Climate change will take center stage next week when a small group of concerned islanders show a documentary about the environmental crisis and invites those in the audience to join a national movement to slow its progress.

The film, “Do the Math,” features Bill McKibben, a noted author, educator and founder of the climate change movement 350.org, as he travels the country on a 21-city tour, speaks to audiences about the science of climate change and proposes strategies to avert disaster.

On Vashon, a discussion will follow the film, according to organizers Julea Lakey and Sheila Brown, who say they are hosting the event in part because they consider climate change the most significant issue of our time.

“To me, it’s the only issue in town,” Brown said, noting that she cares about many other issues as well, but this one has particular significance.

“If we lose our planet and civilization as we know it, we don’t have much to hand down to our children,” she said.

Following the film, the women say they hope to create a local group to join others affiliated with

350.org in taking on the fossil fuel industry and, to the extent possible, halting climate change.

Last November, McKibben, well known in environmental circles, kicked off his Do the Math tour to a full house at Seattle’s Benaroya Hall, where he encouraged divesting financially from fossil fuel companies.

Just weeks later, Seattle was the first city in the nation to pursue this path, when Mayor Mike McGinn directed the city to divest its funds from the the fossil fuel industry, noting this area’s vulnerability to rising seas and extreme weather.

The math behind climate change, as McKibben lays it out in the film, requires just a few numbers to tell a complicated and alarming story.

The number 350, as some know, is the amount of carbon dioxide in parts per million (ppm) that scientists say can be in the Earth’s atmosphere without catastrophic climate change. Any level above that, according to NASA scientist James Hansen, is not compatible with life as we know it.

Unfortunately, McKibben says in the film, we have surpassed that level. When the film was made last year, the CO2 level was at 395 ppm, he said, and rising at a rate of 2 ppm a year. In recent weeks, several news reports have indicated scientists have found carbon dioxide levels higher than 400 ppm.

Beyond the number 350, just three more numbers are sufficient to tell much of the rest of the story, McKibben contends. The first of these is 2 degrees Celsius, or 3.6 degrees Fahrenheit. At a 2009 climate conference in Copenhagen, representatives from a variety of countries signed an accord agreeing to take action to keep the planet’s rise in temperature below 2 degrees.

So far, McKibben said, we are nearly halfway there, with an average global temperature increase of .8 degrees. Scientists have seen results more dire than expected: Summer sea ice in the Artic has decreased by half; oceans are 30 percent more acidic and the atmosphere is 5 percent wetter. The summer of 2012 was the hottest on record, and extreme weather — fires, floods and storms — battered much of the country and other parts of the world. Currently, severe floods are wreaking havoc in Europe and taking lives in several countries.

Central to understanding the challenges ahead, McKibben explains in the film, are just two more numbers. Scientists believe humans can add 565 more gigatons of carbon dioxide to the atmosphere by the middle of the century and still have some hope of staying below a 2-degree temperature rise. However, current oil, gas and coal reserves — the fossil fuels the world is planning to burn — contains 2,795 ppm of carbon, five times the amount that scientists say could reasonably be safe for life on the planet.

“Once you know that number, then you understand the essence of the problem,” he says in the film.

Understanding the picture is grim, McKibben says.

“Humanity cannot survive this,” he said. “If they carry out their business plan, the planet tanks.”

While the film lays out the math and science of climate change, it also highlights the growing movement to try to counter it and suggests avenues for action.

Brown and Lakey say taking action is where islanders come in. While individual conservation efforts are important, what needs to happen, the women say, echoing McKibben, is that many people need to get involved. In the evening’s discussion, the women will share information on taking action, including divesting in fossil fuel companies, as McKibben recommends, working to keep the coal trains and ports out of Washington and investing in clean energy.

They stress that they also hope to hear ideas from islanders on what Vashon individuals or the new group could do.

“We’re not sure what is going to be effective in climate change because we have never faced a challenge on this level,” Lakey said.

“Do the Math” will show for free at 6 p.m. Tuesday, June 18, at the Vashon Theatre.