What percentage of Earth's surface is arid?
It will depend on the exact definition of "arid" and the period of time. But using the widely used and accepted Köppen climate classification, "arid" would correspond to the four climates in climatic group B: "Dry (desert and semi-arid) climates". Using the most up-to-date present climate data (2006) provided by the WORLD MAPS OF KÖPPEN-GEIGER CLIMATE CLASSIFICATION at 0.5° of resolution, that looks like this (in an equal area projection):



We get that the four arid climates (group B), add up to $4.401 \times 
 10^7 {km}^2$ of a total of $1.527 \times 
 10^8 {km}^2$ inventoried in the dataset (A good match for the figure of of global land area of $1.489 \times 10^8 km^2$ listed by wikipedia), leading to a grand total of 28.8% of Earth's land surface corresponding to arid climates.