The most powerful climate policy tool available to the federal government is a single number. It's called the social cost of carbon, and it represents the cost to humanity of emitting greenhouse gas pollution into the atmosphere.

The social cost of carbon adds up all the damage from carbon emissions – the lost crops, flooded homes and lost wages when people can't safely work outside, plus the cost of climate-related deaths. The answer is expressed in dollars.

The current social cost of carbon is $51 per ton of carbon dioxide emitted.

Most climate experts agree that number is too low. That's a problem because it can make it seem like the costs of climate solutions – such as the immediate price tag for building more public transit or expanding wind energy – outweigh the benefits, when in fact many of the benefits to humanity are simply being underestimated.

The Environmental Protection Agency agrees that $51 is too low, and proposes more than tripling it to $190.

"That is an absolutely enormous improvement," says Tamma Carleton, a climate economist at the University of California, Santa Barbara who is an expert on the social cost of carbon. "We don't have other avenues for large-scale climate policy at the federal level. This is our main tool."

But the new number is also controversial, because of the way that the EPA assesses the value of human lives lost due to climate change.

If you make more money, your life is worth more

A major reason the EPA's new social cost of carbon is higher is because this is the first time the federal government has added to its calculations the cost of climate-related deaths outside America, including in developing and low-lying countries that are most vulnerable to the effects of climate change.

But the EPA didn't assign the same dollar value to every life. Instead, a life lost in a lower-income country due to climate change is worth less than a life lost in a higher-income country.

The upshot is that the value of a climate-related death in the United States is equal to about 9 deaths in India, or 5 deaths in Ukraine or 55 deaths in Somalia. It also suggests that the life of a person in Qatar is worth almost twice as much as the life of an American.

"It's inherently inequitable to use this kind of approach," says Vaibhav Chaturvedi, a fellow at the Council on Energy, Environment and Water in New Delhi, India and a leading expert on global climate economics. "All lives are equally valuable."

Chaturvedi argues that the EPA's approach is both philosophically and logically wrong, because America's greenhouse gas emissions endanger people everywhere. In fact, the people who live in low-lying and low-income countries are among the most vulnerable to the effects of climate change, including rising seas and extreme weather.

That's true in India, he says, where climate-driven disasters killed an estimated 2,200 people last year, according to the Indian Meteorological Department. "What makes India very vulnerable [to climate change] is that it's still a very low-income economy," says Chaturvedi. For the EPA to assign less value to the lives of the people most affected by greenhouse gas emissions doesn't make sense, he argues.

The EPA does not apply the same method to lives within the U.S. – the agency applies one value to all American lives, regardless of income.

The EPA declined to answer NPR's questions about its method because the proposed social cost of carbon is currently accepting comments from the public. But an FAQ on the EPA's website explains how the EPA conducts what it calls "mortality risk valuation."

"The EPA does not place a dollar value on individual lives," the FAQ explains. "Rather, when conducting a benefit-cost analysis of new environmental policies, the Agency uses estimates of how much people are willing to pay for small reductions in their risks of dying from adverse health conditions that may be caused by environmental pollution."

Daniel Hemel, a law professor who studies how policymakers assign value to lives saved for the purpose of regulations, says the EPA's social cost of carbon does put a dollar amount on human lives. "You'll hear agencies say 'We're not valuing lives.' I don't know, they kind of are. They're deciding how much it's worth it to spend to save a life," he says.

Getting this number right is important for the future of global warming

If you assigned the same value to lives around the world, the social cost of carbon would be much higher – almost double the number the EPA is currently proposing, says Tamma Carleton, who examined this question for a study published last year.

An even higher social cost of carbon would theoretically push the U.S. government to reduce greenhouse gas emissions more quickly and dramatically. "We'd end up being more concerned about climate change," explains Hemel.

It's unclear why EPA economists didn't choose this route. Hemel speculates that some policymakers might be concerned about proposing a social cost of carbon that is so high, it appears to require the U.S. to take drastic, and politically unpopular, steps to slash greenhouse gas emissions. For example, banning gas-powered vehicles or eliminating domestic fossil fuel extraction.

Chaturvedi argues that the U.S. is missing an opportunity by not embracing the full value of the lives saved around the world if emissions fall. He says an even higher social cost of carbon could spur the development of new renewable energy technology or even methods to remove carbon from the air, which the U.S. could then export to the rest of the world.

Getting this number right is ethically important

The moral implications of the EPA's approach loom at least as large as the practical and political ones.

"To systematically discount the value of deaths outside the United States is a grave moral mistake," says bioethicist Paul Kelleher of the University of Wisconsin. "It's important to get it right because these are life and death decisions."

An estimated 74 million lives could be saved this century if greenhouse gas emissions are eliminated by 2050, a study published last year suggested.

"Every molecule of carbon dioxide matters." The social cost of carbon, Kelleher says, "will make a difference to who lives, who dies, how good their lives are [and] how bad their deaths are," for decades to come.

Hemel worries about the message that the EPA's approach sends at home.

"I think we send a problematic message to Americans when we use a method for assigning values to lives outside the United States that ends up valuing light-skinned people from the global North more than dark skinned people from the global South," he says.