Desert facts: the dry places and their limits
A desert is defined by how little falls on it, not by heat or by sand, which is why the largest one on earth is cold and why several famous ones are not the biggest of anything.
Why does NASA test Mars rovers in the Atacama Desert?
Its soil is nearly lifeless
Parts of the Atacama are so dry that soil samples have shown no detectable microbial life, making it the closest terrestrial analogue for Martian conditions.
Source: NASA Ames astrobiology field campaigns
The Sahara was green grassland as recently as when?
About 5,000 years ago
The African Humid Period ended around 5,000 years ago as orbital changes weakened the monsoon. Rock art in the central Sahara shows swimmers, cattle and hippos.
Source: Palaeoclimate records; Tassili n'Ajjer rock art
What actually defines a desert?
Getting little rain
The usual threshold is under 250 mm a year. That is why Antarctica qualifies, and why only about a fifth of the world's deserts are sandy.
Source: Standard climatological definition
What is unusual about the Gobi Desert compared with the Sahara?
It gets severe winters
The Gobi sits high on the Mongolian plateau and far from any ocean. Winter temperatures drop below -30 C, and snow can settle on the dunes.
Source: Mongolian meteorological records
The Sahel, south of the Sahara, is best described as what?
A semi-arid belt
The Sahel runs about 5,400 km across Africa. Rainfall varies enormously between years, and the boundary with the desert shifts back and forth rather than advancing steadily.
Source: UNEP Sahel assessments
How do plants and animals survive in the Namib Desert, which gets almost no rain?
They harvest fog
Cold Benguela Current water produces coastal fog that reaches far inland. Namib beetles tilt their bodies to condense droplets and drink the runoff.
Source: Gobabeb Namib Research Institute
Which is generally considered the world's oldest desert?
The Namib
The Namib has been arid or semi-arid for roughly 55 million years, far longer than the Sahara in its present form.
Source: Geological studies of Namib aridity
The McMurdo Dry Valleys in Antarctica have not seen rain for roughly how long?
About two million years
Katabatic winds strip moisture and surrounding mountains block ice flow, leaving bare ground in a continent otherwise buried under kilometres of ice.
Source: US Antarctic Program Dry Valleys research
What lies beneath much of the eastern Sahara?
Fossil groundwater
The Nubian Sandstone Aquifer System holds water that fell tens of thousands of years ago when the region was wet. Libya's Great Man-Made River pipes it to the coast.
Source: International Atomic Energy Agency aquifer studies