I FEEL LUCKY to have visited both of Earth’s polar ice caps. Seven years ago I voyaged to Antarctica on an Australian routine expedition ship to resupply one of the scientific stations down south. I experienced that awesome encounter by sea with the great ring of ice that surrounds the southernmost continent. You sail directly south from Australia across the stormiest sea in the world for nine days and then, finally, the ocean stills, for you are among giants. The icebergs. They loom into sight, at first a craggy eminence in the mist and then a faceted jewel revealing its edges and cliffs and threatening fragility. These great icebergs made me shiver with more than cold. In their stateliness and marbled grandeur they provoke a frisson of fear, because of what they come from, because of what they are fragments of. You are heading towards a giant ice-making machine and these are the most trivial of its industrial residue.

Two years ago I had a more intimate encounter with ice at the other end of the globe, when I visited Greenland. Instead of being on a container ship I was on a tiny fishing boat, and we sailed perilously close to fragmenting icebergs in an ice-choked fjord. I learned later that up to five people die every year in that bay from doing this, mostly Inuit fishermen from a small community of five thousand people and eight thousand sledge dogs. I was in Ilulissat, under the eaves of the bergs in Disko Bay, at the mouth of the fastest moving glacier in the world. It is now moving forty metres a day, and its pace has doubled in the past decade. The Sermeq Kujalleq glacier is allegedly the source of the iceberg that sank the Titanic. Now it is a physical and political frontier of climate change. This is where American senators fly in to get a visceral sense of what greenhouse gases are doing.