There is a delta in south-eastern Alaska where a tiny change in the temperature each spring - from zero to only one degree - melts a huge glacier. The tiny change brings great cascades of ice, sometimes twenty storeys tall, crashing down. It brings movement, too: everything is churning. It brings silt down the delta, and seawater back into the bay. The result is a massive wheel of fertility, a huge botanical and biological stew of wildlife and plankton.