About three weeks ago now, researchers announced that the melting of a group of glaciers in Antarctica had become “unstoppable.” The water in those glaciers, once released, will raise sea levels by about three feet over the next few decades or centuries, drowning some low-lying coastal regions and islands and exposing many more to an increased risk of flooding from storm surges. The announcement made the news and now seems to have disappeared.
Aside from protesting this back-to-business attitude (did you all hear this? The shape of the continents as we know them is about to change!), I want to clear up some possible misconceptions.
First of all, the reason scientists are sounding the alarm about this one group of glaciers is that these are the glaciers they looked at. Antarctica is a very big place and research there is difficult, so there are a lot of glaciers they did not look at so closely but that might be melting also. Plus, there are other sources of meltwater (like the glaciers in Greenland) raising sea levels and as the oceans get warmer they expand, also raising sea level. So we’re not really looking at just three feet of rise over the next two hundred years, we’re talking about three feet from this one source, plus more feet from all of the other sources.
Second, not all of this melting is visible to a casual glance. Glaciers can and do melt by getting shorter; you can visit a place and say “OMG, that glacier used to fill this valley and now it stops all the way up there!” But they also get thinner. A glacier can lose a lot of mass and still cover the same area that way. The only way to tell is to measure the changing elevation of the surface, which these scientists did using the ERS-1 satellite.
With these particular glaciers there is also another problem. Glaciers form on land and flow over land, but they do not automatically melt or break up on contact with the sea. If the water is cold enough, they just keep going for a while, forming large floating shelves of ice. But these glaciers are so thick and heavy that they need very deep water in order to float. Until they get out to deep water, they keep flowing along the ground, even though the ground itself is below sea level. The place where a glacier finally separates from the ground and begins to float is called the grounding line and the location of the grounding line is another thing that can change dramatically without the glacier looking much different from a casual glance. If a glacier gets thinner it also gets lighter and needs less water to float. The grounding line retreats inland.
Where the grounding line is matters a lot, matters more, actually, than whether the ice actually melts.
The issue is that flouting ice does exactly the same thing to sea level that extra water does. You can demonstrate this yourself with a glass of water and some ice cubes. Drop the ice in the water and the water level rises–it may overflow the glass. But notice that if the water level is right at the top of the glass (as if it has just overflowed), the ice floating in the water actually sticks up above the top of the glass a little. So what happens if you let that ice melt, does the glass overflow again? No.
The space ice takes up in water is exactly the same as the space it will take up when it melts. Ice floats because water expands when it freezes (water is at its most dense just shy of 40°F or 4°C. Warm it or cool it from that point and it gets bigger) and the tip of the iceberg that sticks up out of the water is always the exact difference in volume between the ice and the same water in a liquid state.
What this means is that ice raises the sea level when it floats in the sea, whether it has melted already or not. So when scientists say that the grounding line of a glacier is moving inland by multiple kilometers a year, that is a very big deal.
Finally,there is the issue of what “unstoppable” means. Does it mean it is time to give up hope, that there is nothing we can do anymore? No, it does not.
First of all, glaciers flow, so how soon a glacier ends up in the sea depends not just on how quickly it melts but also on how free of obstruction its path is. So if the end of a glacier melts, the ice that was behind it can flow more freely and it heads toward the sea faster, unless there is a mountain ridge or something like that to get in the way. Part of what the scientists mean by “unstoppable” is that they don’t see any mountain ridges under these particular glaciers they are studying, so with the front of the glacier melting the ice now has an increasingly clear path to the sea.
But the other thing that they mean is that they honestly don’t believe we’re going to do what must be done to stop the melting; halt greenhouse gas emissions. As Eric Rignot, one of the researchers in question, wrote,
Two centuries – if that is what it takes – may seem like a long time, but there is no red button to stop this process. Reversing the climate system to what it was in the 1970s seems unlikely; we can barely get a grip on emissions that have tripled since the Kyoto protocol, which was designed to hit reduction targets. Slowing down climate warming remains a good idea, however – the Antarctic system will at least take longer to get to this point.
Now, that really doesn’t sound like he means that drastically cutting emissions wouldn’t stop the melting, only that a drastic reduction isn’t going to happen, in his view.
Let’s prove him wrong on that, shall we?