In reply to Anonymous:
The post disappeared but never mind...
> Yes it is.
Same question to you as to niggle. Who has withheld data and what did they withhold?
> No it doesn't. The data is ftped to them and dumped into a folder - all they have to do to share the data is to make that folder public.
Depends on the data. It can take time (and hence money) to put data on the web in a sensible fashion. Sometimes you have to obtain permissions to do so, sometimes the permission isn't granted. Sometimes people can't read the data in the format you use. If you dump your data on an ftp site it might not be useful, then people email you and accuse you of being obstructive.
> This is true because the ‘denialists’ keep finding serious errors in the data. For example the recent discovery that they were missing the minus signs off of METAR data. So exceptionally cold temperatures recorded last winter have been logged as exceptionally warm ones.
The METAR problem was an error in the GISS data set that was spotted and corrected within a couple of days. The error had a very very small effect in the global temperature in one month.
> All the datasets have the same source.
There are large overlaps, sure, but each group uses some independent data. The data set creators use as many stations as they can because it gives a better analysis.
> The data is then adjusted by groups like Hadley and GISS for reasons that they won’t explain
The reasons are explained, in detail in the papers that describe the data sets. There's a brief description of the NCDC adjustments here:
http://www.ncdc.noaa.gov/oa/climate/research/ushcn
> and using algorithms that they won’t publish.
Descriptions of the algorithms are published. In the case of the GISS data set, the full working code has been published on the web.
http://data.giss.nasa.gov/gistemp/sources/
An independent team has recoded the whole lot, tested the code and they get... exactly the same answer as GISS do. See here:
http://clearclimatecode.org/gistemp/
> They effect of their adjustments is to create a warming trend that doesn’t exist in the raw data. If you look at the raw temperature data for the USA (for example) temperatures were higher in the 1930s than the present day.
This is where those explanations in the papers come in handy. The reasons for the warming adjustments in the US are described therein.
http://www.ncdc.noaa.gov/oa/climate/research/ushcn/#biasadj
There are versions of the global temperature curve with and without corrections to the US data. They don't make a lot of difference. The globe still warms.
Other indpendent teams grouped loosely around some skeptic blogs have created a suite of global temperature curves. Their estimates of global average land temperature show a higher rate of warming than GISS, NCDC or HadCRU. See the latest installment here:
http://rankexploits.com/musings/2010/replication/