Could more wind power raise temperatures? It’s complicated

Source: Christa Marshall, E&E News reporter • Posted: Friday, October 5, 2018

A scenario involving the United States getting 100 percent of its electricity from renewables could require more land than previously thought and — in the case of wind — raise surface air temperatures around the country, according to two new studies.

The counterintuitive findings from Harvard University researchers add to an ongoing debate about the role of renewables in addressing climate change. They also stirred broad disagreement and anger among analysts, with one calling the wind temperature study published in Joule “misleading to say the least” and others calling it “robust.”

The Joule paper builds on earlier research finding that wind turbines can raise local temperatures by redistributing heat in the atmosphere with their spinning power. While the study looks at a scenario not likely anytime soon — 100 percent of U.S. electricity powered by wind — a smaller warming phenomenon could still occur with other percentages of the resource, according to the authors.

“Wind beats coal by any environmental measure, but that doesn’t mean that its impacts are negligible,” said David Keith, the study’s co-author and an engineering and public policy professor at Harvard. Keith is known for his work on carbon dioxide air capture systems, including as chief scientist for the company Carbon Engineering.

The study fills gaps in prior estimates, by modeling high levels of wind power in a single country and comparing simulations to actual recent observations of warming at U.S. wind farms, Keith said.

Keith and his postdoctoral Harvard student, Lee Miller, found that a 100 percent wind scenario in the continental United States, which would require about a third of the nation’s landmass, could warm annual surface temperatures about 0.24 degree Celsius on average. Over areas with wind farms, temperatures could increase more than half a degree Celsius, they said.

The reason is that turbines mix the air near the surface, redistributing heat. The warming effect is much stronger at night, they say, when turbines continue bringing warmer air toward the surface after the sun sets.

“During the day, the warming effect from wind turbines is small because there is already a lot of mixing between the surface and the atmosphere,” Miller said. “At night, though, surface-atmosphere exchange is often quite weak,” he said.

Surface temperatures at night were as much as 1.5 degrees Celsius warmer in the simulations.

Several earlier papers documented a similar phenomenon. A 2012 study in Nature Climate Changefound a local warming impact in Texas from turbines, based on satellite data.

Miller said temperatures could still increase even if wind provided a smaller percentage of electricity in the future.

“The climate impact per unit of energy generation would stay about the same,” Miller said. That would mean that 50 percent wind could increase surface temperatures about 0.12 degree, he said.

‘Misleading’ and ‘wholly inappropriate’

Critics of the paper said they did not disagree with the numbers but cautioned that it should not be interpreted to mean that wind causes climate change, a global phenomenon.

Jeff Freedman, a research associate at Atmospheric Sciences Research Center at University of Albany who made the “misleading” remark, didn’t dispute the numbers but emphasized that it’s a local effect, whereas the effects of climate change are driven by global emissions from fossil fuels. He compared the phenomenon to how farmers sometimes use fans to redistribute heat and keep frost off their crops.

“If somebody asks me if the deployment of wind turbines contributes to climate change, I say no. It will distribute the temperature profile on certain nights,” he said.

Similarly, Steven Sherwood, a professor at the University of New South Wales, said in an email that the research was a useful calculation but showed that warming is concentrated around wind farms and would not be centered on major cities. It wouldn’t drive impacts like melting of ice sheets or raised sea levels.

“Wind does not contribute to ocean acidification. So I would characterize these impacts as extremely minor compared to those of the greenhouse gas emissions they would displace. Also, if wind farms were placed over oceans, the impacts would be even less,” Sherwood said.

The American Wind Energy Association released a statement through Michael Goggin at Grid Strategies LLC saying the comparison to the global warming impact of carbon emissions is “wholly inappropriate,” considering that the study is discussing redistributing heat. The modeling is unrealistic in assuming that wind turbine heights would be uniform, according to AWEA.

“The conclusions reached in this study are based on an unrealistic scenario for a future U.S. electricity mix. The study’s assumed level of wind development is 2.4 times larger than forecast wind deployment by 2050 from the Department of Energy,” Goggin said. Wind currently supplies about 6 percent of U.S. electricity.

Others praised the paper, though, and said it is important to have more discussion and research on how potential high levels of renewables would interact with the atmosphere.

Somnath Baidya Roy, an associate professor at the Indian Institute of Technology, Delhi, who worked on the earlier Texas paper, called the analysis “robust.”

“Miller and Keith are very detail-oriented and do rigorous work. The comparison of climate impacts to climate benefits part of the paper will open a new area of inquiry,” Roy said.

The team considered the benefits of reducing greenhouse gas emissions with wind and moving away from fossil fuels. Those effects, however, take time to accumulate and are affected by other global emissions around the world, meaning it would take about a century to offset the local warming, according to the model.

Miller said per unit of energy generation, solar has about 10 times less warming impact than wind. But he and Keith said the study shouldn’t be interpreted as meaning one resource is better than the other.

Wind farms, for example, are not as dense as solar farms, and land between wind turbines could be used for agriculture, according to Miller. Moving wind turbines closer together or running them less at night could minimize the warming effect, he said.

“I would hope that this would inspire further research on the topic … allow us to step back a bit. There’s not a free lunch here,” Miller said.

The second paper, released in Environmental Research Letters today from Miller and Keith, concluded that the transition to wind or solar power in the United States would require five to 20 times more land area than previously thought.

That’s because the “average power density,” or the rate of energy generation divided by the area of a wind generator, was about 100 times lower than some estimates, they said.

Their research was funded by the Fund for Innovative Climate and Energy Research.