Six of 12 extreme weather and climate events during 2012 have shown evidence of human-caused climate change, including Superstorm Sandy, a study released by the Bulletin of the American Meteorological Society revealed Thursday.
Eighteen research teams from across the globe independently analyzed the events that occurred on five continents and in the Arctic. Significant agreements between teams were reached, despite different strategies of assessment, the report said.
The report deduced that "climate-change related increases in sea level have nearly doubled today's annual probability of a Sandy-level flood recurrence as compared to 1950."
"Ongoing natural and human-induced forcing of sea level ensures that Sandy-level inundation events will occur more frequently in the future from storms with less intensity and lower storm surge than Sandy," it reads.
The report did not provide a clear list of which events showed evidence of anthropogenic influence and which did not.
The authors words were carefully chosen, Thomas Peterson, Ph.D., principal scientist at NOAA's NCDC and one of the lead editors on the report, said. Simplifying them into a yes and no table would not do them justice.
The U.S. heat waves of 2012 were one such example. While the events themselves can be mainly explained by atmospheric dynamics, human-induced climate change was found to be a factor in the magnitude of the warmth and the likelihood of their occurrence.
The events in the study were not chosen at random, further complicating the results.
Selecting specific events, rather than selecting at random, "might imply things about the climate that would not be valid without the events being chosen randomly. So caution also needs to be given about what conclusions such an assessment might provide."
The report also examined drought in the United States, low arctic sea ice extent and global rainfall events, among others.
The extremely low Arctic sea ice extent in summer 2012 cannot be explained by natural variability alone, the report confirmed. The ice is expected to continue to decrease in the future, becoming "largely absent by mid-century."
"Scientists around the world assessed a wide variety of potential contributing factors to these major extreme events that, in many cases, had large impacts on society," Peterson said.
"Understanding the range of influences on extreme events helps us to better understand how and why extremes are changing."