"This past year, unlike the US they were not a record globally but they certainly were warm, " said Thomas Karl, director of Noaa's National Climatic Data Centre.

BBC: 2012 was in top 10 warmest on record

The extent of the problem, at least at optical frequencies, has recently been catalogued by Pierantonio Cinzano and Fabio Falchi at the University of Padua, Italy, and Christopher Elvidge at the National Geophysical Data Centre in Boulder, Colorado.

ECONOMIST: Going, going, nearly gone