Well, the Kyoto Convention was based on 1990 as the base year. For the sake of consistency, those who wish to see the next stage of the international framework being built on Kyoto, being an extension of Kyoto, would, for consistency's sake, like to keep 1990. For our part, the U.K. is continuing to use 1990 as the base year from which it calculates the contributions it's making.
On November 19th, 2009. See this statement in context.