NASA's GISS was recently forced to restate its historical temperature database for the US when Steve McIntyre (climate gadfly) found discontinuities in the data that seemed to imply a processing error. Which indeed turned out to be the case (store here).
The importance of this is NOT the actual change to the measurements, though it was substantial. The importance, which the media reporting on this has entirely missed, is it highlights why NASA and other government-funded climate scientists have got to release their detailed methodologies and software for scrutiny. The adjustments they are making to historical temperatures are often larger(!) than the measured historical warming (here, here, here) so the adjustment methodology is critical.
This post from Steve McIntyre really shows how hard government-funded climate scientists like James Hansen are working to avoid scientific scrutiny. Note the contortions and detective work McIntyre and his readers must go through to try to back into what NASA and Hansen are actually doing. Read in this context, you should be offended by this article. Here is an excerpt (don't worry if you can't follow the particular discussion, just get a sense of how hard NASA is making it to replicate their adjustment process):
If I average the data so adjusted, I get the NASA-combined version
up to rounding of 0.05 deg C. Why these particular values are chosen is
a mystery to say the least. Version 1 runs on average a little warmer
than version 0 where they diverge ( and they are identical after 1980).
So why version 0 is adjusted down more than version 1 is hard to figure
Why is version 2 adjusted down prior to 1990 and not after? Again
it's hard to figure out. I'm wondering whether there isn't another
problem in splicing versions as with the USHCN data. One big version of
Hansen's data was put together for Hansen and Lebedeff 1987 and the
next publication was Hansen et al 1999 - maybe different versions got
involved. But that's just a guess. It could be almost anything....It would be interesting to check their source code and see how they get this adjustment, that's for sure.
A basic tenant of science is that you publish enough information such that others can replicate your work. Hansen and NASA are not doing this, which is all the more insane given that we as taxpayers pay for their work.
Hansen cites the fact that Phil Jones gets somewhat similar results as
evidence of the validity of his calculations. In fairness to Hansen,
while they have not archived code, they have archived enough data
versions to at least get a foothold on what they are doing. In
contrast, Phil Jones at CRU maintains lockdown anti-terrorist security
on his data versions and has even refused FOI requests for his data.
None of these sorts of analyses are possible on CRU data, which may or
may not have problems of its own.