Cases where such comparisons are available are rare. Weather measurement stations were built to help with predicting the weather, and the value of added stations lies in being far enough away to measure different weather. It also appears that few weather scientists were willing to live way out in the sticks just so the stations they tended would never suffer from urbanization. So they’ve got to take the measured corrections from the few cases where stations were inadvertently placed too close together and a town grew up around only of them, calculate the offsets there, and then estimate that other stations in similar surroundings get similar offsets.

Except that this is not what the weather scientists were doing for many years. Instead of looking at the stations, they were trying to compare the long runs of data from each station statistically, and deduce the station condition and the required offset from that. That involved using a complex computer program that has been proved to be buggy, and was kept secret for a long time in defiance of all principles of scientific research – but that’s not the basic flaw. The real flaw is that any such methodology will either average the good stations with the bad, to produce data that’s influenced by the bad measurements only not quite as much – or it will assume some baseline trend and “correct” the data to match. So now, someone is beginning to look at the actual stations and what they’re finding is bad…

I warn you that this is really crappy analysis. It’s almost like McKinsey style mooncharts for a dotcom. Or Forester reports with errors carried forward.

Before trumpeting this stuff, you ought to parse it.

Greg

Before correction, the data showed almost no overall trend. With a correction, it shows a warming trend of less than 1 degree over almost 30 years. And remember, the starting point was in a decade when ground stations were showing unusually low temperatures and the alarmists were talking about an impending ice age. That is, if someone had picked a starting point in the 1970′s by choice rather than because the technology just didn’t exist any earlier, I’d suspect them of trying to bias the results by picking an especially cold starting point. If we had satellite data going back further, it’s anyone’s guess as to whether it would show this decade as warmer or colder than the 1930′s.

Over longer periods and greater temperature changes, there are indicators that do not depend on measuring instruments, although their accuracy is lower. E.g., you can follow local accounts of first and last snowfall, when the pond froze over and when it thawed, and when it was judged safe for ice skating. You can find measurements of glacial advance and retreat, and of ice forming and breaking up off Alaska. By such methods, it’s quite clear that North America warmed substantially between 1850 and 1930. Since then, probably the temperatures dropped a little to the 1970′s, and then began a sharp rise for the last 30 years – but it’s not accurate enough to tell whether the world is now warmer or colder than 1937. (Ground temperature readings with the most recent set of fudge factors actually show the warmest years were in the 1930′s.) Another issue with many of these indicators is that they have a long and unknown lag time between a temperature change and the indicator changing; e.g., a glacier might start melting decades before it’s retreat is clearly measured, permafrost might have been warming up since the 1930′s and just started visibly melting, etc.

Going further back, you can also use accounts of where cold-sensitive crops were grown (e.g., vineyards in medieval England), and of subsistence farms being established and then failing in marginal areas, such as the south of Greenland. You can also look at the growth rings in the oldest trees and in pieces of wood that have been preserved from centuries past, and try to untangle how much of the variation in growth per year is due to temperatures versus rain, fertilization, disease, etc. Put these all together and it’s quite clear that western Europe, England, and Greenland were pretty warm in 1000AD, suffered a significant cooling sometime between then and 1700, and have been recovering since then – with the warming trend starting before most of the industrial-age increase in carbon dioxide. What’s impossible to do with such methods is to clearly establish whether the world is warmer overall than it was 1,000 years ago or not. Southern Greenland certainly isn’t – former Norse farmsteads are still covered in ice and snow year round, and no one would need farm implements and cattle barns where the ground never thaws. Some tree ring data seems to show that it’s warmer now – in a few particular locations. There aren’t enough data points for a world-wide average to be meaningful