One of the main accusations made by skeptics is that the temperature record is unreliable. Besides their focus on destroying the hockey stick graph and paleoclimate reconstructions, skeptics/contrarians and denialists alike have insinuated and outright claimed that the record has been corrupted, biased outright or by default, via invalid adjustments, homogenizations, and by the drop out of stations over the years, the movement of stations, UHI effects, microsite effects, etc.
Here’s a selection of statements from the Watts and D’Aleo paper Surface Temperature Records: Policy-Driven Decepton that makes these accusations:
Instrumental temperature data for the pre-satellite era (1850-1980) have been so widely, systematically, and uni-directionally tampered with that it cannot be credibly asserted there has been any significant “global warming” in the 20th century.
Global terrestrial temperature data are compromised because more than three-quarters of the 6,000 stations that once reported are no longer being used in data trend analyses.
Due to recently increasing frequency of eschewing rural stations and favoring urban airports as the primary temperature data sources, global terrestrial temperature data bases are thus seriously flawed and can no longer be representative of both urban and rural environments. [my emphasis]
Recently, several of these claims have been put to the test, and guess what?
If you read over at WUWT very often, you will see these accusations launched repeatedly at anyone who happens to expresses support for AGW. It’s part and parcel of the repertoire of skeptics/deniers/contrarians to undermine the consensus by casting doubt over the temperature record — after all, if the temperature record isn’t reliable, either out of shoddy science or outright fraud, how can anyone claim the globe is warming?
Speaking of WUWT, there is a new essay posted by Steven Mosher, Zeke Hausfather, and Nick Stokes titled The Big Valley: Altitude Bias in GHCN that directly bears on this.
I am impressed by the efforts of these three to analyze the GHCN station drop outs in order to explore the claims made by Ross McKitrick in an earlier paper that due to a drop in altitude of stations, there was a warming of the record. Yet, at the same time, I can’t help but shake my head for all of this has been unnecessary and if the results of these and future investigations hold up, a waste of time that could have been spent on more productive projects. I directly lay the blame at the feet of the main skeptic/contrarians — McIntyre, McKitrick, Watts and their followers who have cast doubt on the temperature record through innuendo and outright claims of fraud and deceit.
Here is McKitrick’s claim that Mosher et. al. investigate:
The sample collapse in 1990 is clearly visible as a drop not only in numbers but also in altitude, implying the remote high-altitude sites tended to be lost in favour of sites in valley and coastal locations. This happened a second time in 2005. Since low-altitude sites tend to be more influenced by agriculture, urbanization and other land surface modification, the failure to maintain consistent altitude of the sample detracts from its statistical continuity. [my emphasis]
Here’s another quote from McKitrick’s paper:
The decline in sample has not been spatially uniform. GHCN has progressively lost more and more high latitude sites (e.g. towards the poles) in favour of lower-latitude sites. Other things being equal, this implies less and less data are drawn from remote, cold regions and more from inhabited, warmer regions. As shown in Figure 1-7, mean laititude declined as more stations were added during the 20th century. From Figure 1-2 we can see that this came about by the filling in of tropical regions after Europe and the US were already well-sampled. After 1980 there was a rise, a sudden, sharp drop, a sharp rise, and then a second, sharp drop in the mean latitude. This indicates that the drops in sampling were not spatially uniform, that the sample has been volatile, and in recent years it has been biased towards low latitudes. [my emphasis]
So two claims — there has been a drop off in the number of higher latitude stations used in the GHCN which has undermined the statistical continuity. Further, this means the record is biased towards warmer lower latitude stations influenced by agriculture and human activity. McKitrick argues, “Other things being equal, this implies less and less data are drawn from remote, cold regions and more from inhabited, warmer regions.”
Here are the conclusions of Mosher et. al.:
The distribution of altitude does change with time in GHCN v2.mean data. That change does not signal a march of thermometers to places with higher rates of warming. The decrease in altitude is not associated with a move toward or away from coasts. The decrease is not clearly associated with a move away mountainous regions and into valleys, but rather a movement out of mountain valley and flatland regions. Yet, mountain valleys do not warm or cool in any differential manner. Changing altitude does not bias the final trends in any appreciable way.
Regardless of the differential characteristics associated with higher elevation, changes in temperature trends is not clearly or demonstrably one of them. For now, we have no evidence whatsoever that marching thermometers up and down hills makes any contribution to a overestimation of the warming trend.
In fact, according to Mosher et. al.:
This test indicates that higher elevation stations tend to see higher rates of warming rather than lower rates of warming. Thus, dropping them, does not bias the temperature record upward. The concern lies in the other direction. If anything the evidence points to this: dropping higher altitude stations post 1990 has lead to a small underestimation of the warming trend.
So — in the end, the claims in McKitrick’s paper amount to much ado about nothing.
Roy Spencer, who appeared in The Great Global Warming Swindle” and is a member of the Heartland Institute, the George C Marshall Institute and the Interfaith Stewardship Alliance, surprised even himself when he did a “Jones style analysis of the Northern Hemisphere land area” in order to explore the claim that the drop out of stations has lead to a spurious warming trend.
He found that “…at face value, this plot seems to indicate that the rapid decrease in the number of stations included in the GHCN database in recent years has not caused a spurious warming trend in the Jones dataset — at least not since 1986″.
Surprise surprise surprise!
Tamino, over at Open Mind, has also done an analysis of the station drop out issue in response to Watts and D’Aleo’s paper Surface Temperature Records: Policy-Driven Deception and found, like Mosher et. al., and Spencer, that it makes no difference to the trend.
Here’s Watts and D’Aleo:
Perhaps one of the biggest issues with the global data is the disappearance of temperature monitoring stations from the networks after 1990. More than 6000 stations were in the NOAA data base for the mid- 1970s, but just 1500 or less are used today. NOAA claims the real-time network includes 1200 stations with 200-300 stations added after several months and included in the annual numbers. NOAA is said to be adding additional US stations now that USHCN v2 is available, which will inflate this number, but make it disproportionately U.S.
There was a major disappearance of recording stations in the late 1980s to the early 1990s. The following figure compares the number of global stations in 1900, the 1970s and 1997, showing the increase and then decrease (Peterson and Vose13).
The results will probably be no surprise, since several others have replicated my results already, with the same outcome. Here’s the temperature history (annual averages) for pre-cutoff stations only, compared to that for post-cutoff stations only:
As stated before, contrary to the claims of D’Aleo and Watts, station dropout did NOT introduce a warming trend. If anything, it introduced a cooling trend.
Three strikes, yer out.