THE END OF ICE

MORE FAST ICE
I want to add a couple of more pieces to the ice melting picture. A year or so ago I saw two NOVA episodes on ice melting fast. One, NOVA: Descent into Ice, looked at glaciers in the Alps and investigated a devastating flood that destroyed a number of towns in the late 1800's. The other, NOVA: Mystery of the Megaflood, discussed the apparent massive flood that shaped large portions of the Northwest United States at the end of the last ice age. In both of these shows there were lakes of water which were held back by dams of ice. Since these lakes can form below the surface of the glaciers, they are not easy to see. The bursting of the dams represents a process which can release large amounts of melted water quickly. A number of you have commented on the recent discovery of new lakes under the Antarctic ice sheet ( BBC: Antarctic Lakes ).

Aside from the possibility of rapid release of water, scientists are interested in the possibility that liquid water flowing through glaciers and ice sheets might lubricate the base on which the ice sits and accelerate the flow of ice down mountains, perhaps to the sea. Let's return to Richard Alley's metaphor of ice sheets as a pile, similar to pancake batter placed in the middle of a pan. The batter will try to spread out. If the batter is on a flat griddle then it will spread faster than if it was on a waffle iron. If the surface is lubricated, say with Teflon, then the spread will be even faster. So if the water gets down to the land surface under the ice sheet, then it can lubricate the motion, speed the flow down the hill. If the surface is bumpy then the transport will be slower than if it is smooth. The motion can fracture the ice, loosening the connection with the land, reducing the friction. The water can both melt ice as it flows through the ice and lubricate the surface on which the ice moves.

The figure below shows a schematic of ice sheet dynamics which brings all of these pieces together.


Figure 1. From the National Snow and Ice Data Center: Schematic of Ice Sheet Dynamics

The top left part of the figure shows the stable ice shelf, grounded to sea floor. It rises and falls with the tide. The weight of the ice, the weight of the glacier is partially supported by the water. There is push back from the sea; this is the buttress holding up the pile, the spatula keeping the batter from spreading. As we saw in the last blog, the shelf can collapse rapidly. Water melts the ice from below; water from ponds of melt water melts it from above. If the sea is warming, the melting is speeded up. The loss of the buttress speeds up the flow of the glacier. This can be further accelerated by water lubricating the surface between the glacier and the land. There is the possibility of glacier collapse, accelerated melting, as ice dams break and lakes are released. This interaction of ice and water is only beginning to be appreciated on global scales.

Occasionally there is a comment that the Earth's climate goes through cycles, and we are seeing nothing different from a cycle. I've thought about this. It is true that there are cycles in climate. But because there are cycles does not mean that there are not also trends. We see cycles and trends mixed together all the time--look at the stock market. There is something different today than in the previous cycles--the large increase in carbon dioxide. We have solid scientific reasons to know that increasing carbon dioxide in the atmosphere would lead to warming. Still, if what was happening today was only a cycle, and we had knowledge of this cycle, and we could predict what was going to happen, should we do nothing simply because it is a cycle? If we can measure this rapid melting of ice, can predict the consequences of related sea level rise, shouldn't we use this knowledge to adapt whether it is a cycle or a trend?

ricky