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Extreme Weather: Human Influence Found in Some 2013 Events, Study Says

Long duration heat waves "are becoming increasingly likely" due to human-caused climate change, but its effect on other types of extreme events — California’s prolonged, severe drought and extreme rain in Colorado, for instance — is a lot less clear, according to a sweeping report that seeks to analyze extreme weather events in the context of climate change.

The difference in confidence, the report's authors said Monday, is that while connecting higher temperatures to heat extremes is relatively straightforward, detecting a human signal in more complex events like droughts or storms is much harder.

"Temperature is much more continuous as opposed to precipitation, which is an on/off event," Tom Karl, director of the National Climatic Data Center, told NBCNews.com. "If you have an on/off event, it makes the tools we have a little more difficult to use."

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The report, an analysis of 22 studies of 16 extreme weather events from around the globe in 2013, marks an attempt to analyze recent events for signals of human-caused climate change and then pass that information on to policymakers.

"The guide to the future is understanding the past," added Karl, whose center is part of the National Oceanic and Atmospheric Administration.

The report was prepared by dozens of scientists and published Monday in the Bulletin of the American Meteorological Society.

Nine of the studies looked at heat waves in Australia, China, Europe, Japan and Korea. Those "overwhelmingly showed that human-caused climate change is having an influence," the authors reported.

Five separate studies clearly showed that the heat wave in Australia, which saw its hottest year in more than 100 years of records, was "largely attributable to human forcing of the climate system," the report stated.

"Five studies all showing the same thing is a very powerful signal," said report co-editor Peter Stott, the leader of the climate monitoring team at Britain's weather agency.

California Drought Crisis Deepens as State Hits Driest Level 2:23

Another study concluded that the kind of heat wave suffered by South Korea last year was "as much as 10 times more likely due to the current cumulative effects of human-induced climate change," the report stated.

In contrast, the report summary stated, human influence on "other types of events such as droughts, heavy rain events, and storms was less clear, indicating that natural variability likely played a much larger role in these extremes."

Here are other key highlights for 2013 from NOAA:

United States

California drought: Three separate studies, which examined Pacific sea surface temperatures and atmospheric anomalies, did not find conclusive evidence for the impact of human-caused climate change. One paper found evidence that atmospheric pressure patterns increased due to human causes, but the influence on the drought remains uncertain.

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South Dakota blizzard: This October storm was unusually strong but not unprecedented for early autumn. But there is evidence to suggest early autumn extreme snowfall events in western South Dakota are less likely to occur as a result of human-caused climate change.

Colorado deluge: A study using a single model found that the probability of another extreme five-day rainfall, like the one that caused widespread flooding in Boulder, is estimated to have decreased because of human-caused climate change. However, the study notes that additional research using more models is needed.

Rainfall across U.S.: Seasonal extremes are primarily attributed to natural variability but with some evidence for human influences on the climate having increased the likelihood of such extremes.

Australia-New Zealand

Australian heat wave: In running multiple global climate models, five separate studies all pointed toward human influence having a substantial increase in the likelihood and severity of the record temperatures.

New Zealand drought: Computer models show human-caused climate change caused conditions that were more favorable for drought.

Deadly floods wash away buildings in India

Asia

Japan, Korea and China heat waves: Studies concluded that human-caused climate change made these heat waves more likely.

India monsoon: With an early arrival of monsoon-like atmospheric circulation, the heavy rains in northern India in June was a once-in-a-century event; however, analyses of observed and simulated June rainfall provide evidence that human-caused climate change has increased the likelihood of such an event.

Britain sizzles in rare heat wave 2:38

Europe

Britain's cold spring: Long-term warming due to human-caused climate change has made such a severe cold 30 times less likely.

Western Europe's hot, dry summer: Along with natural variability of North Atlantic sea surface temperatures, human-caused climate change played a substantial part in that event.

Southern Europe's wet winter: Although the region saw its second wettest winter on record, no evidence of human-caused climate change could be found through modeling.

Danube, Elbe River flooding: No evidence was found that human-caused climate change made the heavy spring rain there more likely.

German cyclone: Cyclone Christian, a strong storm in October 2013 over northern Germany and southern Denmark, was part of the natural, multi-decadal variability that has increased storminess during recent decades.

Pyrenees snow: Natural climatic variability was found to be a main driver of the snowfall that continued into June.

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