Proponents of global warming alarmism place heavy reliance on the temperature record of the United States, probably the world’s best data set. They say that the record shows significant warming in the 20th century. Most global warming realists have accepted this claim, but have questioned whether this particular temperature trend–the Earth has been getting either warmer or cooler, sometimes on a far greater scale, for millions of years–has much to do with human activity.

What is less well known is that the alarmists do not rely on raw temperature data collected by American weather stations. Rather, the alarmists adjust the data before they publish it. This chart from NASA GISS, which you likely have seen before, purports to show the temperature anomaly as measured on land in the U.S. from 1880 to approximately the present:

But that graph, like virtually every discussion of American temperature trends that you have seen, uses “adjusted” temperature data, not what was actually reported from weather stations. This chart reflects raw data from the U. S. Historical Climatology Network, the same data that were the basis–pre-“adjustment”–for the chart above:

Michael Hammer, who created this chart from the raw data, writes:

Clearly the shape of this graph bears no similarity at all to the graph shown in Figure 4 [the first graph above]. The graph does not even remotely correlate to the shape of the CO2 versus time graph. The warming was greatest in the 1930’s before CO2 started to rise rapidly. The rate of rise in 1920, the early 1930’s and the early 1950’s is significantly greater than anything in the last 30 years. Despite the rapid rise in CO2 since 1960, the 1970’s to early 1980’s was the time of the global cooling scare and looking at the graph in Figure 5 one can see why (almost 2F cooling over 50 years). It would appear that the temperature rise profile claimed by the adjusted data is largely if not entirely an artefact arising from the adjustments applied (as shown in Figure 3), not from the experimental data record. In fact, the raw data does not in any way support the AGW theory.

So, what exactly are the adjustments that are made to the raw data by NOAA before they are published? Historically, five adjustments have been made; the only one that tended to reduce temperatures apparently has been eliminated:

It is obvious that the only adjustment which reduces the reported warming is UHI which is a linear correction of 0.1F or about 0.06C per century, Figure 2. Note also that the latest indications are that even this minimal UHI adjustment has now been removed in the latest round of revisions to the historical record. To put this in perspective, in my previous article on this site I presented bureau of meteorology data which shows that the UHI impact for Melbourne Australia was 1.5C over the last 40 years equivalent to 3.75C per century and highly non linear. Compare the treatment of UHI with the adjustments made for measuring stations that have moved out of the city centre, typically to the airport. These show lower temperatures at their new location and the later readings have been adjusted upwards so as to match the earlier readings. The airport readings are lower because the station has moved away from the city UHI. Raising the airport readings, while not adding downwards compensation for UHI, results in an overstatement of the amount of warming. This would seem to be clear evidence of bias. It would be more accurate to lower the earlier city readings to match the airport readings rather than vice versa.

This is really a scandal. Not only does NOAA not correct for the well-recognized urban heat island effect, as I think it obviously should, it goes out of its way to re-introduce the heat island effect where better data are available!

Anthony Watts reports the discovery of more corruption of the historical record by global warming alarmists. The National Climate Data Center has been altering historical records to make past temperatures cooler than were reported at the time. This is being done so as to create an artificial warming trend over the 20th century:

Jeff Masters and [Christopher C. Burt] recently received an interesting email from Ken Towe who has been researching the NCDC historical temperature database and came across what appeared to be some startling inconsistencies. Namely that the average state temperature records used in the current trends analysis by the NCDC (National Climate Data Center) do not reflect the actual published records of such as they appeared in the Monthly Weather Reviews and Climatological Data Summaries of years past. Here is why. Here is a typical example of what Ken uncovered. Below is a copy of the national weather data summary for February 1934. If we look at, say Arizona, for the month we see that the state average temperature for that month was 52.0°F. [Ed.: This is the paper version that was published at the time.] However, if we look at the current NCDC temperature analysis (which runs from 1895-present) we see that for Arizona in February 1934 they have a state average of 48.9°F, not the 52.0°F that was originally published.

So the actual temperatures reported in the 1930s are being reduced, to make it appear as though global warming has been going on since then. A reduction of more than three degrees Fahrenheit is, of course, huge.

These disclosures highlight a key fact with respect to global temperature data: the data sets are utterly lacking in integrity. Global warming alarmists confidently announce that worldwide temperatures have risen by, say .1 degree over a decade. It would be extraordinarily difficult to take measurements at many locations around the globe that would actually demonstrate that proposition. But the real situation is much worse: no one tells you what temperatures were actually measured at the world’s weather stations. Rather, they report claims of global warming based on “adjusted” temperature data–adjusted by alarmists, with the systematic purpose of manufacturing a rising temperature trend. If you subtract the “adjustments,” it may well be that there has been no net warming over the last 100 years at all.

I’ve said it before; here it is one more time: global warming hysteria is not science. It is an unholy combination of religion and politics.