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When the world gives you hydrogen, make juice (a.k.a. electricity).

For centuries, chemists flexed their creativity to harness power from the most abundant element in nature. Their experiments paid off with a slew of technologies, including hot air balloons, rocket fuel, and rechargeable batteries. Today, the US uses another one of those technologies, hydrogen-fuel cells, to generate 250 megawatts of energy daily. And though that’s only a pinprick of the country’s total electric capacity, experts are questioning how carbon-intensive the method is.

Nearly 95 percent of the hydrogen-fuel cell centers in the US rely on natural gas to feed their battery-like circuits. They apply a process called steam-method reforming, where methane from the natural gas (and occasionally, biogas) is forced apart by searing-hot water and pressure to produce hydrogen molecules. That means there’s a heavy greenhouse gas footprint on the front end and a few carbon byproducts on the back end.

[Related: The century-old technology that could unlock America’s renewable future.]

That might make hydrogen less appealing in the grand scheme of climate-friendly energy production—unless you add a little nuance to the discussion. Enter the “hydrogen rainbow,” a color-coded system for describing the many ways to convert the lightest element on the periodic table into energy. Steam-methane reforming and other natural gas-based methods are called gray hydrogen. Anything involving coal is classified as brown and black hydrogen. And nuclear gets the great distinction of being deemed pink hydrogen. (There’s also turquoise hydrogen, which tweaks steam-methane reforming by creating heat from electricity. But it still gets its CH4 from natural gas.)  

While some of these approaches have been in practice for decades, they’re either too inefficient or difficult to scale to be considered real energy alternatives. Which leaves green and blue hydrogen, the freshest and buzziest categories on the spectrum.

Green hydrogen takes energy from renewables, cyanobacteria, or algae to separate hydrogen molecules from water through electrolysis. Though it ultimately depends on wind turbines, solar panels, biomass, or dams, it’s easier to store and transport and has better geographic range than the original power sources. The method is taking off in the European Union, with a handful of facilities breaking ground in the next year or two, and is just starting to see traction in the US. Materials scientists point out that producing green hydrogen can cost four to six times more than gray hydrogen, but those estimates should fall as reservoirs of renewable energy rise.

Blue hydrogen, meanwhile, is another relatively new concept that’s being tested out by fossil fuel companies in Texas, Canada, and the United Kingdom. It isn’t a distinct method of converting hydrogen to energy, but is rather a cleaned-up version of gray hydrogen. Instead of letting steam-methane reformation emit loads of CO2, blue hydrogen uses retrofitted natural gas plants with carbon capture machines to rein in the CO2 emissions from early in the steam-methane reforming process. 

But an analysis released earlier this month by Cornell University climatologists outlines three caveats that make blue hydrogen a bigger polluter than it’s marketed as. First, the authors explain, carbon capture is never 100 percent effective (the technology is still very much in development) and would leave out the byproducts of heating water through combustion. Second, it takes energy to power the machines that trap and pull emissions out of the air: The process itself could produce 25 to 39 percent of the CO2 volume it captures. And third, blue hydrogen only tackles carbon dioxide—leaving out methane gas, another heavyweight when it comes to atmospheric warming.

In all, the analysis found that blue hydrogen produces 75 percent to 82 percent of the same greenhouse gas emissions as gray hydrogen. That contrasts previous claims that it could halve gray hydrogen’s carbon footprint.

The paper also notes that both methods are far less efficient than burning natural gas directly for heat. Much of the fuel piped into US gray hydrogen facilities comes from fracking, which might be responsible for one-third of the increase in global methane emissions over the past decade.

[Related: What companies really mean when they say ‘net-zero’]

“Society needs to move away from all fossil fuels as quickly as possible, and the truly green hydrogen produced by electrolysis driven by renewable electricity can play a role. Blue hydrogen, though, provides no benefit,” the authors wrote in their conclusion. “We suggest that blue hydrogen is best viewed as a distraction, something that may delay needed action to truly decarbonize the global energy economy, in the same way that has been described for shale gas as a bridge fuel and for carbon capture and storage in general.”

So, if you look at the hydrogen rainbow through the filter of sustainability, green is the only color that shines through (with maybe a hint of pink). Other smart solutions could help fill out the spectrum in the future—but if the debate over blue hydrogen makes one point, it’s that you can’t slap a new label on a broken idea and re-sell it.

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