Reducing uncertainty in the instrumental climate record

Instrumental climate records are available for thousands of weather stations in the United States. As a collective, these records constitute the most complete picture of climate for the nation during the 20th century and thus have been applied in numerous assessments of climate change. Despite their popularity, however, instrumental records are "contaminated" by biases resulting from historical changes in observing practice. Consequently, their reliability from a climate change perspective is the subject of debate.; This investigation assesses the utility of instrumental records in addressing climate change issues in the United States. The assessment is accomplished by addressing two complimentary objectives. The first objective is to determine the validity of adjustments that account for contaminants that bias 20 th century temperature trends. The second objective is to determine the usefulness of instrumental records in identifying critical locations for detecting future trends in temperature and precipitation.; In general, the results of this investigation indicate that the adjustments for historical changes in observing practice are robust at the national scale. For instance, no significant problems are evident in the correction for the time of observation bias, the largest such error from a trend perspective. Similarly, there are no major deficiencies in the adjustment for historical changes in station location, instrumentation, and observers. In other words, when properly treated for historical contaminants, instrumental records result in a reasonable depiction of temperature variations during the 20th century.; Adjusted instrumental records also provide guidance in locational analysis for climate monitoring at the national scale. For example, various performance measures indicate that at least 25 stations are needed to capture interannual variations in temperature. In contrast, 135 stations are required if trend detection is a high priority, particularly for precipitation. Several analyses indicate that networks of 135 or more stations, distributed in a quasi-uniform fashion, accurately replicate the temperature and precipitation trends of the 20th century.; In summary, this investigation establishes the efficacy and utility of instrumental records in contemporary climate change applications. Consequently, future climate assessments will have as their foundation a more credible account of the nation's climate during both the 20th and 21 st centuries.