What percentage of the Earth's surface area is desert?

About 20% according to this site.
http://www.blueplanetbiomes.org/desert.htm

But this site says it's over 30%.
http://www.universetoday.com/65639/what-percentage-of-the-earths-land-surface-is-desert/

Different websites and other authoritive sources may give different percentages depending on how they define a desert. There are polar deserts as well as mountain plateau deserts as well as sand or clay or stony deserts. Some deserts are actually relatively rich in specialised forms of plants and wildlife, while others are quite sterile. So ----- the answer rather depends on your own definition!

Between one-third and one-fifth, according to different sources. The higher figure is due to Antarctica, a cold desert which is larger than Sahara, the largest hot desert.

http://education.seattlepi.com/percent-planet-desert-5203.html
https://en.wikipedia.org/wiki/List_of_deserts_by_area