Natural gas can deliver substantial carbon emissions reductions when displacing coal, and the United States has taken good advantage of it in the past several years. But the U.S. experience might be “one moment in time and space,” says Stockholm Environment Institute senior scientist Michael Lazarus, lead author of a new paper [PDF] on gas and its potential climate benefits and pitfalls.

“From all appearances, the newfound abundance and competitiveness of natural gas has been a ‘game changer’ for U.S. energy markets and greenhouse gas emissions, and a positive contributor to the economy,” Lazarus and his co-authors write. “A key question for policy-makers around the world is whether such a phenomenon can endure and spread.”

The paper’s verdict: Gas can yield climate benefits, but there’s an if: It all depends on having policies in place to guide its production and use — policies that many natural gas advocates might not be big fans of, and not just a price on carbon, but also tight regulation of methane leakage, robust renewable portfolio standards and a big emphasis on energy efficiency. And even with those guardrails in place, the paper concludes, the world might risk becoming disastrously tied to gas.

In the end, the question might not be so much, “Can we trust natural gas?” but instead, “Can we trust ourselves with natural gas?”

The title of the paper, “Natural Gas: Guardrails for a Potential Climate Bridge,” alludes to the hope that gas might provide a ready, cleaner alternative to coal which could carry us a decade or two forward, giving emissions-free technologies a chance to evolve. The chart above illustrates the imperative: new coal power plants could, in just the next two decades, use up a big portion of the world’s carbon budget for the next century.

Even some dedicated greens once held to the bridge view – it wasn’t long ago that 350 founder Bill McKibben, in his book Eaarth, was writing that “in the United States, we’ve found some new supplies of natural gas, which is a good ‘bridge fuel’ between dirty coal and clean sun.”

But after the price of natural gas plunged and its use grew dramatically, thanks to horizontal drilling and hydraulic fracturing techniques (fracking), issues of water contamination, methane release and seismic risk largely turned environmentalists away from gas. Others have raised questions about the scope of the role gas is being allowed to play, citing possible cost risks to consumers.

Still, the Obama administration has embarked on a Clean Power Plan that would commit the U.S. even deeper to gas – and developing countries are eyeing gas themselves, importing it and/or developing their own supplies. Greens are supporting the CPP while working to turn its emphasis more toward renewables, while some gas fans say the administration isn’t doing enough to encourage natural gas production.

Into this polarized debate come Lazurus and company, seeking “synthesis” of a full range of variables and disputed facts regarding natural gas.

Their paper ultimately sees three key effects that are pivotal in assessing gas: substitution, methane leakage and scale.

The substitution question is rarely surfaced: What is natural gas literally displacing? It matters, they write, because while natural gas beats coal on GHG emissions by 40 to 50 percent – inherently it is less carbon-intensive, and gas turbines also operate more efficiently – it isn’t always displacing coal. “To the extent natural gas displaces nuclear or renewable energy in power or other applications,” the paper says, “it will increase emissions, and to some extent retard development and possibly lock-out these low-carbon technologies.”

Gas in the transportation sector isn’t a sure-fire winner either. “Displacing oil products with natural gas, the impact is only slightly positive,” the report says, “and possibly even slightly negative, if and where the methane leakage associated with natural gas more than offsets its lower carbon intensity.”

That methane leakage caveat points to a key controversial aspect of natural gas. Methane in the atmosphere loses its warming oomph relatively quickly, but it is so potent initially that over 100 years its global warming potential is 28-36 times that of CO2, according to the EPA. The science on how much is escaping into the atmosphere in the production and distribution of gas has been all over the map, and advocates on both sides are prone to promote the studies that bolster their position. The “Guardrails” paper comes down on what some might consider the low side, a leakage rate of 1.5 percent, just a bit above the EPA’s 1.3 percent figure.

But those numbers, the authors say, “are highly uncertain” and as Lazurus noted in an interview, are for the United States.

“Our study is really thinking globally,” Lazarus said. “It’s not just about us, but also about emerging economies, where it’s reasonable to question whether the infrastructure to regulate effectively and well will exist.”

The third of the three “effects,” scale, speaks to what happens in the broad economy due to increased supplies of natural gas. A study out of the U.S. Energy Department’s Pacific Northwest National Laboratory last fall, for instance, found that “abundant, less expansive” natural gas would lead to greater energy use. Factoring in substitution and leakage impacts as well, that study concluded that expanded natural gas wouldn’t be a climate benefit.

That’s where the guardrails come in. Whether there’s the political will to take advantage of them is an open question, but the paper endorses the view that there are tools available to ensure gas displaces coal, that leakage is minimized, that low prices don’t drive up energy demand and that space remains for cleaner energy technologies to be developed and deployed:

Fortunately, the policies that decision-makers may elect to pursue for broader climate and economic reasons could accomplish many of these goals. These include carbon pricing, energy efficiency standards, measures that internalize some or all of the full environmental and health costs of coal, and technology forcing requirements (such as renewable portfolio standards) for renewable energy, and possibly nuclear or carbon capture and storage (CCS) technologies. In addition, methane leakage needs to be measured throughout the gas supply chain, and efforts to reduce leakage supported through incentives and regulation.

But it’s key to remember: Without these tools, natural gas is a climate fail.

Plus, the paper concludes – rather ominously, it must be said – what do we do with the bridge after we’ve build and paid for it?

Not all gas infrastructure is expensive, but some is. “It is hard to envision an LNG terminal being built for just 15-20 years’ use,” the paper says, creating a risk that we become stalled on an ever-lengthening natural gas bridge.