Climate change caused by human use of fossil fuels played a role in about a half dozen extreme weather events last year, international scientists said Thursday.
A team of experts examined 12 wild weather episodes in 2012, from droughts in the United States and Africa to heavy rainfall in Europe, Australia, China, Japan and New Zealand.
About half of the hand-picked events showed some sign of being worse than expected due to elements like warmer oceans and hotter temperatures brought on by the rise in greenhouse gas emissions and aerosols in the atmosphere.
The report, called "Explaining Extreme Events of 2012 from a Climate Perspective," was published in the Bulletin of the American Meteorological Society.
The peer-reviewed study included 18 research teams from around the world.
"All of the 2012 extreme events considered in this report, based on the authors' analyses, would have likely occurred regardless of climate change," said Thomas Karl, director of the National Oceanic and Atmospheric Agency's National Climatic Data Center.
The goal of the research effort is to understand whether extreme events are likely to occur more frequently in the future, and "whether their intensity is changing because of natural factors or human caused change," Karl told reporters.
Human influence on climate could be partially to blame for heavy rainfalls in Australia and New Zealand and in the record winter drought in southwestern Europe, scientists said.
However, unusual rains in China and Japan, while extreme, did not appear to have a clear link to human-caused climate change.
Nor did the US drought of 2012 appear to be influenced by climate change, even though the same group of scientists reported last year that a harsh dry spell from 2011 did appear to have been worsened by human-caused global warming.
Attribution of extreme events is difficult because climate change may be a contributing factor, but is not the sole factor, said Tom Peterson, principal scientist at NOAA's National Climatic Data Center.
"If you add just a little bit of speed to your daily highway commute, you can substantially raise the odds that you will get hurt someday. But when you do get into an accident, the primary cause may not be your speed itself, it could be wet roads or texting drivers," said Peterson.
While natural variability in climate may be the equivalent of dangerous drivers or slick streets, he likened the added speed on the road are the increases in heavy precipitation and sea level rise that are caused by global warming, he said.
"We know that the world is warming and the primary reason is the burning of fossil fuels," Peterson added.
One of the strongest examples of human influence was seen in an unusual heat wave in the eastern US from March to May 2012. The human contribution to that event was estimated at 35 percent, raising the risk of such a hot spell by 12 times, the report said.
Arctic sea ice hit a record low point of 3.41 million square kilometers, in September 2012, about half of the 1979-2000 average for that time of year.
The phenomenon that could not be explained by natural variability alone, though the extent of human influence was unclear, the report said.
Scientists expect Arctic sea ice extent will continue to decrease and may be all but gone by mid-century.
When it came to Hurricane Sandy, which wreaked havoc along the US East Coast near New York and New Jersey, the storm "required many different factors to come together to create the major impacts that it did," said the report.
"Therefore, Sandy is probably one of the most difficult extreme events of 2012 to fully explain."
In the future, even weaker storms will be able to produce similar devastation due to sea level rise and coastal erosion, the report warned.
Superstorms are also expected to become more frequent in the US and elsewhere, it said.