Cities on the Edge

Some cities sit right where two climate zones meet, so even a small shift hits them first.

Perth, Australia straddles the line between Mediterranean and semi-arid, and its rainfall has fallen sharply since the 1970s. Moscow sits near the boundary of continental and subarctic climates, where milder winters now soften ground that buildings were designed to sit frozen.

For the Sahel, the stakes are highest: when the Sahara's edge creeps south, millions of farmers lose their rains at once.