However, when performing the smoothing procedure, they used the months of January and February of 2008 to represent the whole of 2008. The gives those two months the same weight as twelve months. When the data is smoothed, this propagates the over-weighted influence of those two months into previous months. Because January and February 2008 have been "unusually cool," they pull previous years downward as well, making it look like recent years seem cooler. As noted in the Hadley Center announcement:
We have recently corrected an error in the way that the smoothed time series of data were calculated. Data for 2008 were being used in the smoothing process as if they represented an accurate estimate of the year as a whole. This is not the case and owing to the unusually cool global average temperature in January 2008, the error made it look as though smoothed global average temperatures had dropped markedly in recent years, which is misleading.As has been noted at Climate Audit and Watts Up With That, the same problem existed last year (2007) when a very warm January made recent years look unusually warm. However, the Hadley Center didn't catch the error at that time. It seems that they only notice errors when they erroneously cause temperatures to go down, but not when they erroneously cause temperatures to go up.
Anthony Watts attributes this to "expectation bias," where errors that confirm the expectations of the Hadley Center researchers are missed, while errors that do not confirm their expectations are missed.