The temperature record using the 1221 “official” weather stations in the US Historical Climatology Network, are not useful in determining whether there is any danger of Catastrophic Anthropogenic Global Warming (CAGW), that is man-made global warming. We already have seen that over 90% of these stations cannot measure more closely than the nearest degree Celsius. (In 70% of these stations, the expected error is greater than 2 degrees C.)
But, these numbers have other problems in addition to the poor siting of the instruments.
First, the Goddard Institute for Space Studies (GISS), one of the main US repositories for climate data, continuously adjusts the temperature record. They do this in several ways, but the net effect is to raise current instrument readings, while lowering data from years ago. Lowering the old and raising the new increases the “temperature climate trend.” Look at a couple of analyses here and here.
The tend in rising temperatures since 1910 has increased, just due to GISS adjustments, by about 0.2 degree Celsius! The US Historical Climatology Network (USHCN) also does temperature adjustments.
Their adjustments have made the rising trend since WWII appear almost 0.5 degree Celsius warmer than the raw data would have shown.
Second, there is a phenomenon called the Urban Heat Island (UHI) effect. Concrete and steel retain daytime heat and release that heat overnight. The result is that cities, roads, and airports are warmer at night than if they had remained green areas.
As populations grow, cities grow, soaking up more daytime heat, which makes the night times warmer. Now this is a good thing if you’re homeless trying to keep from freezing to death on winter nights.
But, Alarmists don’t see things the way we see them. They take the high and low for the day and average them. As cities grow, and the UHI effect grows, the trend of the daily averages goes up, so the Alarmists say, “OMG, we’re all going to die!”
The amount of UHI warming changes as a city becomes larger. UHI continues to grow but at a slower rate. Dr. Roy Spencer has done a study showing the effects of population density on UHI.
For a city like Boston, as it grew from about 4,500 people per sq.km to its current 5,000+, it’s UHI effect would have caused a rise in the average temperatures by about 0.5 degree Celsius. That’s not global warming, that’s UHI making the trend look bigger.
Taken as a whole, urban areas which have grown over the last 100 years show a rising temperature trend, while rural areas, not subject to UHI have modest to zero temperature rise trends.
Between the GISS adjustments to the actual instrument data and the non-CO2 related UHI effect raising temperature trends in these cities, there is not much (if any) global warming to get all worried about.