Ah, finally, confirmation as to what the Y-axis means! Just over half way down the page.
"These are anomalies relative to the 1961-1990 average. The global average temperature for 1961-1990 is often quoted as 14.0C. The uncertainty on that figure is around 0.5C, much larger than the uncertainties on annual anomalies."
Oh. Well, so the whole graph IS meaningless! The uncertainty of the thirty year average is greater than the maximum variation.
