There was a flurry of activity at CU Boulder despite the uncharacteristically warm weather Sunday night.

Bill McKibben, an American climate activist, author and journalist was in town for his Do the Math Tour. He’s the founder of the grassroots climate campaign 350.org.

In his traditionally accessible style, McKibben presented three numbers that might add up to global catastrophe. The numbers, the focal point of his recent Rolling Stone article, “Global Warming’s Terrifying New Math,” are central to his presentation as he tours the country to raise awareness about climate change, encouraging people to take action.

McKibben makes clear that “it’s simple math: we can burn less than 565 more gigatons of carbon dioxide and stay below 2°C of warming—anything more than that risks catastrophe for life on earth.”

“But, there’s a problem. Fossil fuel corporations now have 2,795 gigatons in their reserves, five times the safe amount,” according to McKibben. Even if fossil fuel corporations burn a third of this amount, humanity will be pushed to the brink of catastrophe, he continued.

As the UN Framework Convention on Climate Change negotiations were taking place in Doha, Qatar, over one thousand people were listening to McKibben in the Glenn Miller Ballroom at CU where he focused the audience toward the ‘real enemy’, the fossil fuel industry.

His strategy for change is straightforward: divestment.

McKibben’s plan is centered on the concept of divestment—removing resources from companies doing something that people disagree with. In other words, he’s advocating that everyone withdraw consumer and financial support from the companies extracting and using hydrocarbons from the Earth.

The divestment strategy was successful in pressuring South Africa to end apartheid in the 1990s. McKibben is refocusing this strategy to target environmentalists’ biggest foe: Big Oil.

McKibben implored the crowd, with the gentle push of a Protestant minister, to target university financial portfolios, specifically their investments in the fossil fuel industry, hence his appearances on campuses across the country.

He demands that our alma maters divest their holdings in oil, gas, coal or any other fossil fuel related industries.

McKibben believes action is more and more urgent as data confirms the looming rise in temperature, while films, such as James Balog’s Oscar nominated Chasing Ice, depict undeniable evidence of our changing planet.

The Earth’s temperature has been fairly stable for over two billion years, but in the last two hundred years human activities have affected the Earth’s natural cycles. Since the beginning of the Industrial Revolution, atmospheric concentrations of carbon dioxide have increased by over 25 percent as a result of human activities.

Human-induced greenhouse gas emissions and land modifications (soil erosion, desertification and deforestation) have been the biggest culprits.

To put it into perspective, for 160,000 years before the Industrial Revolution, carbon dioxide levels never rose above atmospheric concentrations of 280 parts per million (ppm). By the 1990s, concentrations had increased to 350 ppm. Today, we are approaching 400 ppm and the prospects do not look good for people or the planet.

Human-induced climate change is putting the Earth’s populations and its ecosystems in a vulnerable position.

Climate change has already led to more frequent and unbearable droughts, increased hurricanes, windstorms, tornadoes and floods, some of which are inundating coastal cities. One only need think of the damage resulting from Hurricane Sandy to comprehend the challenges that lie ahead.

McKibben does believe these challenges are surmountable if we take action now by encouraging divestment in fossil fuels and turn toward promoting investment in clean energy, such as solar and wind. But, he leaves the action to us.

(McKibben is traveling across the United States as a part of his Do the Math Tour. His stop in Boulder was one of a couple of dozen occurring nation-wide. Winona LaDuke, a Native American activist, environmentalist and writer joined him. Josh Fox, the Creator of the documentary ‘Gasland’ was also present.)