Do glaciers calve because they are growing or melting?

Question

Why do they always show glaciers calving as evidence of global warming? It suddenly occurred to me that glaciers can only calve if they have reached the sea and are moving downward; which means that they must be growing! What's the deal? When glaciers melt don't the "retreat"? I'm confused.

Answer 1

You've already got some good answers for your question, but there seems to be some confusion about the dynamics of glaciers in your question and also in the answers.

A glacier forms anywhere that more snow accumulates than melts, over a period of many years. Snow builds up in the zone of accumulation, turns to ice under great pressure, and begins to flow downhill to a lower elevation where it either melts or calves into the sea at the zone of ablation (also called wastage). If the glacier metls before it reaches the sea (or a lake) then there won't be any calving. The rate of flow and the rate of calving are proportional for glaciers which reach the sea.

Therefore the rate which a glacier flows downhill and ultimately calves into the sea depends on many factors which we don't completely understand, but mainly the steepness of the surface that the glacier is moving on. Same goes for a bike or a skateboard, the steeper the hill, the faster you go. That's gravity.

Other factors controlling flow/calving rate are the thickness of the ice, the volume (mass) of ice, neve, firn, and snow pushing behind it, air temperature, ice temperature, how smooth the bedrock below the glacier is, how straight the route is, and how much water is acting as a lubricant below the glacier (but that is disputed by some glaciologists).

There is also a phenomenen called a jokullaup or glacier surge, where a glacier that normally moves slowly can suddenly advance hundreds of meters in one day (jokullaup is an Icelandic word for running glacier). Some examples of surging glaciers are the Bering Glacier in Alaska, the Tulsequah Glacier in British Columbia, and the Brúarjökull Glacier in Iceland. What causes a jokullaup event is not known, but a great amount of calving can take place over a few days.

Most glaciers move tens or a few hundred meters per year, but that shouldn't be confused with advancing and receding. An advancing glacier is one that moves ahead more than it melts, and a receding glacier is one that melts more than it moves, as measured at the toe or foot of the glacier. By definition, all glaciers move, and they only move in one direction. If it is stagnant it is called an icefield. Glaciers do not "retreat" (as that implies they are flowing backwards) - its just a function of the rate of flow minus the rate of melting or calving. Typically, a glacier advances during the winter and recedes during the summer, but to be technically correct the amount of advance (or receding) should be measured over a full year, or multiple years.

A glacier that makes it as far as an ocean or a lake will not advance or recede - the toe of the glacier will be at the calving point. All of the ice will calve into the water as the ice moves ahead - like an endless conveyor belt. Except for very large glaciers, it is not possible for the glacier to advance into the ocean, as the ice will melt or break off. If a glacier that has been calving begins to recede, then it won't be calving becuase it is no longer in contact with the water.

Calving glaciers make for impressive video footage, but glaciers will calve no matter what kind of a cooling or warming trend the earth is in. Using a calving glacier as evidence of global warming is junk science.

Answer 2

The pictures are dramatic images that the producers think help get the story across - like all those stock footage shots, not directly related to the story, that they show on the TV news for dramatic effect.

Ice flows downhill unless something stops it. In the mountains, when terrain features block most of the flow, the ice gets squeezed out through the low spots or valleys like toothpaste from the tube - this is a glacier. The pressure of the ice on top is nearly constant, so the glacier ice is being squeeze out at constant rate. How far down the valley the ice gets thus depends on the melting rate at the front of the glacier. If it melts faster than the ice is being squeezed in at the top, the glacier retreats. This is happening with the vast majority of mountain glaciers in the world.

If the ice flow is big enough and the local temperature is low enough, the ice flows all the way down to flat land . This is happening in Greenland and Antarctica. In both places, the flat land is on the continental shelf and is below sea level. The ice flowing in has been thick enough to touch the bottom anyway, so it sticks there and forms a cork, with the glacier piling up behind it. The part in the water that is still touching the bottom is called an ice shelf.

Warming oceans and sea level rise act like water flowing into a drydock with a ship sitting inside on blocks. Eventually the water gets high enough to float the ship. This has happened to very large ice shelves in both Greenland and Antarctica and all of the remaining ice shelves are at risk. When the ice shelf floats free, it removes the plug that has been holding the rest of the ice flow back and the downhill motion accelerates. Moving more of the ice into the ocean raises sea level more, which floats more ice shelves, which allows more of the ice to slide into the sea. In Antarctica, this has resulted in icebergs bigger than New Jersey floating away from the continent. That doesn't look as dramatic as a glacier calving at the water's edge, though.

Answer 3

Glacier Calve

Answer 4

That's a good question!

Yes, glaciers calve because they are sliding down to the sea, and it seems reasonable to think that if they weren't growing, at least some, they would stop doing that.

But most of the ice in that glacier is thousands of years old! Even if it stopped growing it might keep sliding for a long time.

I think scientists are looking at the way they calve. When it's warmer they might break off more often, in smaller pieces. It might not take a huge rise in temperature to do that.

In other places, glaciers are receding. In the film 'An Inconvenient Truth' they show pictures of maybe a dozen glaciers today and years ago, and you can really see a difference! It's one of the most convincing pieces of evidence in the film.

Answer 5

They would calve more if growing.
When they shrink, eventually the front of the glacier will be far away from anybody of water, and rivers and falls will result.