Human-Caused Climate Change Fueled Some of 2013's Extreme Weather

As humans continue to burn fossil fuels and pump greenhouse gases into the atmosphere, the planet is progressively warming and having a discernible impact on some of the world's weather, particularly to heat waves across the globe.

A new report released Monday in the Bulletin of the American Meteorological Society finds that long-lasting heat waves "are becoming increasingly likely" due to human-caused climate change, but its effect on other types of extreme events - such as California's prolonged, severe drought and extreme rain in Colorado - is a lot less clear.

The report, titled "Explaining Extreme Events of 2013 from a Climate Perspective," looks at possible links between 16 bouts of weather last year and man-made climate change. After analysis of 22 studies, the report's authors concluded that heat waves - all five studied were in Australia, Europe and Asia - were more likely and more severe because of burning fossil fuels.

"It's very hard to imagine how you could have had those temperatures in a world without climate change," contributor Peter Scott told the San Francisco Chronicle.

Many countries, including Australia, saw some of their hottest years on record in 2013.

But weather extremes beyond heat waves, such as droughts, heavy rain events and storms, could not conclusively be linked to human-caused climate change. Instead, the authors pinned these extremes on natural variability in the weather.

"For temperature there's a very clear signal of climate change that's emerging and has emerged," Scott, with the UK Met Office Hadley Centre, told Climate Central. Whereas for precipitation, there is much larger variability and "the signal [from climate change] is weaker in many parts of the world compared to the natural variability."

However, one of three independent California groups in the study found that the high-pressure ridge associated with the state's drought was "very likely" linked to global warming.

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

While scientists struggle to identify the true origin behind California's extreme drought - on top of other weather events around the globe - the state continues to wrestle with three below-average rain years, prompting increasing concern about water supplies, thirsty crops and intensifying wildfires.