Attributing Global Warming to Extreme Weather Events
Ten years ago from a scientific perspective relating climate, trying to attribute to global warming for producing a particular Category 3 hurricane in the North Atlantic (as Hurricane Sandy that affected New York City in 2012) would have been considered risky.
Today, it is well understood that during the last century the earth's seas have been absorbing solar energy that has not returned to the atmosphere. As a result, the seas' mean temperature is increasing. Climate scientists are now investigating how global warming is affecting what would have considered extreme weather events in certain geographical areas. Swanson (2013) summarizes the idea of the relationship between the greenhouse effect, global warming, and extreme weather events, pointing out that the likelihood of these extreme weather events is incrementally tied to manmade global warming-too much carbon dioxide being emitted into the atmosphere . This is not an abstract idea anymore. It is felt on every level. 
Global Warming and the Arctic's Ice Melting
As a result of global warming, temperatures in the Arctic Ocean have been rising. This higher temperature is extending the summer melting of Arctic ice well into the fall and winter. Arctic areas that were formerly covered by white reflecting snow have now been replaced by dark land and sea areas that do not reflect sunlight. Estimating the amount of glacier mass lost in Greenland, Jay Zwall of NASA's Goddard Space Flight Center, pointed out that each year from the received snowfall "20 percent more" ice is coming out. Biello (2006)  Given that this sunlight is no longer reflected, this energy is now absorbed by the ocean, causing extra warming of the sea and reinforcing the Arctic ice shelf melt effect.
This melting is causing significant changes in the Jet Stream (narrow, rapidly moving westerly winds flowing west to east over the northern zones of the US, Canada, Europe, and Asia), which is the most important pressure (difference) gradient in the atmosphere affecting the northern middle latitudes' winter weather.
Fundamental changes in the Jet Stream are caused by Global Warming
The Jet Stream may be seen as waves in crests and valleys moving around the middle of the Northern Hemisphere bending smoothly north and south. The temperature gradient (difference) between the Arctic latitudes and the North Atlantic latitude decreases in autumn, when the Arctic Ocean liberates the additional sun energy absorbed from incremental ice melting produced by global warming. Then, the difference on air pressure between the two pressure fields is also reduced, and the speed of Jet Stream west-to-east winds is also reduced.
Two pressure fields are present in the Northern Hemisphere. The Arctic Oscillation, or AO, -positive or negative- pressure field covering from parallel 70 ° north latitude to the North Pole, and the North Atlantic Oscillation, or NAO, -positive or negative-pressure field that covers from parallel 70 ° north latitude to the subtropics. Now, given that the NAO pressure field affected by global warming is more likely to be negative in autumn and winter, it is more likely that the Jet Stream can be altered in winter.
Extreme Warm US & Cold European Winters 2011-2012
As mentioned before, global warming produces an effect on the extent of Arctic ice melting. More of the sun's energy is absorbed in the summer by the sea, and then this heat is discharged by the Arctic Sea in the autumn, reducing the pressure (difference) gradient between the Arctic Oscillation pressure field and the North Atlantic Oscillation pressure field, and the Jet Stream slows. The pressure gradient between the AO and the NAO becomes fragile, making easier a larger Jet Stream bend extending farther north or south.
During the winter 2011-2012, extremely warm temperatures developed in the northeast United States. The Jet Stream bent farther north than usual over the northeast middle states of the US, which allowed the semitropical warm air to go as far north as the US-Canada border, and it stayed there for a long time. Beside, La Niña (which means the pressure oscillation that originates in the South Pacific) was also present. This climate phenomenon tends to deviate the Jet Stream northward over the northeast US
Simultaneously, in Eastern Europe, the coldest winter in 25 years occurred in 2011-2012. The pressure gradient (difference) between the NA and NAO was weak because of the additional heat liberated by the sea in autumn derived from the Arctic ice melting caused by global warming. The Jet Stream extended farther south, causing Arctic air to reach Eastern Europe's locations and lock in there longer than usual, producing a freeze of large proportions. Fischetti (2012) summarizes all this, indicating that as more Arctic ice melts in summer this will result in more prolonged Jet Stream bends and the longer they will remain in place, making winters warmer or colder than usual. 
No normal winters anymore
Scientists are investigating how the Arctic ice mass has been melting as a result of global warming. The question now is: Why is the Arctic ice melting more rapidly than computer models can predict from global warming?
As global warming continues, the process of Arctic ice melting and its effect on the Northern Hemisphere's pressure fields are likely to continue pushing the Jet Stream pattern farther north or south in big bends. Normal, regular winters will not be the norm anymore.
By Alfonso de Garay
 Swanson, Jeanene. 2013. "Cloudy, with a Chance of Typhoons," Storm Warnings: Climate Change and Extreme Weather, the editors of Scientific American, 12/11/13 ebook
 Biello, David. 2006. "Greenland's Glaciers Are Going, Going …"
Scientific American online, October 19, 2006
 Fischetti, Mark. 2012. "Northern Hemisphere Could Be in for Extreme Winters,"
Scientific American online, October 30, 2012