A peer-reviewed report published by the Bulletin of the American Meteorological Society on Monday, Sept. 29 investigated the factors behind extreme climate and weather events in 2013 and found a link between some of these events and the influence of man-made activities on the climate.

The report, "Explaining Extreme Events of 2013 from a Climate Perspective," which was prepared by 92 scientists from 14 countries, is an analysis of 22 studies that examined 16 of the extreme weather events that took place in four continents in 2013.

"There is great scientific value in having multiple studies analyze the same extreme event to determine the underlying factors that may have influenced it," said report lead editor Stephanie Herring, from NOAA's National Climatic Data Center, who noted of the implications of the report. "Results from this report not only add to our body of knowledge about what drives extreme events, but what the odds are of these events happening again-and to what severity."

The five heat waves analyzed in the report, revealed [pdf] a clear association between human-caused climate change, which occur primarily through the burning of fossil fuels, and the increased odds and severity of these events. Human influence, for instance, were seen to have played a crucial role in the record temperatures in Australia last year as and in the extremely hot summers experienced in Japan, Korea and China in 2013.

The influence of man-made climate change on droughts, storms, and heavy rain events, however, is not as clear. The impact of anthropogenic activities on these phenomena is sometimes evident but is not often clear which suggest the possibility that natural factors may have played a more central role in their occurrence. Southern Europe, for instance, had its second wettest winter last year but despite the extreme precipitation, no link to human-caused climate change had been found.

The studies that examined the surface temperatures and atmospheric anomalies of the Pacific Sea also found no conclusive evidence that human activities have something to do with the rainfall deficits that currently occur in California. Although one paper provided evidence that human influence impacted the atmospheric pressure patterns, its effect on the drought in California remains unclear.

"We know that this is, in terms of observational record, it's an unprecedented event, in terms of persistently high annual scale extreme geopotential heights, we know historically that atmospheric condition has been associated with low precipitation over California," said climate scientist Noah Diffenbaugh, from the Stanford University.

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