No trees. Frozen ground. A growing season measured in weeks. Tundra covers about 10 percent of Earth's land surface, mostly in the Arctic and on high mountain peaks where conditions are too harsh for forests.

The word comes from Russian, meaning treeless mountain tract. Arctic tundra rings the northern hemisphere, from Alaska across Canada, Scandinavia, and Siberia. Alpine tundra appears at high elevations on mountains anywhere in the world. Both share the same defining features: permafrost, low precipitation, and a short summer.

What lives there

• Mosses, lichens, sedges, and dwarf shrubs
• Caribou, musk oxen, Arctic foxes, lemmings
• Migratory birds that arrive for the brief summer
• Insects that complete their life cycles in weeks

Permafrost, the permanently frozen layer beneath the surface, shapes everything. It prevents deep roots. It traps water, creating bogs and ponds in summer. It stores massive amounts of carbon, locked in frozen organic matter. As the Arctic warms, permafrost thaws and releases methane and carbon dioxide. Tundra is not empty. It is a carbon vault that is slowly opening.