Mark Zuckerberg's tour of American states hit a roadblock on Saturday when he visited Montana's Glacier National Park to see the effects of global warming.

Zuckerberg was prevented from meeting with climate change expert Daniel Fagre after the U.S. Department of Interior denied Fagre's visit to the national park, Mic reported. The Facebook CEO and Fagre, a research ecologist at the United States Geological Survey, were expected to discuss the negative impacts of climate change, a topic Zuckerberg is passionate about. USGS insiders suspect the Department of the Interior canceled the trip to minimize attention on the issue of global warming.

But Zuckerberg did meet with several National Park Service Rangers on Saturday and saw the effects of global warming. "The impact of climate change is very clear at Glacier," Zuckerberg wrote in a Facebook post. "In the last hundred years, the average global temperature has risen 1.5 degrees. But in the high elevations of Montana where Glacier is, the temperature is warming at 3x the global average--enough to melt glaciers."

Zuckerberg, who is by Forbes's estimate the fifth richest person in the world, used his Harvard commencement address this year to encourage Millennials to tackle climate change. He was also one of the most prominent business leaders to denounce President Trump's decision to withdraw from the Paris climate accord in June. "Withdrawing from the Paris climate agreement is bad for the environment, bad for the economy, and it puts our children's future at risk," Zuckerberg wrote in a Facebook post in June. "Stopping climate change is something we can only do as a global community, and we have to act together before it's too late."

So far, Zuckerberg is seven months into his tour of the 30 states he's never visited and has met with local residents to learn about their lives. The goal is his personal challenge for the year, an annual tradition for the entrepreneur. That sounds like a smart way for Zuckerberg to position himself for a future presidential run, but he's insistent that's not his intention. However, the decision to cancel Zuckerberg's meeting with Fagre appears to be all about politics.

But if Zuckerberg were to run, he'd be tough competition for President Trump. Zuckerberg would scoop up 40 percent of the votes if he were the Democratic presidential nominee, according to a Tuesday poll from the Democratic firm Public Policy Polling. Trump would get the other 40 percent and the remaining 20 percent of respondents were unsure.