Scientists have come up with an astounding number: the amount of Arctic ice that has melted from land (not the Arctic Ocean) since 2003 is over 2 Trillion tons (that’s over 2,000,000,000,000 tons in just five years). What effect does this monumental loss have? Well, it’s very different than melting sea ice, which has also been dramatic since 2003. When sea ice melts, sea levels only rise a tiny bit. That’s due ice’s displacement of water volume as it floats. When land ice melts, every bit of it raises sea levels. For instance, the melted ice from Greenland alone has raised global sea levels by 0.5mm/yr.
Climate change denyers and delayers will point out that 0.5mm/yr isn’t very much. If that were all the melting to occur, they might have a good point. Unfortunately, the climate isn’t a one-dimensional system. You see, the rate at which the Greenland ice sheets are melting is increasing. Here’s an example using made-up numbers to illustrate the point. Let’s suppose in 2003, 100,000,000 tons of ice melted. Due to our continued forcing of the climate system, in 2004, 150,000,000 tons of ice melted; in 2005, 200,000,000 tons of ice melted; in 2006, 260,000,000 tons of ice melted; in 2007, 340,000,000 tons of ice melted. The rise in sea-level seen to date will be slower than what we are likely to see in the future. How about 0.6mm/yr? How about 1mm/yr? How about 2 or 5mm/yr? The oceans are already moving shorelines inland. Coastal towns and islands have and are disappearing as I write this. That sad state will only grow worse with time.
Oh, the same heat that is causing the land-based ice sheets to melt? It’s expanding the volume that the already existing ocean takes up. Since liquids expand with increasing temperature and since the oceans are collecting a good deal of the heat that has been added to the climate system, even if land-based ice were to stop melting immediately, sea level rise would still occur as the ocean responds to that heat content. The heat content of the oceans is expected to have an impact on ocean volume on a multi-decadal to multi-century time-scale. The Earth of the future will look quite different than the one we’re familiar with today. That, of course, will have a direct impact on a majority of human and animal populations. Most populations live near the coast. As the character of that coast changes, populations will have to adjust also.
The total melt since 2003 is going to be presented at the American Geophysical Union meeting tomorrow. I will try to look at the paper and monitor reaction to it thereafter.
[Update 12/18/08]: Just a quick note about the ice that melted from the Arctic Ocean the past few years. The recent Arctic sea-ice retreat is larger than in any of the 19 IPCC climate models. Remember that delayers and deniers claim those models overestimate the effects of climate change. The opposite is true: real-world observations, ice extent among them, are outpacing the most severe model results used in the latest IPCC reports. The IPCC policy recommendations are therefore also out-of-date.