changes in arctic ice sheet

The European Space Agency has reported that, for the first time on record, enough arctic ice shifted and melted in August that a ship could sail to the North Pole. You can see a somewhat less technical, but perhaps clearer, version of the article here. The article quotes Mark Drinkwater, director of the ESA's Ocean/Ice unit, as saying

This situation is unlike anything observed in previous record low ice seasons. It is highly imaginable that a ship could have passed from Spitzbergen or Northern Siberia through what is normally pack ice to reach the North Pole without difficulty.

If this anomaly trend continues, the North-East Passage or ‘Northern Sea Route’ between Europe and Asia will be open over longer intervals of time, and it is conceivable we might see attempts at sailing around the world directly across the summer Arctic Ocean within the next 10-20 years.

Not entirely uncoincidentally, the October Scientific American has an article that looks at how mass extinctions in the past might have happened. While it's commonly accepted that an asteroid or comet impact killed the dinosaurs, the details for earlier mass extinctions are fuzzier. The gist of this article is that, once atmospheric carbon dioxide reaches around 1000 ppm or so, anaerobic bacteria in the oceans begin bubbling up large quantities of hydrogen sulfide gas, killing things both in the ocean and on land. The models predict that the situation eventually stabilizes and returns to normal after a few hundred thousand or million years.[1] If current carbon dioxide growth rates continue without change, we should hit the 1000 ppm number some time around the year 2200.

[1] The problem with numbers in the hundreds of thousands of years range is it's very hard to conceive of what they mean in human timescales. If we assume agriculture developed around, say, 12000 BCE, and round off for convenience to 15,000 years, in a 100,000 year time-span you could go from discovering agriculture to learning how to form a civilization to inventing writing to walking on the moon and jamming on iPods a little over six times. In a million years, you could do it 67 times, give or take a dark age.