Arctic sea ice
Since the summer minimum of 2007 the Arctic sea ice extent has consistently been significantly lower than the previous normal during the summer melting season. However what is less well known is that the volume of sea ice has steadily diminished year on year even though we have not broken the final extent record (to time of writing in 2012). The volume calculations by Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS) indicate that we have already lost at least 75% of the ice from the Arctic and can expect virtually complete loss within the next few years.
The sea ice is critical to the functioning of the climate as we know it. It is a key driver of atmospheric and oceanic circulation currents that are responsible for driving the weather patterns that fall within our usual range of experience. By keeping the Arctic region cool the temperature gradient that drives the jet stream is maintained as is the driving the oceanic circulation loops that transport heat northwards.
Unfortunately the loss of the sea ice is also a positive feedback process. Ice is white and therefore reflects a lot of heat back into space. As the ice melts it leaves behind darker sea water. This darker sea water is much more able to absorb heat from the sun, causing warming of this region and accelerating the melting of the ice. Clearly this tends to amplify the effect of losing more of the ice than usual and to make it less likely that the ice can recover.
Furthermore this is not the only function that the ice is fulfilling within the earth system. It takes a lot of energy to melt ice and as we know if you place ice in a glass of water it will maintain the temperature around freezing point as the ice melts, after which time the temperature will rapidly increase. In short - in areas in the Arctic sea where the ice is gone it is possible for the temperature to rise rapidly. Ocean temperature is a key driver of weather although in this case it is worth noting the area was not previously even ocean at all and therefore the change is far more fundamental.
Due to the loss of albedo and the rapid rise in temperature it is virtually impossible that the ice will ever recover following the significant summer time losses, particularly as we continue to increase the greenhouse gas load of the atmosphere and to commit the system to further warming. We are already experiencing peturbations in our usual weather patterns as a result of heat exchange between ocean and atmosphere being accelerated due to the thinner ice acting as a less efficient insulator. There is increasing evidence that the weakening of the thermal gradient driving the jet stream (as a result of the Arctic warming) is causing a rapid and dramatic increase in extreme weather.
With sufficient bad luck with the weather it is possible for the Arctic ice to melt out within any year from 2012 onwards and the probability that it does so grows significantly year on year until it is very likely by 2015. We are already experiencing increasingly severe problems even without total ice loss.