A new research report says Hurricane Sandy's impact was exacerbated by rising temperatures and sea levels -- both consequences of greenhouse gas emissions. Darren McCollester 2012

Man-made climate change may have contributed to half of the extreme weather events of 2012, scientists said in a new report in the Bulletin of the American Meteorological Society (AMS).

The report (PDF), published Friday by scientists with the National Oceanic and Atmospheric Administration (NOAA), is based on studies of about a dozen extreme weather events from around the world in 2012 -- from northern Europe’s unusually wet summer to Hurricane Sandy.

The scientists were aiming to answer the question from which their field of study, known as “climate attribution science,” was born: How many of these events were caused by man-made climate change as opposed to natural variability?

Their conclusion, based on factors such as sea-surface temperatures, changes in incoming solar radiation and many others, is that human influence varies from one event to another. But in about half of the extreme events studied, they found that man-made climate change was a contributing factor.

Attempting to decipher whether or not climate change is the dominant factor in these events is a complex process.

Using an analogy, the AMS report explained: “Adding just a little bit of speed to your highway commute each month can substantially raise the odds that you’ll get hurt some day. But if an accident does occur, the primary cause may not be your speed itself: it could be a wet road or a texting driver.”

Likewise, while climate models indicate that compounded human effects increase the frequency of extreme-weather events -- “much like speeding increases the chances of having an accident” -- natural variability may still be the primary factor in any individual event.

Hurricane Sandy's intensity, for example, was reportedly compounded by both human effects and natural variability.

Global warming caused by human atmospheric pollution makes storms more frequent and intense, the report said. Hurricane Sandy happened to hit the East Coast at peak storm-tide levels and close to local high tide -- what researchers behind the report call natural variability. Further aggravating the intensity of the storm was Sea Level Rise (SLR), which has been shown to make flooding events worse.