The letter says: “We know that high inequality undermines all our social and environmental goals. It corrodes our politics, destroys trust, hamstrings our collective economic prosperity and weakens multilateralism. We also know that without a sharp reduction in inequality, the twin goals of ending poverty and preventing climate breakdown will be in clear conflict.”
Looks like it briefly went down in 1995 and then started climbing around 9/11 and the war boom. Didn’t the dot com bubble happen around 1995? I figure that could account for the random downturn.