A new PNAS study questions research behind 100-percent renewable energy commitments led by Mark Jacobson.
100-percent daisy power, please. Charlie Riedel/AP Photo

As cities vow to meet the Paris climate commitments, rival researchers debate the true costs of going 100-percent renewable.

From San Diego, California, to St. Petersburg, Florida, dozens of U.S. cities are working rapidly toward 100-percent renewable energy targets, doubling down on solar and wind installations, investing in electric bus fleets, and swapping out streetlights with LEDs.

But a new study is questioning the scientific foundations of such “100-percent” commitments—and sparking heated debate.

On Monday, the Proceedings of the National Academy of Sciences published a thorough take-down of two influential 2015 studies. Those studies—authored by Mark Z. Jacobson, a professor of civil and environmental engineering at Stanford University—argued that 100-percent renewable energy is technologically attainable for the U.S., and more affordable than it sounds.

The new paper is co-authored by Christopher Clack, a former research scientist at the National Oceanic and Atmospheric Administration and founder of the energy software firm Vibrant Clean Energy LLC, plus 21 engineers and energy policy scholars. It claims Jacobson’s 2015 analyses contain “errors, inappropriate methods, and implausible assumptions.” Clack and his co-authors ultimately argue that a “broader portfolio” of energy resources would be a better, cheaper pathway towards decarbonization.

PNAS also published a retort from Jacobson, who counters every claim Clack et al make; he writes that their analysis is “riddled with errors and has no impact” on his original conclusions. (A bitter blow-by-blow also unfolded on Twitter; yes, Mark Ruffalo was involved.)

So who’s right? And what energy mix is best for the country? These aren’t simple answers to parse.

Jacobson’s 2015 papers were somewhat audacious in their claims. They charted a solution for renewable energy’s oft-cited intermittency and reliability problems through nationwide investments in hydroelectric generators, low-cost underground and water-based energy storage systems, and programs that incentivize utility customers to spread out their energy use. Crucially, Jacobson demonstrated that the technology exists for the U.S. to completely transition away from fossil fuels, without expanding nuclear power, biofuels, or carbon capture and storage—technologies that shrink carbon emissions relative to coal and gas, but carry greater environmental footprints and risks than renewables.

In a follow-up study, he charted separate “roadmaps” to take all 50 states to the point where “every driver in America rides an electric car, every stove in every house and restaurant cooks with electricity instead of gas, [and] every plane flies by cryogenic hydrogen,” as my former colleague Julian Spector wrote in 2015. That level of technological changeover sounds incredibly expensive. But once you account for billions of dollars saved through reduced air pollution, averted natural disasters, and un-lost lives, Jacobson’s research claimed that renewables would be cheaper for society on the whole.

Clack’s crew mainly takes issue with Jacobson’s underlying assumptions, and say that going all-in on renewables would be far more expensive than Jacobson suggests. For example, the paper states that Jacobson overestimates the country’s potential hydropower output; Jacobson counters that his analysis assumes the addition of turbines to increase discharge. Clack asserts that Jacobson is wrong to assume geothermal storage could take hold across the U.S. quickly and cheaply; Jacobson says, hey, Denmark did it. Clack says it’s unreasonable for Jacobson to expect that industrial consumers would respond to incentives to cut peak energy demands; Jacobson points to studies that argue “demand response” programs could be a lucrative enterprise for them.

“This [first] study shouldn’t be held up as beacon,” Clack says over the phone. “We should be bringing all of our tools to bear on this problem.” He adds that if  the costs of a 100 percent renewable target aren’t reasonable at the national level, it shouldn’t be taken as gospel for cities, which have less economy of scale to gain cost efficiencies. His paper points to nuclear energy, bioenergy, and carbon capture and storage as tools the U.S. should deploy, in addition to renewables.

But even Clack’s paper acknowledges that shooting for 100 percent is technically feasible. What it ultimately takes issue with is Jacobson’s assertion that it is the best approach, considering the costs—and that is question that draws input from far beyond the domain of science. Ethicists and religious leaders weigh in on these questions, particularly around nuclear power. Nuclear has become a shibboleth dividing scientists and policy makers working to determine America’s climate future, and with different sorts of interests.

To wit: On Clack’s paper, two of the 22 co-authors direct UC San Diego’s Deep Decarbonization Institute, which advocates for the expansion of nuclear energy. Another two authors received funding from, or consulted for, major oil companies. Most of the others have published research that supports an energy mix that includes nuclear power, bioenergy, and carbon capture from burning natural gas or coal—“technologies we exclude [in the analysis] for good reason,” writes Jacobson in an email, citing “well-documented” environmental risks and impacts.

Who is right, and who is wrong? “There is no one true answer to whether a given approach is going to be the best approach,” says Alex Steffen, a noted climate author, speaker, and futurist. “When we're talking about researchers who are both technically accomplished and are doing good work, the balance of credibility has a lot more to do with whether or not you'd believe their underlying assumptions.”

No one disagrees that 100-percent renewable is technologically attainable. The will of elected leaders and market realities obviously can’t be discounted, but these dynamics are defying expectations lately. Witness the growth of renewables’ generating capacity, which has far outpaced predictions from just five years ago. This debate comes as the U.S. renewable energy market hit a major milestone—10 percent of the nation’s electricity generation came from wind and solar in March, a record.

Plus, the political calculus of environmental progressivism has shifted dramatically and fast. Some 250 mayors, including a fair number of Republicans, have pledged to defy President Trump’s withdrawal from the Paris accord and stick to its carbon commitments. More than 30 cities have pledged to transition to renewable energy within decades through the Sierra Club’s Ready for 100 campaign, and several have already shown it is possible.

The complicated reality behind this show of force is that there might not be a single “correct“ strategy to achieve the common goal of decarbonizing the grid. It is a highly politicized science in the U.S. Energy is a charged field. Healthy skepticism and rigorous analysis is crucial on all sides of the debate as the country scrambles to cut emissions—now without the support of the federal government.

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