It’s cold in the United States at the moment. Very cold. Temperatures in places where lots of people live are down below -20C (before worrying about the wind chill).This isn’t just hypothermia weather, this is ‘exposed skin freezes in minutes’ weather, and hasn’t been seen on such a large scale for decades. So why isn’t this evidence against global warming?
It will be a month or two before we have the global data, but the severe cold snaps in recent years have been due to cold air being in unusual places, rather than to the world being colder that week. For example, November 2013 was also cold in the North America, but it was warm in northern Russia; the cold had just moved (map from NASA).
The cold spells in Europe in recent years have been matched by warm spells in Greenland and northeast Canada. You don’t hear about these as much, because hardly anyone lives there. The ‘polar vortex‘ being described on the US news is an example of the same thing: cold air that usually stays near the pole has moved down to places where people live. That suggests the global temperature anomaly maps for December/January will show warmer-than-usual conditions in other parts of the far northern hemisphere.
For contrast, look at the heat wave in Australia last January, when the Bureau of Meteorology had to find a new colour to depict really, really, really hot. This map is from the same NASA source (just a different projection)
Not only was all of Australia hot, the ocean south of Australia was warmer than typical. This wasn’t a case of cold air from the Southern Ocean failing to reach Australia, which causes heat waves in Melbourne several times a year. It doesn’t look like a case of just moving heat around.
No single weather event can provide any meaningful evidence for or against global warming. What’s important for honest scientific lobbying is whether this sort of event is likely to become more common as a result. The Australian heat waves definitely are. The situation is less clear for the US winter cold: the baseline temperatures will go up, which will mitigate future cold snaps, but there is some initial theoretical support for the idea that warming of the Arctic Ocean increases the likelihood that polar vortices will wander off into inhabited areas.
[note: you can also see in the Jan 2013 picture that the warm winter in the US was partly balanced by cold in Siberia that you didn't hear so much about]