On Monday, a new paper broke open the academic fight over how the United States can get to a 100 percent renewable grid. It’s a re-analysis and rebuttal of a study, which is the academic equivalent of starting a brawl. Also on Monday, the author of original study came out swinging with an academic takedown of the rebuttal.

There is no doubt that we can eventually hit a 100 percent renewable grid, the debate is whether we need more than solar, wind, and hydro power to get there. Christopher Clack, an environmental mathematician at University of Colorado-Boulder and lead author of the new paper in the Proceedings of the National Academy of Sciences attacks a Stanford University paper for its main hypothesis of a 100 percent renewable energy grid. The debate has been going since 2015, and this is just the latest chapter. In the midst of all the fighting, the scientists seem to agree that we can get to 80 percent renewable by 2050. It’s getting that last 20 percent and whether we keep nuclear energy that forms the real sticking point.

“It is important to understand the distinction between physical possibility and feasibility in the real world,” Clack writes. “There is a difference between presenting such visions as thought experiments and asserting, as the authors do, that rapid and complete conversion to an almost 100% wind, solar, and hydroelectric power system is feasible with little downside.”

In 2015, a paper was published by Mark Jacobson, an environmental engineer at Stanford University, which stated that the U.S. could get to a totally green grid by 2050 to 2055 without having to use alternative fuels like biofuel, nuclear, or batteries. At the time, it was criticized for cutting out possibilities like nuclear energy or trade-offs between a renewable grid and helping to manage climate change. Jacobson wrote a rebuttal of this criticism, saying that the criticism shows how other studies failed to measure “true nuclear and carbon capture costs.” Other analyses have suggested that we will need all of our current technology to hit eighty percent decarbonization, which is needed to keep global warming at the level set by the Paris climate agreement.

“It is one thing to explore the potential use of technologies in a clearly caveated hypothetical analysis; it is quite another to claim that a model using these technologies at an unprecedented scale conclusively shows the feasibility and reliability of the modeled energy system implemented by midcentury,” Clack and his team write in the academic version of throwing down.

Essentially, Clack says that Jacobson’s paper doesn’t analyze the costs of the transition and rely too heavily on scaling up existing renewable technology too quickly. In particular, Jacobson recommends installing 156,000 wind turbines off the coasts of the U.S. by 2050, which is 52 times more offshore wind turbines than have been installed in Europe over the last 20 years. One of the main problems Clack has, is that Jacobson uses the estimated impact of a nuclear war every 30 years as part of the data to de-incentivize the use of nuclear power. And importantly, in 2016 the U.S. relied on nuclear power for 19.7 percent of our energy generation.

In Jacobson’s published response to Clack he calls the analysis of his study is full of “inaccurate claims.” He also writes that Clack is ignoring the social costs of biofuels and nuclear power in order to decrease carbon alone. Despite the criticism, Jacobson ends his rebuttal by stating that Clack’s paper has no impact on his work and that the United States can reach a 100 percent renewable grid using just solar, wind, and water energy. “In sum, Clack et al.’s analysis is riddled with errors and has no impact on [my] conclusions,” he writes.

However, in Clack’s analysis and the analysis of the international Deep Decarbonization Pathways Project have found that without things like nuclear, carbon capture and storage, and bioenergy it will be impossible to get to a grid with zero emissions. It’s too early to know if Clack or Jacobson are correct, but the fight between major clean-energy experts itself suggests that there are perhaps multiple ways towards a renewable future. We are just going to have to decide how much we want to find a way to replace nuclear energy.