While 2012 was a rough year around the globe, it wasn’t because of the Planet X/Mayan calendar doomsday reasons people feared. Instead, it was a year of extreme weather: drought and heat waves in the United States; record rainfall in the United Kingdom; unusually heavy rains in Kenya, Somalia, Japan and Australia; drought in Spain; floods in China. And, of course, Hurricane Sandy.

One of the first questions asked in the wake of such an extreme weather event is: “Is this due to climate change?” In recent years, a brand of research called “climate attribution science” has sprouted from this question, examining the impact of extreme events to determine how much — often in fractional terms — is related to human-induced climate change, and how much to natural variability (whether in climate patterns such as the El Niño/La Niña-Southern Oscillation, sea-surface temperatures, changes in incoming solar radiation or a host of other possible factors).

In a report published online last week in the Bulletin of the American Meteorological Society, scientists with the National Oceanic and Atmospheric Administration (NOAA) tackled this question head-on. The report — the second such annual report — analyzes the findings from about 20 scientific studies of recent extreme weather events that occurred around the world, seeking to parse the relative influence of anthropogenic, or human-influenced, climate change. The overall message of the report: It varies.

“About half of the events . . . reveal compelling evidence that human-caused change was a [contributing] factor,” said NOAA National Climatic Data Center Director Thomas Karl at a press conference accompanying the release of the report. In addition, noted climate scientist Peter Stott of the British Met Office, these studies show that in many cases, human influence on climate has increased the risks associated with extreme events.

Some highlights from the report:

In Union Beach, N.J., debris surrounds the front steps of a home destroyed in Hurricane Sandy last November. (Bloomberg News)

December 2011: Two days of extreme rainfall deluge New Zealand’s Southern Island, producing landslides in what scientists call a 1-in-500-year event.

Conclusion: Total moisture available for this extreme event was up to 5 percent higher as a result of anthropogenic greenhouse gas emissions.

September 2012: Arctic sea ice reaches a record low of 3.4 million square kilometers. A study examined three factors: warmer-than-usual surface atmosphere conditions (related to global warming); sea-ice thinning prior to the melting season (also related to global warming); and an August storm that passed over the Arctic, stirring up the ocean, fracturing the sea ice and sending it southward to warmer climes.

Conclusion: Global warming was primarily responsible, due equally to the thinning sea ice and warm atmospheric conditions.

Summer 2012: Heavy rainfall in eastern Australia.

Conclusion: A La Niña episode — long associated with wetter-than-normal conditions in Australia — in 2012 likely accounts for most, but not all, of the heavy rainfall. Sea-surface temperatures north of Australia — driven by global warming — could also play a role, increasing the chances of above-average rainfall by as much as 5 percent in the future.

Hurricane Sandy: Although not among the most powerful windstorms to hit the U.S. East Coast, the storm’s real impact came from storm surge and inundation: It broke 16 records for storm-tide levels along the coast.

Conclusion: The storm coincided with peak high tide in New York Harbor — but future sea-
level rise will exacerbate this inundation, making a Sandy-level event more likely, even if the storm itself is less severe.

ScienceNow, the daily online news service of the journal Science, produced this story.