I have seen a lot of comments over the past couple of years in regards to the U.S. temperature record and I want to go over this subject once again. I recall blogging about this a few years ago.
Most of the comments state that there is a warm bias, mainly due to advancing urbanization and poor siting of weather stations.
We also hear that the massaged data is purposely adjusted upward to show the warming trend.
Keep in mind, the area of the United States only takes up a very small percentage of the entire Earth, so even if the warm bias was true, it would have little if any impact on the global temperature trends.
So what does the National Climatic Data Center (NCDC) have to say about this in their U.S. temperature monitoring section?
1. There is no evidence that the U.S> temperature trend is inflated by poor siting of stations that are part of the U.S. Historical Climatology Network (USHCN). Over a thousand stations make up the USHCN.
2. There is indeed a bias associated with unadjusted data from poor exposure sites relative to data from good exposure sites. However, this bias is consistent with previous studies documenting the impact of the widespread conversion to electronic sensors during the last 25 years because the majority of poor exposure sites were subject to this instrument change.
3. These instrument changes actually led to an artificial negative (cool) bias in maximum temperatures and only a slight positive (warm) bias in minimum temperatures.
Overall, the temperature adjustments leave a small, overall residual negative (cool) bias in the adjusted U.S. average maximum temperature, according to the NCDC.
Change from afternoon to morning......
The greatest bias in the U.S. temperature record occurred with the systematic change in observing times from afternoon, when it is typically warm, to morning (cooler). This shift led to an increasing cool bias over the last several decades, which of course needed a major correction to be applied, according to the NCDC.
Rural stations versus urban.....
The NCDC identified Global Historical Climatology Network (GHCN) stations were rural and which were urban. Then they created global temperature time series from the rural only stations and compared that to our full dataset. The result was that the two showed almost identical time series (actually the rural showed a little bit more warming) so there apparently was no lingering urban heat island bias in the adjusted GHCN dataset.
Globally adjusted temperature anomalies versus raw.......
The top image below is the adjusted version of the GHCN and the bottom is the raw (unadjusted) version of the GHCN. Both clearly show a long-term, global warming trend and throws out the argument that the observed warming is due to the adjustment process.
Adjusted. Images courtesy of the NCDC.
Again, you can read the entire NCDC FAQ section on this interesting subject right here.
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