Methane lifetime in the atmosphere is 11 years, if we "only" put as much in as eleven years ago it will go flat. Pretty hard to keep the curve up if we do not get help from the permafrost which has an enormous amount of methane locked up. Looks like it will blow any day now though!

2000 is 11 years after the Soviet empire began to collapse in 1989. A lot of very dirty state-subsided industries in Eastern Europe went offline in the subsequent decade. Maybe that's what we are seeing?