What percentage of land on earth is desert?I don't just want to know the largest desert but exactly how much of the earth's land surface is...

A true desert is both dry and has limited plant and animal life. Although people tend to think of deserts as very hot sand dunes a la "Lawrence of Arabia", deserts can be cold, or rocky. The percentage of true desert has increased in the last 125 years, from about 9% to 14%. According to the Gale School link cited below,

True deserts cover about 14 percent of the world's land area, or about 8,000,000 square miles (20,800,000 square kilometers). Another 15 percent of the Earth's land surface possesses some desertlike characteristics. Most deserts lie near the tropic of Cancer and the tropic of Capricorn, two lines of latitude lying about 25 degrees from the equator.