How much of the earth's surface is desert?
Approximately 33% of land area or 9.5% of total area.
According to the definition:
"Deserts are mainly defined by their precipitation deficit. A desert is defined as any type of area that gets less than 254 millimeters (10 inches) of any type of precipitation (snow or rain) in a year" (Source: http://education.seattlepi.com/percent-planet-desert-5203.html).
Based on this definition, almost one third of all land area is classified as desert. This area has been increasing due to global climate change and loss of some wetlands all around the world.
Since 71% of the Earth is covered by water then about 9.5% of the Earth's surface is desert.