Effects Of Melting Sea Ice

Arctic Sea Ice Blog, via Early Warning:

But what happens in the Arctic doesn't stay in the Arctic. When sea ice cover disappears, the changing interaction between sea and atmosphere can shift atmospheric patterns. The results may be felt all over the Northern Hemisphere. As we have seen, a smaller ice pack, combined with an ever earlier melting season, means more and more sunlight is soaked up by dark ocean waters. These warmer waters then release heat and moisture to the atmosphere during fall and winter—an effect already being observed and measured.
This change in turn may already be disturbing the jet stream, the high-altitude wind that separates southern warm air from cold Polar air. A destabilized jet stream becomes more 'wavy', allowing frigid air to plunge farther south, a possible factor in the extreme winters that were experienced all around the Northern Hemisphere in recent years.
Another side-effect is that as the jet stream waves become larger, they slow down or even stall at times, leading to a significant increase in so-called blocking events. These cause extreme weather simply because they lead to unusually prolonged conditions of one type or another. The recent prolonged heatwave, drought and wildfires in the USA are one example of what can happen; another is the cool, dull and extremely wet first half of summer 2012 in the UK and other parts of Eurasia.

These changes will likely impact agriculture in the Midwest.  Considering that the Midwest is one of the largest, richest rain-fed agricultural regions in the world, and one of the contributing factors to the great wealth and productivity of the United States, this impact could be major.  And the part of the story about the potential climate change impact of thawing permafrost is also pretty scary.