Americans in their 20s and 30s could lose trillions of dollars in potential lifetime earnings as climate change disrupts the global economy and weakens U.S. productivity, according to a new report by NextGen Climate.

If countries fail to reduce greenhouse gas emissions and limit the amount and pace of global warming, a 21-year-old college graduate today could lose $126,000 in lifetime wages and $187,000 in long-term savings and investments, the report found.

This would outrank the lost income due to student debt or wage stagnation.

As an entire generation, U.S. millennials — all 75.4 million of them — could lose nearly $8.8 trillion in lifetime income without climate action, NextGen said. Those losses could keep climbing for the children of millennials and beyond.

“Global warming may very well be the biggest threat over the lifetime of a single generation,” Tom Steyer, the billionaire liberal climate activist and founding president of NextGen, told reporters Monday on a press call.

The NextGen report defines millennials as all Americans ages 18-34 in 2015, which is the same definition the Pew Research Center uses.

NextGen is prominent in Democratic politics

NextGen is a nonprofit environmental advocacy organization funded by the NextGen Climate Action Committee, a super PAC that spent $74 million in 2014 to support Democratic candidates in Senate and gubernatorial races, according to OpenSecrets.org.

In June, Steyer formally endorsed Hillary Clinton, the Democratic presidential nominee, citing her promises to confront the global climate crisis and make the U.S. a “clean energy superpower of the 21st century.”

NextGen plans to spend at least $25 million on voter turnout in Pennsylvania, Iowa, Ohio, New Hampshire, Nevada, Illinois and Colorado during the 2016 election cycle. "We really see climate as our candidate and think it’s important to educate voters on where the candidates stand on climate change and clean energy..." said NextGen spokeswoman Suzanne Henkels in an email to Mashable.

Billionaire activist Tom Steyer addresses the March to Break Free from Fossil Fuels in Los Angeles, May 14, 2016. Image: AFP/Getty Images

The new report from NextGen is part of the group’s broader effort to reach younger voters ahead of the 2016 presidential elections. The advocacy organization is active at 200 college campuses in battleground states like Ohio and Colorado.

Donald Trump, the Republican presidential nominee, has repeatedly rejected the scientific conclusion that human-caused greenhouse gas emissions are causing global warming and opposes the Obama administration's efforts to reduce the use of fossil fuels.

“The difference between Hillary Clinton and Mr. Trump couldn’t be more stark, especially when it comes to climate and clean energy,” Steyer said on the press call.

Academic underpinnings

Despite NextGen’s Democratic bent, the report’s main findings — that climate change will deprive millennials of potential earnings — is based on a nonpartisan study by researchers at Stanford University and the University of California, Berkeley.

Image: NextGen Climate

That study, published in the journal Nature last October, estimated that climate change could cause 10 times as much damage to the global economy as previously estimated.

The researchers said that higher global temperatures could reduce average global incomes by nearly a quarter, compared to a world without climate change.

The higher economic costs would come from extreme heat limiting outdoor work and altering agricultural output, as well as sea level rise eating away at highly populated and economically productive coastal cities, among other impacts.

That study and others have found that a hotter world could also see a widening gap between rich and poor countries.

People affected by flooding in eastern Bihar state, India, wait to be rescued, Aug. 21, 2016. Image: AFP/Getty Images

According to Gernot Wagner, a fellow at the Harvard University Center for Environment and consultant for Environmental Defense Fund, the NextGen report picks a small element of the Nature paper and “translates it into dollar figures so that everyone can understand it.”

Wagner, who co-authored the book “Climate Shock: The Economic Consequences of a Hotter Planet,” did not participate in the NextGen study or Stanford-Berkeley paper.

“You don’t have to stretch the science or the economics in order to argue that climate change is a problem, and that climate action is necessary,” Wagner said by phone.

“It shouldn’t strike anyone as surprising that climate change costs a lot of money.”

In the 2015 Nature paper, researchers examined 50 years of historical data from 166 countries and compared each country’s own economic performance during warm spells and cooler spells.

A farmer ploughs his dusty field in Sheldon as a severe drought continues to affect California, May 25, 2015. Image: AFP/Getty Images

They determined that a nation’s overall economic productivity — labor supply, labor productivity and crop yields — peaked at an annual average temperature of about 55 degrees Fahrenheit (13 degrees Celsius).

But productivity dropped “abruptly” beyond the range of 68 to 86 degrees Fahrenheit (20-30 degrees Celsius), according to the Stanford and Berkeley team.

For the U.S. in particular, they found gross domestic product could drop 5 percent by 2050 without climate action. By the end of the century, Even steeper declines — up to 36 percent of per capita GDP — could occur by 2100.

The NextGen report borrows these projections to help determine how crop-killing heat waves, more destructive weather events, sea level rise and other climate change-related consequences could affect the future income potential of U.S. millennials.

A girl holds a USA sign during the final day of the 2016 Democratic National Convention in Philadelphia, July 28, 2016. Image: ROBYN BECK/AFP/Getty Images

“This is the first time that losses have been applied at the individual level and to generations that will bear the brunt of our actions or inactions,” Heather McGee, a self-identified millennial and president of Demos, a left-leaning research and advocacy group, said on the Monday press call.

“The millennial generation and our children are the ones who will face both the crisis and potentially the opportunity to be part of transforming our economy and our society,” she added.