Earth Observatory, Columbia University

Some news reports took the extreme cold weather as evidence against climate change.

By analysing 60 years of snowfall measurements and satellite data, researchers concluded the anomalous weather conditions were caused by an unusual combination of an El Nino event and the rare occurrence of a strongly negative North Atlantic Oscillation (NAO).

El Nino events result from a periodic warming of the tropical Pacific Ocean coupled with changes in the atmosphere. El Ninos move storm systems in the Northern Hemisphere towards the equator. They occur every few years and can be predicted up to a few seasons in advance.

The North Atlantic Oscillation (NAO) is climatic phenomenon resulting from shifts in atmospheric pressure between two regions above the North Atlantic Ocean. Large changes can only be forecast aweek or two weeks in advance.

When the NAO enters a strongly negative phase, cold air repeatedly comes down from the Arctic. This affects eastern North America, and Western Europe.

Image caption Snow that covered most of the UK was caused by a rare weather system bringing down cold air from the Arctic

Richard Seager, a meteorologist with the Lamont Doherty Earth Observatory at Columbia University in the US, is one of the lead authors of the paper.

"The NAO was probably as negative as it's ever been in the instrumental record, which goes back to the early 1800s. This was a once-in-a-century type of event," he told BBC News.

It was this combined with the El Nino event that caused the severe snowfall in North America.

"The NAO on its own doesn't cause