves and other station changes that may affect the homogeneity of the site's data, and, finally, how the By themselves, these criteria, however, are not adequate in addressing specific station characteristics such as how well the instrument siting adheres to internationally accepted standards of exposures (described fully in WMO 1996). In particular, it is important to know the site of stations relative to various structures and surfaces. Generally, near-surface air temperature observations should be representative of the free-air conditions over as much of the vicinity as possible, at a height approximately 1.5 m above the ground. The site should be level, without locally significant topographical variations or steep slopes or hollows, and should offer free exposure to both sunshine and wind (not to close to trees, buildings, or other obstructions; WMO 1996).It thus becomes critical to conclusively determine how much of any potential regional change in observed air temperatures might be due to land-use changes at the site itself. In the late spring and early summer of 2002, we visited 57 of the temperature-measuring COOP sites of the plains of eastern Colorado, giving particular emphasis to the USHCN sites (10 total). As a group, these sites revealed a wide variety of site exposure characteristics. Many stations were located at the observer's residences; the temperature sensors, of which the vast majority are now electronic Maximum Minimum Temperature Sensor (MMTS) devices (see Fig. 1), are often unsatisfactorily close (within 2-3 m) to buildings (Fig. 2). This siting practice reflects an effort to minimize the costs of providing electrical power to the sensor, while at the same time providing a measure of open ventilation to the sensor.In another common situation, the temperature sensor is in a relatively open location, but the surface under and around the sensor is a patchwork of different land-cover types. Frequently, lawn, asphalt, gravel, bare dirt, and concrete were all in relatively close proximity to each other. Sites that met all the WMO site exposure requirements (e.g., Fig. 3) were in the minority. I BAf15-APRIL 2005The poorest exposures usually had excessive vegetation around the temperature sensor (Fig. 4) or were locally urbanized and thus not representative of the immediately surrounding region (Fig. 5).The majority of the USHCN stations we visit...