What percent of earth is covered with karst areas?

You pretty much have the answer to your question stated in the longer text following the question already. The figure for the amount of karst terrain in the US is typical for the rest of the world as well, so you can simply multiply this percentage (20%) by the amount of land area on the surface of the earth (30%) and see that roughly 6% (.2 x .3=0.06 or 6%) of the surface of the earth is karst topography.