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Ice-Free Summers Coming To Arctic
rocketjam writes "CNET reports that researchers from the University of Arizona and other universities have concluded that the Arctic will likely see ice-free summers within a century due to the increasing rate of global warming. The melting will raise ocean levels worldwide, flooding coastal areas where a substantial proportion of the world's population live. The increasing rate of ice melt is already having an impact on people and animals in the Arctic. Currently, researchers cannot foresee any natural forces that will counteract the trend."
Here a a few stats from a quick Google search or three-
The total area of Greenland is around 2,175,600 km2 (840,000 sq mi), of which about 84 per cent, or some 1,834,000 km2, is ice cap.
The average thickness of the Greenland ice sheet is over 2000 m.
The area of the oceans is what, 360,000,000 km2?
Melt all of Greenland's ice and is that 10 meters?
Ouch. Er, glug...
Isn't the icecap frozen fresh water? Maybe someone who really knows can tell us if it makes a difference that it is frozen fresh water floating on salt water.
Yes it does matter. While it is generally true that ice does not change the level of the water it's floating in as it melts, it isn't quite true if the ice contains a different concentration of salt (after it melts) than the water.
Here's a thought experiment:
Fill a water balloon with fresh water and freeze it. Drop it into in a bucket of water from the ocean. The ice inside the balloon floats, just like ice that is not in a balloon, because ocean water is 2.5% denser than fresh water, and fresh water is roughly 10% denser than fresh ice.
Now wait until it melts. Soon the water balloon is full of fresh water again. Has the level of the ocean water in the bucket changed? No. There has been a phase transition inside a floating body, changing its density, but as long as 1. it still floats and 2. its mass hasn't changed, the water level in the bucket doesn't care. The only thing that matters is the mass of the object (i.e. the mass of the displaced salt water), and the fact that the object continues to float.
But if you look at the balloon of meltwater floating in the bucket, you'll notice that it isn't totally underwater. The water line forms a little coin-sized circular "island" at the top of the balloon. This is because the bucket has ocean water in it. If the bucket had fresh water, you wouldn't see a part of the balloon sticking up above the water at all. The balloon might even sink.
Now rip the balloon. This will affect the water level. Why? Because when the balloon breaks, that little crescent of water, that was previously sticking up above the water line as an "island", isn't held together by the balloon anymore and it's free to spread across the surface of the salt water in the bucket, raising its level. Really, the salt water level isn't rising- the shape of the floating object (a blob of fresh water) changes, so that there's a layer of fresh water on top of the salt water. But we say that the water level rises anyway.
Again, if the bucket had fresh water, this wouldn't happen, because the balloon would be totally underwater even if it were floating and there would be no "island".
Remember it's only a tiny little bit of water in the island, and the amount is determined by the density ratio between the fresh water and the ocean water. The density of ocean water is about 2.5% higher than that of fresh, and that determines the extent of the balloon's rise above the water level.
This doesn't take into account secondary effects- we haven't taken into account the effects of mixing. The water might shrink a little bit as the brine and fresh fractions mix. (Similar to how mixing one part alcohol and one part water yields slightly less than two parts of 100 proof, because the water and alcohol molecules fit into each other somewhat.) But physical effects like that are not predictable by a thought experiment, and I'm guessing in the case of fresh vs. salt water that they'd account for much less than a percent of a volume change from what we'd expect. So to an elementary first-order approximation, we'd expect the water level of ocean water to rise when fresh ice melts in it.
How much will it rise? Probably by an amount equivalent to approximately 2.5% of the volume of the total fresh meltwater, divided across the entire surface area of the salty ocean water.
Ice on land is far more threatening to global sea levels. The effective meltwater contribution from landed ice is 100% by weight, not just a few percent as with floating ice.