Pennsylvanians will need to adapt quickly to already occurring climate change that is causing more frequent and intense rainstorms and flooding, and hotter, longer and potentially more deadly summer heat waves, according to the third U.S. National Climate Assessment released Tuesday.

In other regions of the U.S. the assessment also predicted dire, long-term consequences for failing to act quickly to reduce man-made greenhouse gas emissions that are fueling a changing climate. Those include heat waves and drought in California and the Southwest, wildfires and a degraded forest ecosystem in the West, a decline in species diversity and an increase in invasive species, and higher levels of air pollution in warming urban areas.

John Holdren, science adviser for the Obama administration, which recently has begun pushing its climate change policies, including limitations on greenhouse gas emissions, said the assessment is “the loudest and clearest alarm bell to date signaling the need to take urgent action.”

The periodic federal climate assessment, compiled by 13 federal agencies with input from more than 300 scientists, advisers and government administrators, is mandated by a 1990 law and updates assessments released in 2000 and 2009. The 840-page report makes a strong case that climate changes already are occurring in many parts of the U.S. and the impacts on ecosystems, infrastructure, economics and public health will continue to grow.

The assessment predicts that the Northeast, a region that includes Pennsylvania, will experience warmer winters and less snowfall. In mountainous regions, including much of Pennsylvania and West Virginia, more intense precipitation events will mean greater flood risk, particularly in valleys, where people, infrastructure and agriculture tend to be concentrated.

And along the Atlantic seaboard, continuing sea level rise could triple the frequency of flooding and severely damage water, sewer and electrical systems and human health, said Radley Horton, research scientist at Columbia University and the lead author of the assessment’s Northeast region chapter. He said flooding along the Atlantic seaboard could cause “multiple systems failures in a cascade effect,” and warned that the extreme heat waves could be deadly for young people, the elderly and the disadvantaged.

“I see a real risk, a growing risk of very negative outcomes for residents of the Northeast,” said Mr. Horton, during a teleconference Tuesday afternoon that was hosted by Climate Nexus, an organization focused on climate change policy and clean energy solutions, and attended by a half-dozen other authors of the assessment.

They urged states to implement mitigation and adaptation strategies to address what the report calls “climate disruption,” but noted that while most states have adopted climate action plans, implementation is in the early stages for most.

“We are being presented clearly with evidence that it’s time to make a change in the pathway we are on,” said Kirstin Dow, professor of geography at the University of South Carolina and lead author of the assessment’s Research Agenda for the Climate Change Chapter. “And there are opportunities to go a better way.”

Mr. Horton said in the Northeast region, West Virginia is the state that has done the least. It has not started to develop a climate action plan. The other states in the region either have action plans or are moving in that direction.

Pennsylvania, which produces 1 percent of the world’s greenhouse gas emissions and 4 percent of the U.S. total, approved an almost 800-page climate action plan in December 2009. The state Department of Environmental Protection issued an updated report at the end of last year that removed a provision in the 2009 plan that set an air pollution reduction target of 30 percent by 2020, pays little attention to renewable energy or proposed expansion of natural gas usage and mandates no state action.

Patrick Henderson, Gov. Tom Corbett’s energy executive and deputy chief of staff, stated, “The statute lays out the specific requirements put forth by the General Assembly that are to be included in the action plan, along with recommendations to assist in mitigating climate change and further reducing emissions...”

U.S. Sen. Bob Casey, D-Pa., issued a statement saying the assessment is a reminder that climate change is already having significant impacts on the nation and urging development of “sensible policies to stave off more dramatic consequences over the next century.”

The assessment highlighted Pennsylvania’s efforts to generate 18 percent of its electricity from wind, solar and geothermal power sources by 2021, and Pittsburgh’s pledge to reduce energy use by 20 percent by 2020 in 1.77 million square feet of its buildings. Allegheny College in Meadville, Crawford County, committed to the same goal for 1.3 million square feet of building space.

David Dzombak, a professor and chair of the Civil and Environmental Engineering Department at Carnegie Mellon University, said the department began an initiative this year to look at climate change’s impact on water and the environment.

“Climate change is occurring and every region of the country will be affected in different ways,” Mr. Dzombak said. “In the Northeast it means that storm water management challenges already facing the region will be even more challenging, the number of cooling days will increase and the number of heating days will likely decrease.”

To reduce climate change impacts, Mr. Dzombak said greenhouse gas emissions must be significantly reduced.

“To make a dent, a large amount of carbon dioxide reduction needs to occur,” he said. “But there’s already large momentum toward warming, and given carbon dioxide’s long-term residency in the atmosphere, there will be warming, and adaptation will be required. That’s what we’re working on.”

The assessment was hailed by environmental organizations and scored by industry groups, Republican leaders and conservatives as alarmist and another example of government overreach.

Katharine Hayhoe, lead author of the report’s climate change science chapter and director of the Climate Science Center at Texas Tech University, acknowledged that while there remain some climate change skeptics who are philosophically or ideologically opposed to acknowledging climate change, most people understand it’s real but don’t think action is a priority.

“They know it’s an issue, but they’ll worry about it later,” she said. “For those, the report shows how the climate is changing here and now, no matter what part of the country we are living in.”

First Published: May 6, 2014, 2:31 p.m.
Updated: May 7, 2014, 4:15 a.m.