HARRISBURG — Penn State researcher Michael Mann, one of the world’s leading climate scientists, responded with concern to this week's United Nation's landmark climate change report, saying changes are already being felt worldwide.

The United Nations report, released Monday, said global warming could reach a tipping point in just 12 years if the world doesn’t take forceful steps to reduce the amount of man-made carbon dioxide released into the atmosphere.

“Warmer oceans mean stronger storms, with bigger storm surges, that are more moisture-laden, producing worse inland flooding. (Hurricane) Harvey last year and Florence this year were textbook examples,” Mann said.

Climate change is also affecting the jet stream so that storm systems linger longer amplifying their damage, Mann said. That was a factor in making the flooding from Harvey and Florence among the worst flooding events in U.S. history, he said.

“The impact of climate change on extreme weather events is no longer subtle — we’re seeing it play out in real time now,” he said.

The U.N. panel of climate experts that issued the report warned carbon dioxide emissions need to be cut drastically around the globe by 2030 to stave off the worst effects of coastal erosion, natural disasters and other consequences.

Turning the tide, the report said, will require immediate, draconian reductions in emissions of heat-trapping gases and dramatic changes in the energy field.

President Trump, who has called climate change a hoax designed to help China make U. S. manufacturing noncompetitive, cast doubt on the U.N. report. He cited unidentified other reports that determined the environment is “fabulous.”

The U.N. report is based on more than 6,000 scientific references from 91 authors across 40 countries.

Global average temperatures have already risen an average of 1 degree Celsius, or about 1.8 degrees Fahrenheit, above levels before the industrial age, according to the U.N. report.

If temperatures rise another half-degree, the report said, they could melt Antarctic ice sheets that could contribute to warmer, rising sea levels that intensify coastal storms and flooding and endanger marine life.

Rising sea levels may not seem like much of an issue in parts of the state far from the ocean, however, projections suggest that portions of Philadelphia could be under water within the next century, said Tom Schuster, senior campaign representative for the Sierra Club’s Beyond Coal campaign.

The rest of the state isn’t immune from the perils of climate change, he said. Flooding and landslides have already become more common. Last month, a landslide was blamed for a pipeline explosion in Beaver County, Schuster noted.

Changes in climate will create other problems, as well. If the temperature rises as much as expected, it would essentially eliminate the skiing industry in Pennsylvania, and agricultural sectors like dairy farming and timber would be dramatically affected, he said.

Kelly Flanigan, Global Warming Solutions Associate for PennEnvironment, said that increased temperatures will make conditions unbearable in the state’s urban areas, regions already struggling with some of the worst air quality in the nation.

Across the state, climate change will create other problems, such as making conditions better for bugs, and subsequently worse for the rest of us.

“Mosquito season in Pa. is already longer than it was 20 years ago and we can likely expect that to continue to increase,” Flanigan said. “We also have the highest rate of Lyme Disease in the country and, as winters become shorter, the tick populations just aren’t dying off as readily as they used to.”

The U.N. panel said putting a price on carbon dioxide emissions is key for getting global warming under control.

Flanigan said that her group supports legislation, like that passed in California, which would require Pennsylvania to move to 100-percent renewable energy and away from fossil fuels. Bills with bipartisan support have been introduced in both chambers of the General Assembly, but neither has moved.

Politically, climate change briefly popped up on the radar in the state’s gubernatorial campaign as well. Republican Scott Wagner, who like Trump has spoken dismissively about the dangers of climate change, caused a stir when he called an activist “naïve” when she questioned him about the issue over the summer.

Later, Wagner said he believes climate change is happening but said that “it’s not realistic” to think that the state can move completely away from using fossil fuels. Instead, he suggested there needs to be "a dialogue" about what steps ought to be taken to confront climate change in the state.

Thursday, Beth Melena, a spokeswoman for the campaign of Gov. Tom Wolf called climate change “most significant environmental threat facing the world.”

The state, “as a major energy producer, must continue to reduce its contribution to this threat,” Melena said. That includes promoting clean energy and improved energy efficiency, as well as stricter oversight of emissions of potent greenhouse gases, such as methane, she said.

Christian M. Wade, Massachusetts state reporter for the CNHI News Service, contributed to this report.