To return to the original topic of this post...melting ice might be an indication that climate in those areas is getting warmer, but it is not in itself an indication of
global warming. I am not
nitpicking here -- it is impossible to explain
weather and
climate without understanding
convection! If you observe it's warmer in one part of the globe, you have to rule out the most likely explanation, that the heat simply migrated in from some other part of the globe, leaving your region hotter and the other regions cooler, with no change in global average temperature. You can't use local and regional phenomina to prove or disprove global warming.
The only way to determine if we're having global warming is to measure global temperatures.
Floating Ice that Melts Doesn't Raise the Sea Level
Another common claim of global warming believers is the observed ice melt proves that rising sea levels (from whatever cause) are going to be a problem. But if all this ice is melting, why hasn't there been a significant, observed increase in global sea levels?1
Part of the answer is that most of the above examples are either ice that is already floating (Arctic Sea Ice and others), or else non-polar, mountain glaciers that on a human scale are immense, but compared to the volume of the earth's ocean, are literally less than a drop in the bucket (Glacier National Park and others).
Why doesn't floating ice that melts increase the sea level? By definition, anything that floats displaces its own weight in water. In other words, if the floating ice melts, the resulting water would be exactly equal to the amount of water displaced while the ice was floating. The sea level, therefore, already includes the total amount of water in all floating ice.
You can easily verify this in your own kitchen, just make a glass of ice water (to my fellow Americans with our love of ice: be sure none of the ice is grounded on the bottom of the glass!), mark the water level in the glass, then go node for a while until all the ice melts. The water level will not have changed.
Non-floating (Grounded) Polar Ice
It's true, a few of the examples above are grounded ice that are part of ice packs that, if fully melted, would significantly raise the sea level. The Larsen Ice Shelf, both Larsen A and Larsen B, and the Prince Gustav are all floating ice, and so not an example of this. Neither is the Antartic Sea Ice (and I must point out the correct spelling is AntarCtic). The Pine Island Glacier is a better point of discussion, because some of this ice is grounded. Grounded ice can be partially submerged, but still resting on the underwater rock surface, in which case the ice is already displacing part of its weight in water. If this ice melts, only part of the resulting water will raise sea levels. Other grounded ice can be completely above water; if this ice melts, all of the resulting water will raise sea levels.
An important consideration for sea levels2 is, if global warming were to raise the temperature 6 degrees Celsius (one of the worse case global warming scenarios), how much grounded ice is actually capable of melting? The obvious answer is, only that portion of the Antarctic ice in regions with average temperatures within 6 degrees of freezing can possibly melt. In other words, grounded Antarctic ice whose average temperature is below -6 degrees Celsius, will not melt. For ice right on the -6 degree "line", some melting may occur in the warmer summer months, but it will freeze back in the long winter months.
I haven't been able to find figures, but I have to imagine that on a continent, Antarctica, on which summer temperatures rarely get above -18 Celsius (http://www.infoplease.com/ce6/world/A0856633.html), the percentage of ice on that continent that actually sustains a temperature of -6 Celsius is very small.
But there may be a less obvious answer. Scientists have known that solid ice in Antarctica migrates from the pole to the coast. There is currently research under way to measure the migration rate, which varies tremendously depending on what's in between the ice and the ground it rests on, and other factors. Currently, the formation of new ice due to snowfall is thought to roughly balance the rate at which ice migrates into the ocean or melts. Scientists are trying to measure if an increase in temperatures (but still below freezing on most of the continent) cause the migration rate to increase, or will cause the snow accumulation rate to decrease, resulting in less total water locked in the Antarctic ice cap, and therefore more water in the sea.
What's It All Mean?
The effects of global warming on sea level may be significant, or not. What's clear is when considering these issues, it is necessary to consider all the effects, not just the obvious ones, and to carefully examine if our experiences and intuitions about the weather and local climate may be applied the global climate. I am not a global warming denier. Humans are affecting the globe in many, many ways. There is some intriguing evidence that global warming might be a problem, but it's just too soon to tell. The question of sea level change is just one more example of how much more research is required before we really know what the deal is with the effects of global warming.
References and a Note
Much of the information in this writeup comes from an excellent summary of the available science at
http://www.radix.net/~bobg/faqs/sea.level.faq.html), and from the PBS Nova program "Warnings from the Ice", http://www.pbs.org/wgbh/nova/warnings/. Although the Nova producers generally present the doom and gloom scenario, if you look at what the actual scientists (fully half of whom don't believe global warning is a problem!) are saying, you will get a sense of how tricky it is to really understand what is going on.
1. Update January 2004: Upon further research and at the suggestion of some other noders, I need to change this setence to read, "But if all this ice is melting, why has there been such a tiny and uncertain observed increase in global sea levels?" According to the UN's IPCC, in their 2001 report (Chapter 11, Executive Summary, http://www.grida.no/climate/ipcc_tar/wg1/409.htm) the sea level is estimated to be rising at 1.5 +/- 0.5 millimeters per year. That's a tiny amount!
Keep in mind, the point of making this statement about sea levels wasn't to make assertions about the actual sea level, but to make assertions about how difficult it is to measure it and to determine how much of this change is due to ice melt. Upon reading the details of the UN report, one learns that the amount of increase due to ice melt is so tiny, it's smaller than the amount of the increase due to thermal expansion (see Note 2 for perspective on how tiny this is). One further learns that the amount of sea level change is so tiny, and the rate of change so slow, that before making definitive conclusions, one actually has to factor in how much of the apparent changes in sea level are merely artifacts of changes in the shape of the earth itself! It turns out, the earth's shape is to this day still slowly rebounding from the distortions caused by the weight of glaciers from last ice age, some 20,000 years ago.
Think of it this way: You're happily displacing your weight in water in a bath tub. Some water is dripping from the tap, but it's nearly matched by the amount leaving the tub due to evaporation and the limits of current drain plug technology. You decide to determine if one day the tub will overflow. After accounting for your breathing and many other confounding factors, you notice a slight rise in the water level. The observed water level change is so small, you can't even hazard a guess on the overflow question until you consider how changes to your body mass and density due to the meal you ate hours ago might be affecting your observations of the water level!
The UN scientists make a valiant attempt to deal with all these uncertainties. My point is that by the time the science makes its way into the popular consciousness, we've lost sight of the tiny scale of these changes, and we're no longer mindful of the importance of uncertainties, caveats, and qualifications. We can barely make a reliable 24 hour weather forecast; do you really feel confident about our ability to model the viscosity and morphology of the entire globe over the course of many millennia?
2. Under global warming, the sea levels could also increase because the sea water itself is warmer, and things generally expand when they get warmer. But check any physics book, liquid water is a substance that expands and contracts very little with respect to temperature. Scientists calculated the sea level change solely due to warmer water and concluded it would be about 1/10 of 1 meter per degree of global warming, and that it would take decades or centuries after the atmosphere warms for the ocean to warm and expand. Hardly a catastrophe.