Evaluating the impact of U.S. Historical Climatology Network homogenization using the U.S. Climate Reference Network

Hausfather, Z., K. Cowtan, M. J. Menne, and C. N. Williams Jr., 2016: Evaluating the impact of U.S. Historical Climatology Network homogenization using the U.S. Climate Reference Network. Geophys. Res. Lett., 43, 1695–1701, doi: 10.1002/2015GL067640.

Abstract. Numerous inhomogeneities including station moves, instrument changes, and time of observation changes in the U.S. Historical Climatological Network (USHCN) complicate the assessment of long-term temperature trends. Detection and correction of inhomogeneities in raw temperature records have been undertaken by NOAA and other groups using automated pairwise neighbor comparison approaches, but these have proven controversial due to the large trend impact of homogenization in the United States. The new U.S. Climate Reference Network (USCRN) provides a homogenous set of surface temperature observations that can serve as an effective empirical test of adjustments to raw USHCN stations. By comparing nearby pairs of USHCN and USCRN stations, we find that adjustments make both trends and monthly anomalies from USHCN stations much more similar to those of neighboring USCRN stations for the period from 2004 to 2015 when the networks overlap. These results improve our confidence in the reliability of homogenized surface temperature records.

Synthesis

  1. By Ed I. Tor One and Ed I. Tor Two

    Reviewer 3 seems to have strong priors on how the data should look like, but failed to provide evidence for his/her claims. The grades of assessment 3 are thus ignored.

    Assessments 1 and 2 seem to mostly agree in their assessments of the quality of the work. They disagree on the newness of the study. We would argue that the study is more than a replication study. While the results confirm what we already know, the methods to arrive at this conclusion are new and elegantly use the new state-of-the-art US climate reference network measurements.

    This synthesis is still somewhat preliminary. Reviewer 3 was asked for his/her knowledge of the US network an additional assessor with this expertise would be welcome.

    Impact on the larger scientific community. [70]
    Contribution to the scientific field of the journal. [80]
    The technical quality of the paper. [90]
    Importance at the time of publishing. [-]
    Importance of the research program. [-]

Reviews

  1. The study provides a validation of the relative homogenisation method (PHA) used by NOAA to generate the US Historical Climate Network (USHCN). It compares the USHCN data before and after homogenisation with the data of the US Climate Reference Network (USCRN). The USCRN was designed to be highly stable for the study of climate change.

    The main result is that homogenisation makes the temperature data of the USHCN stations more similar to the USCRN stations. An interesting side note is that there are remaining small, but statistically significant differences in the maximum temperature; the USCRN shows more warming. Homogenisation only partially removed the biases.

    The main limitation of the study was that they could only use a relatively short period (2005-2015), which reduces the sensitivity of the analysis.

    References
    Williams, C. N., M. J. Menne, and P. W. Thorne, 2012: Benchmarking the performance of pairwise homogenization of surface temperatures in the United States. J. Geophys. Res., 117, D05116, doi: 10.1029/2011JD016761.

    Impact on the larger scientific community. [70]
    The finding that relative homogenisation improves temperature station data, but only partially removes trend errors, fits to the validation study of Williams et al. (2012) and strengthens these conclusions.

    Contribution to the scientific field of the journal. [80]
    It is an important paper because it provides a validation of relative homogenisation based on observed data. This strengthens our trust in homogenisation methods by providing evidence independent of benchmarking with simulated data based on our current best understanding of inhomogeneities.

    The technical quality of the paper. [90]
    No problems found. Extensive Supplemental Information. Analysis code published. Exemplary.

    Importance at the time of publishing. [-]
    Not relevant. New paper.

    Importance of the research program. [-]
    Not relevant. Single paper.

  2. Example review by Monk Mockton

    The authors study the important question of the reliability of warming estimates by comparing the temperature anomalies of the US Climate Reference Network with warming estimates from the default US climate station network. The authors find that the homogenisation of the data of the default network makes the warming estimates more similar to those of the reference network at a station level. This is evidence that homogenisation improves warming estimates in dense networks.

    Impact on the larger scientific community. [60]
    The climatological community already expected homogenisation to improve the quality of trend estimates.

    Contribution to the scientific field of the journal. [70]
    A well executed study, but it is mostly a replication study and confirms what we already know.

    The technical quality of the paper. [90]
    I agree with reviewer 1 that the technical quality of the study is an example for the community with extensive Supplemental Information and published analysis code.

    Importance at the time of publishing. [-]
    Not relevant. New paper.

    Importance of the research program. [-]
    Not relevant. Single paper.

  3. Example review by anonymous

    The conclusions are not robust because we do not have photos of all the COOP stations involved. The results must be wrong because the USHCN should have shown more warming due to urbanization and homogenization should thus have reduced the warming estimates.

    Impact on the larger scientific community. [10]
    This study distracts the scientific community from the central problem of urbanization.

    Contribution to the scientific field of the journal. [10]
    The conclusions must be wrong. The authors should have studied why the influence of urbanization on the USHCN is not found.

    The technical quality of the paper. [20]
    Figures are well made and language okay, but the analysis must have an error.

    Importance at the time of publishing. [-]
    Not relevant. New paper.

    Importance of the research program. [-]
    Not relevant. Single paper.

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