BOSTON — If there’s a subtext to this year’s meeting of the American Association for the Advancement of Science, the largest gathering of scientists of the year, it’s anxiety for the future.

John Holdren, the top science adviser to President Barack Obama, who spoke Friday at the conference, summed it up like this:

“I’m worried — based on early indications — that we can be in for a major shift in the culture around science and technology and its eminence in government. We appear to have a president now that resists facts that do not comport to his preferences. And that bodes ill on the Obama administration’s emphases on scientific integrity, transparency, and public access.”

Donald Trump has yet to select people for several top science jobs in the administration — such as NASA administrator, director of the Centers for Disease Control and Prevention, and director of the National Institutes of Health.

But with the appointment of Scott Pruitt to head the Environmental Protection Agency, he’s signaled that his administration will be making big changes to environmental regulation. One of the first bills he signed as president killed an Obama-era rule that made it harder for coal companies to dump waste in streams.

One of the names floated for Trump’s science adviser is Will Happer, a former Princeton physics professor who recently told ProPublica the science on global warming was “very, very shaky.” I asked Holdren if a science adviser whose opinions conflict with the scientific consensus on climate change is better than none at all. “Absolutely,” he told me. “Because somebody who knows about some domains of science and values science would still offer advice on those topics.”

“Happer is a distinguished physicist,” Holdren says. “He has views that I think are wildly wrong on climate change and immigration. And he’s not particularly diplomatic. But it would still be beneficial to have someone like Happer whispering in the president’s ear on the importance of basic research. ...”

In the meantime, Holdren offered five points of advice for the hundreds of scientists assembled:

Number 1: Don’t be discouraged or intimidated. Two: Keep doing your science ... don’t change what you do or how you think about what you do or its importance. Three: Besides your own science, become more broadly informed about science and scientific issues. Four: Tithe at least 10 percent of your time to public service ... including activism. And last: We as a community need to think carefully about how to focus and utilize our activities to try to insure the continuation, momentum, and the integrity of science in this new era.

Scientists are becoming more politically engaged in the Trump era, and it shows here at AAAS. Later in the day, Harvard science historian Naomi Oreskes got a standing ovation after speaking on how scientists can — and should — be “sentinels” for the public, and shouldn’t fear a loss of credibility for getting more politically engaged.

Soon, Congress and the president will be making another set of decisions that are worrying for scientists — the 2018 fiscal year budget. Republicans want to cut taxes while possibly expanding defense and infrastructure spending. That is costly, and it may mean cutting back on discretionary spending, including scientific research and development.

Science funding often enjoys bipartisan consensus. But in this unsure era, researchers are extra anxious to make their voices heard.