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.”
There is a noticeable uptick in the wealth share of the top 1% after 2010, while their share had fallen en been quite stable between 1995 and 2010.
https://equitablegrowth.org/the-distribution-of-wealth-in-the-united-states-and-implications-for-a-net-worth-tax/