Last Tuesday morning the world’s most important teenager was in the middle of the Atlantic Ocean, northwest of Flores Island in the Azores, making 12 knots to the west, 2,138 miles by line of sight from her destination of New York.

If you have not heard of Greta Thunberg yet, you will when she steps off a 60-foot, solar- and wind-powered Malizia II racing yacht and onto the streets of New York after making a carbon-free ocean crossing. On September 23, Thunberg will speak at the United Nations Climate Action Summit, at the invitation of UN Secretary-General António Guterres.

During the last year the 16-year-old Swede has become the world’s most effective spokesperson for global action on climate change. Starting in Europe but now spreading around the world, Thunberg organized Fridays for Climate, school strikes in which students don’t attend classes but take part in demonstrations to demand actions to reduce climate change.

Thunberg doesn’t pull punches. When she spoke to the world’s elite at the World Economic Forum in Davos, Switzerland, in January, she roasted them. “Some people say that the climate crisis is something that we will have created, but that is not true, because if everyone is guilty, then no one is to blame. And someone is to blame,” Thunberg told them. “And I think many of you here today belong to that group of people.”

“I think it’s very insane and weird that people come here in private jets to discuss climate change. It’s not reasonable,” Thunberg told CNN. She took a 32-hour train ride from her home to Davos and camped out in a tent in freezing weather rather than stay in a luxury hotel. She doesn’t fly. Hence her trip by boat to the UN.

When Thunberg speaks, she commands the room like no adult can. She urges the people who control the planet’s future to pay attention to the message of climate scientists and the global carbon budget.

One of the reasons she’s effective is she gets her climate science exactly right. In her April 23 speech to the United Kingdom Parliament, Thunberg said, “Around the year 2030, 10 years, 252 days and 10 hours away from now, we will be in a position where we set off an irreversible chain reaction beyond human control.”

But few people in positions to do anything about it even know about the global carbon budget. “Did you hear what I just said?” Thunberg asks. “Is my English OK? Is the microphone on? Because I’m beginning to wonder.”

The latest Intergovernmental Panel on Climate Change report calculates that to have a 66 percent chance of keeping global average temperatures from rising more than 1.5 degrees Celsius (2.7 degrees Fahrenheit), humans can release no more than 115 billion metric tons (gigatons) of carbon in the form of carbon dioxide between 2018 and 2100. Considering that the burning of fossil fuels, clearing forests and the degrading soils combined now release nearly 12 gigatons per year, Thunberg’s numbers are on the mark.

The 1.5 degree C increase was identified by the multinational Paris Agreement on climate as the maximum allowable temperature increase needed to avoid the most catastrophic climate disruptions. But the situation is even more dire than Thunberg says. The timeline is likely closer to six years. Because incorporating potential accelerated releases from natural systems—like the thawing Arctic permafrost or the increasingly fire-prone Amazon rain forests that my Woods Hole Research Center colleagues and I work on—only speeds up this already short timeline.

The other reason Thunberg is effective is that she speaks—in the voice of an informed but terrified 16-year-old—for future generations. “I know many of you don’t want to listen to us—you say we are just children.…In the year 2030 I will be 26 years old. My little sister Beata will be 23. Just like many of your own children or grandchildren. That is a great age, we have been told. When you have all of your life ahead of you. But I am not so sure it will be that great for us.”

“You lied to us. You gave us false hope. You told us that the future was something to look forward to,” she told the MPs in London.

Thunberg will spend time in the United States and then negotiate low-carbon, ground-based travel to the UN Climate Change meeting that starts December 2 in Santiago, Chile.

Thunberg is right to focus on the Earth’s carbon budget and she’s right to be scared. Fossil fuel companies sit on the equivalent of at least 760 gigatons of carbon in potentially burnable fossil fuels worth upward of $26 trillion. Leaving the vast majority of that in the ground is now humanity’s greatest challenge.

“[The] future was sold, so that a small number of people could make unimaginable amounts of money,” Thunberg says.

The way I look at it, she’s now trying to buy that future back through sheer force of moral will.

Overwhelming and consistent climate science that connects carbon dioxide emissions to rising temperatures and higher temperatures to disruptive climate change has not spurred nations of the world to reduce fossil fuel emissions, which reached an all-time high in 2018. Maybe a scared teenager with more than one million followers on Twitter can do what adults have not.

Your microphone is on, Greta. Keep talking.

Mr. Neill is a senior scientist at the Woods Hole Research Center and a member of the Falmouth Water Stewards.

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