The warnings come with unsettling regularity:
Climate change threatens 1 million plant and animal species.
Warmer oceans could lose one-sixth of their fish and other marine life by the end of the century.
Global warming is a major risk to the economy.
The world's leading scientists have made it clear that to prevent the worst effects of climate change, there needs to be "rapid, far-reaching and unprecedented" changes to our energy systems. In simpler terms: We need to stop adding carbon dioxide to the atmosphere.
Federal action to curb greenhouse gas emissions in the U.S. seems unlikely in the near future. The Trump administration is going the other way, rolling back regulations to cut emissions.
But a growing number of cities, states, electric utilities and businesses are recognizing the risk presented by climate change and making their own pledges to reduce emissions.
Just in the past year, three states — including California, the world's fifth-largest economy — plus the District of Columbia and Puerto Rico made commitments to get all of their electricity from carbon-free or carbon-neutral sources in the next few decades. More than 100 cities and counties have signed similar pledges, and at least three presidential candidates are proposing climate plans that call for the U.S. to become carbon-neutral by 2050.
"It feels to me like we're headed toward a decarbonized energy system," says Rolf Nordstrom, president of the Great Plains Institute, a nonprofit energy research group. "Now it's just down to how fast and what that energy mix looks like."
Despite the growing push to reach "zero carbon," there are big questions around whether these goals are possible and how much they would actually slow climate change. Here's an attempt to answer some of them.
Who's doing it?
Four states — Hawaii, New Mexico, California and Washington — as well as Puerto Rico and Washington, D.C., are leading the decarbonization charge. All of those places have enacted legislation requiring that they get all of their electricity from renewable or clean sources by 2050 at the latest.
Nevada and Colorado went one step short of that, establishing a goal, not requirement, of 100% carbon-free energy by 2050.
At least 9 other states have proposed similar mandates or goals, according to EQ Research, a renewable energy consulting firm.
In the meantime, local governments in those states and others are taking action on their own.
More than 100 localities across the country — from Buncombe County, N.C., to Spokane, Wash. — have committed to getting all of their electricity from clean or renewable sources. The Sierra Club has a nifty list of cities making similar pledges.
Electric utilities, responding to customer demand (and eager to have support for new infrastructure), are getting in on the action, too. Xcel Energy, which serves eight U.S. states, announced its own 100% clean energy goal by 2050. A handful of other utilities have followed suit.
Business is also pushing for clean energy. The group RE100 says that about 180 private companies, including giants like Google, Nike, Facebook and Johnson & Johnson, have committed to 100% renewable goals.
Why are they doing it?
Climate change not only poses a risk to human health and life, but it could cost the U.S. economy hundreds of billions of dollars. State, city and business leaders aren't blind to that fact.
For more on what's at stake, see the United Nations' Intergovernmental Panel on Climate Change report. The threat is real.
Shortly before signing her state's carbon-free goal earlier this year, New Mexico Gov. Michelle Lujan Grisham pointed to that report, which said greenhouse gas emissions need to be sharply reduced by 2030 to avoid the worst effects of climate change.
"We are clear that we have basically a decade to begin to turn things around, and New Mexico needs [to] and will do its part," she said.
But the need to address emissions is hardly the only factor driving the wider shift toward renewable energy sources.
It's a convergence of social, economic, technological and political factors.
The latter being fueled, in no small part, by the inaction at the national level.
"There's the complete abdication of responsibility and leadership at the federal level, within the Trump administration and other leaders in Congress," says Jeff Deyette, the director of state policy in the climate and energy program at the Union of Concerned Scientists. He says cities and states are stepping up to fill that void and, in some cases, score political points.
The "blue wave" of last year's midterms saw cities and states elect lawmakers who vowed to take action on climate change. Environmental groups have been focusing their efforts on the state and local level.
Renewable energy is also becoming more affordable and more popular than fossil fuels in some places. A survey by Consumer Reports last year found that roughly three-quarters of respondents felt that increasing renewable energy is a worthwhile goal.
On the technical side of things, advancements are being made in energy storage and energy efficiency, making an eventual renewable energy grid more realistic.
Is "zero carbon" even possible?
This depends on whom you ask.
There are some energy analysts and academics, like Mark Jacobson, director of the Atmosphere and Energy Program at Stanford University, who say the answer is a resounding yes.
"It's technologically and economically possible," he says. "But there are social and political barriers."
Jacobson says it would cost the U.S. about $9.5 trillion to transition the country's entire energy economy — transportation, electricity, agriculture and industry — to carbon-free sources.
That may seem like a lot, he says, "but if you actually transitioned to clean, renewable energy for everything, you eliminate the health and climate costs [associated with fossil fuels]." In the long run, he argues, the country would be saving money.
Other energy analysts and academics aren't so sure that it's feasible to reach zero carbon, using only renewable energy, on an economywide scale. It's theoretically possible, they argue, but the challenges to get there are many.
Most cities, states and businesses are taking a narrower focus, aiming to just get all of their electricity from carbon-free sources (electric generation being only one piece of the greater emissions pie).
But that's not without challenges too.
Powering an entire state like California, with its nearly 40 million residents, with renewable or clean energy would require huge amounts of energy storage and massive infrastructure projects, which don't always have local support.
For cities or counties making renewable commitments, "many of them don't really have the tools to implement their vision," says Leah Stokes, an assistant professor of political science at the University of California, Santa Barbara. "Utilities often work at the state level, not the local level."
Atlanta is a good example of this.
Are 'zero carbon' goals the most effective way to cut greenhouse gases?
"All evidence points to no," says Sanya Carley, an associate professor at Indiana University's School of Public and Environmental Affairs. "The most efficient and cost-effective way to reduce carbon emissions would be directly pricing carbon and putting a price tag on the cost of those emissions."
In other words, something like a carbon tax would bake the environmental and health costs of greenhouse gas emissions into the existing market economy. It would be a rapid, large-scale way to incentivize reductions across all sectors.
A 2018 study by the Massachusetts Institute of Technology found that putting a price on carbon and returning the revenue from it to the public would reduce greenhouse gas emissions. The higher the cost, the greater the reductions.
Renewable energy goals or zero-carbon commitments are "less efficient, less cost-effective, but usually more politically feasible," Carley says.
That's why we're seeing more of them.
But they should still help slow climate change, right?
Every little bit helps.
But remember that most of these efforts are focused on cutting greenhouse gas emissions from the electric sector, and electricity generation accounts for less than a third of the country's total emissions. Transportation is now a larger source.
The other problem is scale.
"[Climate change] is a global problem, so that's the scale at which you'd want your solution," says Noah Kaufman, an economist at Columbia University's Center on Global Energy Policy. "In lieu of that, second best would be at the national level, but we've pretty much failed to pass any serious climate policy."
A piecemeal approach won't do it, he says.
An annual statistical review from energy giant BP, released last week, found that global carbon emissions rose by 2 percent in 2018, the fastest growth in seven years.
"My guess is that when our successors look back at Statistical Reviews from around this period, they will observe a world in which there was growing societal awareness and demands for urgent action on climate change, but where the actual energy data continued to move stubbornly in the wrong direction," wrote Spencer Dale, BP's chief economist in a review of the findings.
He described it as "a growing mismatch between hopes and reality."
AUDIE CORNISH, HOST:
We're going to spend some time now looking at what it will take to reach an ambitious goal.
(SOUNDBITE OF MONTAGE)
LINDA DOUGLASS: By mid-century, we must be carbon neutral.
BERNARD TYSON: We are already about 30% towards that carbon neutrality goal.
NANCY PELOSI: Carbon-free economy by 2045.
CORNISH: That's Linda Douglass of Bloomberg, Bernard Tyson of Kaiser Permanente and, of course, Nancy Pelosi at a climate summit last fall.
ARI SHAPIRO, HOST:
This summer, we'll be exploring what exactly carbon neutral means and whether getting to zero carbon, as some people put it, is even possible. For an introduction to this challenge, we're joined by NPR's Nathan Rott, who covers climate change and the environment.
NATHAN ROTT, BYLINE: Hey, Ari.
SHAPIRO: So carbon neutral, zero carbon, carbon free - what do these phrases actually mean? What are we talking about?
ROTT: So in the most broad and basic terms, the idea is to stop adding carbon dioxide to our atmosphere. CO2 is the chief contributor to climate change, and we're emitting it from all sorts of stuff - cars, planes, industry, power plants. World scientists are telling us we basically have until 2050 - mid-century - to get all of our carbon emissions to net zero if we hope to avoid the worst impacts of climate change.
SHAPIRO: What would those worst impacts look like?
ROTT: Look. We're already seeing some of these effects - increases in sea level rise, human displacement, endangered food supplies, economic losses, spread of disease.
SHAPIRO: I get the idea.
SHAPIRO: So high stakes and tight timeline - is it even possible to get to zero carbon by the middle of the century in just a few decades?
ROTT: So you're going to get a range of answers from people on that question in the energy world. On one far end, you have folks like Mark Jacobson, a professor at Stanford University, who says it is very doable. He's even drawn up plans for more than 130 countries to do just that. But other scientists and energy analysts are going to tell you that it's going to be very, very difficult - politically, technologically, economically. You know, we may even have to find ways to pull carbon out of our atmosphere.
SHAPIRO: What are the costs that we're talking about here? I mean, how expensive would it be to get the world carbon neutral?
ROTT: It would be very expensive - you know, trillions of dollars here in the U.S. alone, though a lot of people would argue that any costs we pay now will be far cheaper than paying for the costs of climate change down the road. But, you know, what strikes me with this whole idea is just how much of our day-to-day lives would need to change to truly be carbon free or carbon neutral. So to give you an idea, I just walked around my apartment earlier to see how many parts of it are powered by carbon-emitting fossil fuels.
So you have the oven - natural gas. The hot water heater, my dryer - natural gas.
So you add to that my heater, my gas-powered car, the airplanes that you and I both take to do reporting trips, Ari - all of that would need to change.
SHAPIRO: So cities and states have made an effort to get to zero carbon. Big companies are promising to do the same. We're going to hear about some of that later this summer. Would those be enough to bring about the kind of change that you're talking about?
ROTT: No. That's because most of the efforts that we're talking about are just trying to take on one piece of the larger greenhouse gas emissions pie, and that is electricity. We've seen six states or territories and more than a hundred American cities commit to getting 100% of their electricity from renewable or clean energy sources - think solar, wind, hydro, in some cases, nuclear. And, you know, look. That would be really big if they can accomplish that. But electrical generation only accounts for 28% of the total emissions in the U.S. So energy economists, policy academics you talk to, they'll tell you that the best way to cut emissions across all sectors would be to put something in place like a carbon tax.
SHAPIRO: We've heard about a carbon tax for years, and Congress hasn't acted on it. Under the Trump administration, that seems even more politically unrealistic.
ROTT: Yeah. I mean, most folks will tell you that a carbon tax right now is not politically feasible. These zero-carbon plans from cities, states, businesses that we're talking about are feasible in some places. And that's why we're seeing them. I talked to Leah Stokes, who studies climate policy and these zero-carbon efforts at the University of California, Santa Barbara. And she says that, look. These efforts will help cut emissions, but...
LEAH STOKES: Climate change is already happening. We are already seeing impacts, like forest fires and drought and heat waves. And we need to be moving faster.
ROTT: And look. We're going the wrong way. Emissions from the U.S. are up. BP came out with a report that found global carbon emissions actually grew last year at their fastest rate since 2011.
SHAPIRO: Wow. That's NPR's Nathan Rott, kicking off our exploration of what it would take to get to zero carbon emissions. Thanks,
ROTT: Thank you, Ari. Transcript provided by NPR, Copyright NPR.