As climate activist Greta Thunberg neared the United States on a boat which set sail from Britain on August 14, the global climate action movement called on adults to join in the climate strike begun in part by the 16-year-old Swedish student.

350.org co-founder Bill McKibben wrote Monday in a letter to supporters that the choice to join in the global climate strike taking place in dozens of cities on September 20 should be a simple one.

"If a kid says help, you help," said McKibben.

"The climate crisis represents an assault on justice...but also an assault on the future. A future that some have a larger share in simply because they'll be alive longer. For the rest of us—those who will die before climate change reaches its burning zenith—the strike is a chance to show that our affection for our children and their children is sincere."

—Bill McKibben, 350.orgThe climate strike will take place a year after Thunberg, who is expected to arrive in the U.S. on Wednesday, began a global mass mobilization with a one-person protest outside Swedish Parliament.

Thunberg told the world last September that she would not attend school until lawmakers in her country committed to acknowledging the climate emergency and taking concrete steps to end the burning of fossil fuels. The climate crisis is pushing the Earth closer to warming more than 1.5 degrees Celsius above pre-industrial levels—a threshold climate scientists say would cause millions to be forced from their homes due to sea level rise, 37 percent of the world population to suffer from extreme heatwaves every year, and more than 400 million to experience dangerous water scarcity.

Thunberg's action kicked off climate strikes all over the world, with an estimated 1.4 million students walking out of their classrooms to demand governments end their support and complicity in the extraction of oil, gas, and coal.

As McKibben wrote, Thunberg's "argument was that if the world's adults weren't willing to prepare the planet for her generation, they had no right to demand that her generation spend their youth preparing for the future. Kids across the planet saw the logic."

On September 20 and during the Week of Action which will follow, hundreds of thousands are expected to holding sit-ins at pipeline projects, planting trees, or attending rallies and marching in cities around the world to make a number of demands—passage of a Green New Deal, a shift to renewable energy and a just transition, and greater commitments from UN countries to cut their carbon emissions.

"We strike now, because now is our last chance; because change must occur today; because the future is written in the present," said Fridays for Future NYC in New York, where Thunberg will speak at one of the strike's biggest events. "'Business as usual' is no longer excusable. 'Middle ground' solutions are no longer justifiable. September 20th is not the end goal, but a catalyst for breaking down the status quo, and creating a just, sustainable world in its place."

Seventeen-year-old Jamie Margolin, a co-founder of Zero Hour, directly appealed to adults to join young people in the September climate strike.

"The climate crisis looms over every single decision I make, and impedes any and all plans I try to make for the future," wrote Margolin, who is helping to organize the action.

"It's time for adults to join us, and help us with this immense weight we are carrying," he added. "We the youth are calling on people of all ages to join in—and we need all of us to heed the call to action."

350.org promoted the strike on social media, calling on adults to join the mobilization after the world's children have spent a year making the case for concrete climate action. The youth movement has had some success, including a number of official climate emergency declarations by local and national governments and a commitment from the European Commission to spend $250 billion over seven years to fight the climate crisis.

"Solving the climate crisis will involve disrupting business as usual," wrote McKibben. "Adults like you and me should consider joining this walkout as a statement that they’re committed to disruptive, transformative change."

Join millions of us in the streets on #ClimateStrike 20-27 September to demand climate justice and action to end the age of fossil fuelshttps://t.co/MTIAHxZ9sN#FridaysForFuture https://t.co/5yKee48AR7 — 350 dot org (@350) August 26, 2019

So far, the Sept 20-27 Global #ClimateStrike in the US has: 250+ events

10,000+ RSVPs

Actions in 48 states + Puerto Rico Sign up today. #StrikeWithUs to demand climate justice for all: https://t.co/Rfb5zHG9UI Also, North Dakota? South Dakota? You guys coming too?pic.twitter.com/JBM60hZ0KC — 350 dot org (@350) August 26, 2019

McKibben and 350.org are devoting time and resources to help promote the strike because, McKibben wrote, "Elders need to act like elders."

"The climate crisis represents an assault on justice (those who have done the least to cause it suffer the most) but also an assault on the future," he added. "A future that some have a larger share in simply because they'll be alive longer."

"For the rest of us—those who will die before climate change reaches its burning zenith," McKibben wrote, "the strike is a chance to show that our affection for our children and their children is sincere."