Covering roughly 33% of our planet's land surface, deserts are a defining geographical phenomenon. What is a desert? Deserts are defined as dry regions receiving under 12 inches of precipitation yearly and are framed when regional climate changes bring about long-lasting drought conditions. The 5 biggest desert in the world: Antarctic Polar Desert: 14 million km² (5.4 million square miles). Arctic Polar Desert: 13.7 million km² (5.3 million square miles). Sahara Desert: 9.2 million km² (3.5 million square miles). Arabian Desert: 2.3 million km² (800,000 square miles). Gobi Desert: 1.295 million km² (500,000 square miles).