It's been horribly cold in the United States the past several days. Here in North Carolina temperatures overnight fell to about 10 Fahrenheit (-12 Celsius), which is quite cold enough for me if you were about to ask, though a lot of more northerly states have lately had temps well below zero Fahrenheit. And as usual, some people take this as evidence against global warming.
Of course, global warming doesn't mean the weather never gets scary cold anywhere or that climates don't continue to vary locally and globally. It means that the overall trend for global average temperature is upward.
Last winter, you may recall, we had a similar bitter cold spell in the U.S. (and learned the expression "polar vortex"), but globally 2014 was a warm year. In fact, the Japan Meteorological Agency has completed a preliminary analysis (link) indicating that the world's overall average temperature last year was the hottest ever recorded. Separate data (link) indicate that greenhouse gas emissions also hit a record in 2014. In fact, eastern North America and central Asia were the only regions with below-average temperatures last year.
That said, a single year record year doesn't by itself prove anything one way or the other. But we've had a whole series of record-breaking or near-record-breaking hot years during my lifetime. The hottest 10 years in our temperature data all occurred in the period 1998-present.
But we have not broken a record for coldest year since 1909, more than a century ago.
To see this graphically, take a look at this silent one-minute video released by NASA last November. It shows record-breaking hot and cold years going back to the 1880s.
It's also worth noting that 1998 was an anomalously hot year, much warmer than the years immediately before or after, very probably because of an extremely strong El Niño. There was barely any El Niño in 2014, but it still seems to have surpassed even 1998.
Taken together with research published in the journal Science (link) in March of 2013, 2014 may not only be the hottest year in well over a century of large-scale direct meaurements but also very possibly the hottest in 4000 years. A New York Times article (link) provides a good non-technical summary of that earlier research.
For more about the Japan Meteorological Agency's analysis, see reports in Slate (link) and Scientific American (link), the latter a reprint from Climate Central, a good source to check for ongoing coverage (link).