Images of colossal chunks of ice plunging into the sea accompany almost every news story about climate change. It can often make the problem seem remote, as if the effects of rising global temperatures are playing out elsewhere. But the break-up of the world’s vast reservoirs of frozen water – and, in particular, Antarctic ice shelves – will have consequences for all of us.
Before we can appreciate how, we need to understand what’s driving this process.
Ice shelves are gigantic floating platforms of ice that form where continental ice meets the sea. They’re found in Greenland, northern Canada and the Russian Arctic, but the largest loom around the edges of Antarctica. They are fed by frozen rivers of ice called glaciers, which flow down from the steep Antarctic ice sheet.
Ice shelves act as a barrier to glaciers, so when they disappear, it’s like pulling the plug in a sink, allowing glaciers to flow freely into the ocean, where they contribute to sea level rise.
If you cast your mind back to 2002, you may remember the sudden demise of Larsen B, an ice shelf on the Antarctic Peninsula – the tail-like land mass which stretches out from the West Antarctic mainland – which splintered over just six weeks.
Before Larsen B broke up, satellite images showed meltwater collecting in huge ponds on the surface, the precursor to a process called “hydrofracturing”, which literally means “cracking by water”.
Ice shelves are not solid blocks of ice: they’re made up of layers with fresh snow at the top, which contains lots of air gaps. Over many seasons, layers of snow build up and become compacted, with the bottom of the shelf containing the densest ice. In the middle, there is a porous medium called firn, which contains air pockets that soak up meltwater every summer like a sponge.
In the Antarctic summer, ice shelves get warm enough to melt at the surface. That meltwater trickles into the firn layer, where it refreezes when temperatures dip below freezing again. If the rate of melting every year is greater than the rate at which that firn can be replenished by fresh snow, then those air pockets eventually fill up, causing the ice shelf to become one solid chunk.
If that happens, then the following summer when melting occurs, the water has nowhere to go and so collects in ponds on the surface. That is what we can see in the satellite images of Larsen B before it collapsed.