A study of a dozen of last year's wildest weather events finds that in about half the cases, manmade global warming increased the likelihood of their occurrence.

Researchers with the United States and British governments concluded Thursday that the other cases reflected the random freakiness of weather.

They said climate change had made these events more likely: U.S. heat waves, Superstorm Sandy flooding, shrinking Arctic sea ice, drought in Europe's Iberian peninsula, and extreme rainfall in Australia and New Zealand.

They found no connection for the U.S. drought, Europe's summer extremes, a cold spell in the Netherlands' winter, drought in eastern Kenya and Somalia, floods in northern China and heavy rain in southwestern Japan.

The study appears in in the Bulletin of the American Meteorological Society.