Have you ever wondered, when you see an assertion along the lines of “The Earth has warmed by 1.62 degrees over the last 100 years,” how anyone could know that? The literature of global warming alarmism is littered with faux precision; the truth, as you might imagine, is that it is very difficult to get reliable data for the whole Earth over a period of decades if not centuries.

Climate realists are generally willing to assume, for the sake of argument, that the Earth has warmed somewhat in recent decades. In fact, though, it is not obvious that even this modest claim is true. Satellite data show no net warming for as long as such data have been collected, i.e., back to 1979. Ocean measurements show no net warming over that period, either; the evidence for warming is based on land measurements. But the accuracy of land measurements depends on proper siting and maintenance of weather stations. One obvious factor is the urban heat island effect: many weather stations are located in cities, which grow warmer as more people and buildings accumulate. Thus, increasing temperatures at such stations may be measuring urban development rather than the climate. We all know that the urban heat island effect is real–“chance of frost in outlying areas”–yet the data that alarmists rely upon do not take it into account.

Apart from that broad issue, the siting and maintenance of weather stations is crucial to the accuracy of the data they generate. This was nicely illustrated in a recent post on Watts Up With That:

Governor Jerry “moonbeam” Brown has created a comical clone of the Skeptical Science website at his state office website, and announced it today in lake Tahoe with this missive via the Sacramento Bee’s Capitol Alert: “Global warming’s impact on Lake Tahoe is well documented. It is just one example of how, after decades of pumping greenhouse gases into the atmosphere, humanity is getting dangerously close to the point of no return,” Brown said in a prepared statement. “Those who still deny global warming’s existence should wake up and honestly face the facts.” Well documented? I suppose Gov. Brown has never seen the kind of problems associated with the official NOAA weather stations….

The author notes that the weather station at Tahoe City does show a sharp warming trend beginning around 1980:

Conclusive proof of global warming, right? Well, not so fast. It turns out that other stations in the Sierras show no warming at all over the same period. Here is one such station, in Tahoe National Forest:

This is a newer station, but its temperature record shows no warming trend:

This is the record from a ranger station just 15 miles from the one at Tahoe City, going back to 1949. No warming trend starting in the 1980s:

So what is going on here? Is Lake Tahoe really warming dramatically, or not? This is the weather station that shows the warming trend that so alarms Governor Brown:

Note the trash burning barrel just five feet away from the weather station. That was removed after a global warming skeptic pointed it out. But that isn’t what drove the sudden temperature increase in the early 1980s. Rather, it was the construction of the adjacent tennis court and apartment complex, which occurred at that time:

As you can see there was a significant jump in temperature starting around 1980. A jump that is not evident in other sites around Lake Tahoe and the Sierra. Some investigation revealed that 1980 was about the time a concrete tennis court was installed next to the surface station. According to the condo property manager that an investigator spoke to said the court was installed in the “early 80′s”, though she was not there at the time. This tennis court heats up during the day and gives up energy at night, warming the area. According the groundskeeper, he picked up trash during the day and burned it at the end of the shift, leaving a warm burn barrel to increase the night time temperatures.

This kind of issue exists with respect to an enormous number of weather stations, according to a survey conducted by Anthony Watts, the proprietor of Watts Up With That:

The official record of temperatures in the continental United States comes from a network of 1,221 climate-monitoring stations overseen by the National Weather Service, a department of the National Oceanic and Atmospheric Administration (NOAA). Until now, no one had ever conducted a comprehensive review of the quality of the measurement environment of those stations. During the past few years I recruited a team of more than 650 volunteers to visually inspect and photographically document more than 860 of these temperature stations. We were shocked by what we found. We found stations located next to the exhaust fans of air conditioning units, surrounded by asphalt parking lots and roads, on blistering-hot rooftops, and near sidewalks and buildings that absorb and radiate heat. We found 68 stations located at wastewater treatment plants, where the process of waste digestion causes temperatures to be higher than in surrounding areas. In fact, we found that 89 percent of the stations – nearly 9 of every 10 – fail to meet the National Weather Service’s own siting requirements that stations must be 30 meters (about 100 feet) or more away from an artificial heating or radiating/ reflecting heat source. In other words, 9 of every 10 stations are likely reporting higher or rising temperatures because they are badly sited.

So don’t be fooled by the fictitious precision with which climate alarmists talk about the temperature record.