When Hillary Clinton and Bernie Sanders debated Sunday night in Flint, Michigan, their similarities often seemed larger than the differences. Both are outraged by the Flint water crisis. Both agree that climate change is one of the biggest threats that we, as a global society, face.

However, a significant difference persists between the two Democratic candidates on the issue of fracking – the practice of pumping huge volumes of water, containing some chemicals, deep underground at high pressures to crack open shale layers and release oil and natural gas.

Fracking is currently responsible for one of the biggest transformations that we’ve seen in the energy space in some time. By unleashing vast reserves of so-called “unconventional” gas, for instance, it’s a key factor behind a run of starkly low natural gas prices, which recently touched just $1.61 per million BTUs, or British thermal units.

Sanders, nevertheless, wants a ban on fracking. Or as he put it in Michigan, “I do not support fracking.” He added, “I talk to scientists who tell me that fracking is doing terrible things to water systems all over this country. We have gotta be bold now. We gotta transform our energy system to energy efficiency and sustainable energy.”

Clinton’s view was more nuanced, though some observers find the overall tone harder on fracking than she has been in the past. On Sunday, she said she doesn’t support fracking where it is causing environmental damage, is opposed by states themselves (such as in Clinton’s own New York), or where the chemicals used aren’t disclosed. But she came far short of opposing it outright, later adding, “I’ve already said we are taking away the subsidies for oil and gas, but it is important that people understand that a president can’t go ordering folks around. Our system doesn’t permit that.”

On her campaign website, Clinton says that “Domestically produced natural gas can play an important role in the transition to a clean energy economy,” striking a tone that seems very consistent with President Barack Obama’s “all of the above” energy approach, and with the widely publicized notion that natural gas can serve as a “bridge fuel” enabling a shift to completely renewable energy, since it releases much less carbon dioxide than coal when burned.

“You have some folks in the U.S. who want to see an outright ban on fracking, and then you have other people who want to see more substantial regulation of the practice, and I would say that as between the two of them, that represents the majority of Americans on the topic,” says Mark Brownstein, who heads the Environmental Defense Fund’s work on natural gas and methane emissions from the sector.

In other words, at a time when the next president will likely oversee sweeping and consequential changes in how we get energy, the difference between Clinton and Sanders on natural gas – one of the key upstarts in the space of late – is very real.

So let’s look more closely at some of the key issues at stake here:

• Water contamination. When it comes to charges about fracking’s contamination of water supplies, few deny that at least some problems have occurred in specific places. But the extent of the issue, and whether it is systemic or merely occasional, remains very much contested.

A draft assessment on the topic, completed by the EPA last year, failed to find “widespread, systemic impacts on drinking water resources in the United States.” Rather, it found that while there were some water contamination problems, “the number of identified cases, however, was small compared to the number of hydraulically fractured wells.”

Granted, the new report also estimates that some 25,000-30,000 new wells “were drilled and hydraulically fractured annually” in the U.S. over the period from 2011 to 2014. So, even a small failure rate could lead to numerous impacts.

Some environmental groups have heavily critiqued this report. The passage above also triggered criticism from the EPA’s Science Advisory Board. In its latest review draft of the EPA’s report, the board singled out this much-cited passage and said that it requires “clarification and additional explanation,” and that the EPA should “discuss the significant data limitations and uncertainties” in relation to this and other major findings.

“I think there’s a mismatch between the details of the EPA report and their headline message,” says Rob Jackson, a professor of Earth science at Stanford. “Their own work has documented hundreds of cases of water contamination and water issues. On the other hand, it depends on how you define ‘widespread.'”

Other environmental consequences of fracking got less attention at the debate, but are also worth weighing. For instance, there are concerns about air pollution near fracking operations.

• Methane emissions. Most significant from a climate perspective, there are also worries that fracking and the broader unconventional gas revolution are leading to more accidental releases of methane, a potent but short-lived greenhouse gas.

“Because of the methane issue, and because it’s really the only way to slow climatic warming on the timescale of the next few decades, I think shale gas is probably worse overall than coal,” said Robert Howarth, a researcher at Cornell University who published one of the first estimates of fugitive methane emissions believed to be coming from shale gas drilling.

The EPA recently revised upward its estimates of how much methane is being emitted by U.S. oil and gas operations, notes Environmental Defense’s Brownstein. And he expects that the number is still underestimated.

“Current methods of reporting don’t take into account emissions from sporadic, but significant emissions events that come from equipment failures, equipment malfunctions, leaks that occur over time. Aliso Canyon is a dramatic example of that kind of event,” Brownstein said.

The Obama administration has proposed new rules to limit fugitive methane emissions from oil and gas operations, but as Jackson notes, these only cover operations on public lands. “I think the challenge with Hillary’s position is, she’s not in a position to regulate the industry strongly except on federal lands,” he said.

• Methane versus carbon dioxide. Without denying the significance or the scale of this problem, it must also be said that methane is not the same as the most worrisome greenhouse gas, carbon dioxide. Indeed, the two gases are so different that, despite many pat statements suggesting that methane is “more powerful,” the truth is that they are extremely difficult and complex to even compare.

Once emitted, some proportion of carbon dioxide stays in the atmosphere for thousands of years. In contrast, methane has a much stronger immediate warming effect, but it only lasts in the atmosphere for a little more than 10 years.

This means both that reducing methane has a more immediate impact on warming, but also that when it comes to the long-term, geological fate of the planet, today’s methane emissions are of considerably less consequence than carbon dioxide emissions, because they lack the staying power.

However, Howarth argues, in the shorter term methane really matters. “The climate simply responds too slowly to change in carbon dioxide emissions,” he said. “We’re going to sail right through the Paris target if we concentrate just on carbon dioxide.”

• Gas versus coal. But weighing the issue fairly, it’s also quite clear that fracking is having at least one environmental benefit, from a climate change perspective.

Namely, fracking, by enabling the unconventional natural gas revolution in the U.S., has greatly increased overall natural gas supply and, therefore, driven down cost. “Affordable and abundant natural gas has ushered in an era of substantially lower prices than they otherwise would have been without the unconventional revolution,” noted a 2014 report by IHS.

Cheap natural gas, in turn, are displacing carbon-intensive coal in the electricity sector. While the burning of coal provided 39 percent of U.S. electricity in 2014, that dropped to 34 percent in 2015 – and U.S. greenhouse gas emissions declined accordingly, according to an analysis by Bloomberg New Energy Finance. And so far in 2016, prices have plunged even lower than they did in 2015.

Just how good this news is, though, depends on a consideration of all the long-term consequences of cheap gas – not all of which are immediately clear to us. “An energy policy that takes coal offline, especially old coal plants, is a winning energy policy,” said Stanford’s Rob Jackson. “What we don’t know is how much natural gas will displace renewables, compared to complement renewables. That’s what we need to know.”

In sum, the environmental issues around fracking remain real, yet also contested and complex. This is not an easy or simple issue and it is not one where you can really give a black and white answer. And maybe that’s not so surprising, in that this is a relatively new phenomenon that is sweeping the U.S. energy arena, in many cases faster than science can keep up with it.

More will, undoubtedly, come into focus in the coming years – under the next president.

In light of all this, the difference between Clinton and Sanders is one that voters should definitely be aware of. Clinton is more of a pragmatist who is closer to President Obama, whose “all of the above” energy policy is just as critiqued by some environmentalists as his climate change actions are praised. Sanders wants a much faster transition to renewable energy, rather than using a “bridge fuel” such as natural gas (or, for that matter, nuclear power) in the interim.

That’s a debate that will continue to play itself out constantly from the center out toward the environmental left – and it may ultimately be one of the most important ones there is for the U.S.’s energy future.

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