What is the smallest ocean?

The Arctic, about a tenth the size of the Pacific and the shallowest - much of it sits over continental shelf rather than deep basin. It is also the only ocean substantially covered by ice, and that ice is the reason it matters out of proportion to its size.

Sea ice reflects most of the sunlight that lands on it; open water absorbs nearly all of it. So melting ice replaces a white surface with a dark one, which absorbs more heat, which melts more ice - a feedback that is why the Arctic is warming roughly three to four times faster than the global average. Summer sea ice extent has fallen by something like forty per cent since satellite records began in 1979.

The geopolitics has followed the ice. Routes that were impassable are opening: the Northern Sea Route along Russia cuts the Asia-Europe voyage by thousands of kilometres, and the Northwest Passage that killed the Franklin expedition is now navigable in some summers. Five states have coastlines on it and are submitting overlapping seabed claims under the Law of the Sea - Russia planted a titanium flag on the sea floor at the pole in 2007 - and the argument is about oil, gas and shipping lanes that only exist because of the melting.