Due to a historical quirk (of unknown origin), the World Meterological Organisation releases its summary for each year based on the Dec to Nov ‘meteorlogical year’ means (rather than the more usual calendar year). Anyway, the WMO summary is now available, as is the NASA GISS analysis and the CRU summary. The point upon which all the analyses agree is that 2005 was exceptionally warm and that it continues the long term mean warming trend. All show record warmth in the Northern Hemisphere since 1860, while GISS gives 2005 as the warmest year globally as well (CRU/WMO have it second after 1998). As the summaries indicate, the differences in ranking are on the order of a few hundredths of a degree (smaller than the accuracy of the analysis) and so a definitive ranking is not possible. Differences in how the separate analyses deal with missing data are responsible for most of the apparent variations. Note too that the convention for the base periods for the anomalies differ between the analyses (1961-1990 for CRU/WMO, 1951-1980 for GISS), but this does not affect the rankings.
Update 7pm: The GISS analysis curiously appears to have gone off line….
Update 8am 16 Dec: The GISS summation is still not back up, but the raw data and new figures do seem to be available http://data.giss.nasa.gov/gistemp . Note that as pointed in comment #5, the WMO/CRU/Hadley Centre analysis is for Jan-Nov, and not for the met. year as stated above (though the GISS analysis is). Don’t ask us why!
Final Update 11pm 16 Dec: The GISS analysis is back!
(traduit par T. de Garidel)