Polar Regions
Two cold ends of the planet, and how differently they are built.
The polar regions lie beyond the Arctic and Antarctic Circles, roughly 66.5° north and south, where the sun stays above or below the horizon for at least one full day a year. Beyond that shared definition they are structurally opposite.
Arctic and Antarctic are not mirror images
| Arctic | Antarctic | |
|---|---|---|
| Structure | Ocean ringed by continents | Continent ringed by ocean |
| Ice | Mostly floating sea ice | Ice sheet up to 4 km thick on rock |
| Coldest | About −68 °C | Below −89 °C |
| Land mammals | Polar bear, fox, caribou | None |
| People | Indigenous peoples for millennia | No indigenous population |
Sea ice versus land ice
The distinction matters for sea level. Sea ice already floats, so melting it does not raise sea level — it displaces its own weight. Land ice does, because it adds water that was not in the ocean. Losing Arctic sea ice changes albedo and ecosystems; losing Greenland or Antarctic land ice changes coastlines.
Permafrost
Ground frozen for at least two consecutive years, underlying much of the Arctic. It holds large quantities of organic carbon that decomposes and releases greenhouse gases as it thaws, and it is a foundation problem: buildings and pipelines on thawing permafrost subside.