There is a long and detailed post at Judity Curry's blog about how the adjustments are done, and why:
Having worked with many of the scientists in question, I can say with certainty that there is no grand conspiracy to artificially warm the earth; rather, scientists are doing their best to interpret large datasets with numerous biases such as station moves, instrument changes, time of observation changes, urban heat island biases, and other so-called inhomogenities that have occurred over the last 150 years. Their methods may not be perfect, and are certainly not immune from critical analysis, but that critical analysis should start out from a position of assuming good faith and with an understanding of what exactly has been done.So far, so good, although I'm quite sceptical about how Urban Heat Island adjustments are made. I'd like to see these broken out separately, because recent temperatures should be adjusted downwards for UHI, which means that other adjustments (for Time Of Day, etc) would have to be significantly higher than they would appear to be. But that's just me.
But this whole topic underlines how much better the satellite data are:
- It's a truly global measure because it will record temperatures over open ocean and other places you can't put a weather station. No infilling is needed.
- The same instruments measure all locations, so are immune to many of the biases that weather stations are prone to.
- Time Of Day issues should not be a concern.
- Any instrumentation changes are the result of replacing a satellite and so appear globally, not in only localized data.
But my biggest problem is with the overall philosophy, summed up in this paragraph:
Diligent observers of NCDC’s temperature record have noted that many of the values change by small amounts on a daily basis. This includes not only recent temperatures but those in the distant past as well, and has created some confusion about why, exactly, the recorded temperatures in 1917 should change day-to-day. The explanation is relatively straightforward. NCDC assumes that the current set of instruments recording temperature is accurate, so any time of observation changes or PHA-adjustments are done relative to current temperatures. Because breakpoints are detected through pair-wise comparisons, new data coming in may slightly change the magnitudeof recent adjustments by providing a more comprehensive difference series between neighboring stations.And so to the real dispute here. I don't so much object to the data as reported as to the assumption that it is valid for a scientist to modify observational data after it was recorded. That just feels wrong to me. And it's what this whole ZOMG THERMAGEDDON thing is about.
And it is the most important reason that I like the satellite data.