Doyle Rice

USA TODAY

Man-made climate change is worsening some extreme weather events, according to a report made public Friday by a top science group in Washington.

The report from the National Academies of Sciences, Engineering and Medicine found the clearest links between climate change and heat waves, droughts and heavy rain and snowstorms. Scientists found less evidence for climate change impact on other weather events, such as tornadoes, hurricanes and wildfires.

This is "the first definitive ranking of what events can be attributed to climate change," said Marshall Shepherd, a University of Georgia meteorologist and report contributor. The National Academy of Sciences (NAS) "brings a gold standard to the assessment of the science," he said.

The NAS has advised the federal government on science since the 1860s.

Man-made climate change is caused by the burning of fossil fuels such as gas, coal and oil, which release greenhouse gases such as carbon dioxide (CO2) into the atmosphere. This extra CO2 causes temperatures of the atmosphere and oceans to rise, allows the atmosphere to hold more water vapor (which can add extra fuel to storms), and causes global sea levels to rise.

Extreme event attribution is a new area of climate science that looks at how much human-caused climate change influences extreme weather events, when compared with other factors, such as natural sources of climate and weather variability, such as El Niño.

“An increasingly common question after an extreme weather event is whether climate change ‘caused’ that event to occur,” said committee chair David W. Titley, a Pennsylvania State University meteorology professor.

"While that question remains difficult to answer given all the factors that affect an individual weather event, we can now say more about how climate change has affected the intensity or likelihood of some events," he said.

The report is titled “Attribution of extreme weather events in the context of climate change" and can be used to assess and manage risk, guide climate adaptation strategies and determine greenhouse gas emission targets, according to the NAS.

“The attribution of the increasing frequency and intensity of heat waves to human-caused climate change has raised awareness that climate change is happening now and already affecting some extreme events,” said David Karoly, a professor of atmospheric science at the University of Melbourne in Australia.