The number of U.S. patents granted for clean energy technologies has dropped over the past couple of years, after more than a decade of growth.

Those findings come from a new report by The Brookings Institution. According to the think tank, while the total number of cleantech patents granted by the U.S. Patent and Trademark Office doubled between 2001 and 2014, growing at a rate of 7 percent annually, the number of U.S. cleantech patents granted between 2014 and 2016 actually declined by 9 percent each year.

Over the same period, the number of U.S. cleantech patents owned by foreign companies grew. Japanese, South Korean, and German companies dominated patenting for certain cleantech categories, like transportation and energy storage, between 2011 and 2016.

It’s too early to tell whether these trends are temporary or are actually here to stay. But the data “raises concerns about the long-term competitiveness of the U.S. cleantech sector,” said Devashree Saha, senior policy associate for Brookings' Metropolitan Policy Program. Saha worked on the report.

“This is a picture of momentum that has begun to potentially drift. We’re not ready to say the sector has lost its mojo, but there are now questions about it,” said Mark Muro, senior fellow and policy director for Brookings' Metropolitan Policy Program, who also worked on the report.

The trend could be a result of funding declines in the wake of the stimulus, which saw the Obama administration invest tens of billions of dollars into clean energy infrastructure projects and R&D between 2009 and 2014.

At the same time, many venture capitalists who invested in cleantech startups in the early and mid-2000s lost money and ultimately pulled back from the sector. There are a few cleantech investors still around (and even some new ones), but not anywhere close to the amount a decade ago. That could account for a smaller portion of the drop in patents from cleantech startups and entrepreneurs.

Now the Trump administration wants to slash budgets for clean energy. A drastic change to federal funding “could make this flattening a more permanent downward trend in the next few years,” Saha said.

Trump’s “skinny” budget proposal would eliminate ARPA-E, the federal government’s energy innovation arm. “There are critical [federal] programs that have major impacts on these industries and ecosystems,” said Muro. "There’s a clear need to sustain the surge” in cleantech innovation.  

“In the last decade the country has developed a pretty solid, effective platform for supporting clean and low-carbon technology development, so it would be a shame to lose that momentum,” said Muro.

States and the private sector could pick up some of the slack. But many agree that fundamental energy research and development is best done by the federal government.

The Brookings report lays out a few recommendations for Congress. They include continued support for the Department of Energy’s Lab to Market initiative and the ARPA-E program, as well as the 17 national laboratories in the U.S.

“Congress has the opportunity now to come together around a short list of minimum viable supports for cleantech innovation and growth," the report concludes.

Muro said there’s also a need for the private sector to argue more forcefully in favor of government research and development in order to provide a funnel of innovation for groups like Breakthrough Energy Ventures, founded by Bill Gates.

At Bloomberg’s global energy summit in New York this week, Michael Liebreich noted that global clean energy investing across the board (including project finance and tech investing) was down 17 percent compared to last year. That’s partly because of lower prices for solar and wind.

But outside of the U.S., many countries continue to invest heavily in energy research, development and deployment. China invested $17.9 billion into clean energy in the first quarter of 2017, while the U.S. invested $9.4 billion.