New study upends conventional thinking about what’s driving down carbon emissions

Study comes on the heels of a Bloomberg New Energy Finance report that highlighted a similar trend.

Emissions-producing diesel trucks and cars pass non-polluting wind turbines along the 10 freeway near Banning, California. CREDIT: David McNew/Getty Images
Emissions-producing diesel trucks and cars pass non-polluting wind turbines along the 10 freeway near Banning, California. CREDIT: David McNew/Getty Images

Breaking with conventional wisdom, renewable energy resources were as important as natural gas in driving down carbon dioxide emissions in the United States over a seven-year period beginning in 2007, according to a new peer-reviewed study.

U.S. energy-related carbon dioxide emissions dropped roughly 10 percent between 2007 and 2013. Renewables contributed between a fourth and a third of this reduction. Switching from coal to gas in the power sector and changes in energy demand played similar roles in emissions reductions over that period.

Over the period of 1990 to 2007, energy-related CO2 emissions were rising. In 2007, emissions started a sharp turn downwards and, by 2013, annual CO2 emissions had decreased by 10 percent. Over this same period, total renewable energy capacity increased significantly in the United States.

The study, “Factoring in the Forgotten Role of Renewables in CO2 Emission Trends Using Decomposition Analysis,” comes on the heels of the release of a Bloomberg New Energy Finance report, which found that in 2017 renewable energy and energy efficiency were the primary drivers behind a drop in power sector emissions, which helped drive overall U.S. greenhouse gas emissions to their lowest levels since 1991.

The study was published in Energy Policy, an international peer-reviewed journal addressing the policy implications of energy supply and use from their economic, social, planning, and environmental aspects. Authors of the study included Kristina Mohlin, a senior economist with the Environmental Defense Fund (EDF); Jonathan Camuzeaux, an energy expert in EDF’s Office of the Chief Economist; Adrian Müller, a professor in the Department of Environmental Systems Science at the Swiss Federal Institute of Technology; Marius Schneider, a consultant with the South Pole Group, a consultancy that focuses on reducing emissions; and Gernot Wagner, a consultant with the EDF’s Office of the Chief Economist.

The researchers looked at nationwide energy-related CO2 emissions, covering emissions from the residential, commercial, industrial, and transportation sectors. Most of renewables’ impact on reducing carbon emissions came from the electric power sector. But there was also a contribution from biofuels in the transportation sector, Mohlin, the lead author of the study, told ThinkProgress.

In a Monday blog post, the authors of the study wrote that their findings show past incentives to support the expansion of renewable energy have been successful in reducing U.S. emissions. Furthermore, decreasing costs for renewable energy offer hope for continued progress despite the Trump administration’s refusal to address climate change, the authors said.

Wagner, who worked with Mohlin on the study, told ThinkProgress in an email that their study “clearly complements” the BNEF report that determined 2017 was the first time renewable energy and energy conservation playing a bigger role in reducing CO2 emissions than a switch from coal to natural gas to generate electricity. BNEF researchers also found that a near-record deployment of renewable energy resources across the country, carbon emissions from the power sector dropped 4.2 percent in 2017.

Mohlin and her fellow researchers did not have a chance to look at energy output and emissions numbers for all economic sectors since 2013 for their peer-reviewed study. For the power sector alone, preliminary results indicate natural gas may have played a larger role than renewables for the decrease in CO2 emissions between 2013 and 2016, she said. But that early assessment of natural gas’ role may change once the data is released for Mohlin and her group to analyze.

Unlike other studies, the research published in Energy Policy sought to separate out different factors influencing total CO2-emissions in order to better understand the role of renewable energy. This technique, known as decomposition analysis, allowed the authors to look at each of the following individually: total energy demand, the share of natural gas in the fossil fuel mix, and the share of renewables and nuclear energy in total energy production.

Experts have generally attributed the decrease in carbon emissions to the economic recession and to a huge surge in cheap natural gas displacing coal in the U.S. energy mix. “But those same experts mostly overlooked another key factor: the parallel rise in renewable energy production from sources like wind and solar,” the authors explained in their blog post.

The study shows that during this period between 2007 and 2013, renewables decreased total CO2 emissions by 2.3 percent to 3.3 percent, roughly matching the 2.5 percent to 3.6 percent contribution from the shift away from coal to natural gas. Nuclear energy helped to reduce CO2 emissions by 0.6 percent to 1.5 percent during the  period.

With renewables playing a bigger role in the U.S. electric generation mix, researchers are spending more time analyzing their impact on reducing carbon dioxide emissions. A growing body of research is emerging that identifies the significant role played by renewable energy in reducing emissions. “As the cost of renewables continues to decrease, it becomes increasingly relevant to track their role in CO2 emission trends,” the researchers said.

Power-sector emissions are also declining compared to other parts of the U.S. economy. For decades, the power sector had been the nation’s biggest source of carbon emissions. But as emissions from the electricity sector plummeted again in 2017, transportation retained its place as the largest carbon-emitting sector for the second year in a row.

In the peer-reviewed study, the researchers emphasized that their results do not diminish the role of coal-to-natural gas switching. “However, it is important to keep in mind the role of methane leakage along the natural gas supply chain, which significantly reduces the net climate benefits of natural gas and the magnitude of which has previously been underestimated,” the researchers wrote.