Climate change may be a controversial subject to many, an urgent one to others, and an afterthought for everyone else, but of those people thinking about it — or trying not to think about it — it’s typically adults having the conversation.

And though children aren’t normally the face of climate change action, that’s exactly why a then-nine-year-old Felix Finkbeiner of Germany chose to do something about it. It’s easy to talk, it’s easy to argue, and it’s even easier to just turn off your brain and live in your bubble, but it takes a lot of work to put words into actions. So, feeling like many adults were only talking about climate change issues instead of taking action, Felix felt obligated to step up to the plate.

“We children know adults know the challenges and they know the solutions,” he said at the United Nations General Assembly. “We don’t know why there is so little action.”

He cites the idea of differing perspectives on the meaning of the word “future” as one possible explanation.

“For most adults, it’s an academic question. For many of us children, it’s a question of survival,” he said. “Twenty-one hundred is still in our lifetime.”

Another reason could be climate denial, and the last explanation proved especially insightful.

“If you let a monkey choose if he wants one banana now or six bananas later, the monkey will always chose the one banana now,” he said. “From this, we children understood we cannot trust that adults alone will save our future. To do that, we have to take our future in our hands.”

At the time of his speech, Finkbeiner had been four years into heading an environmental cause that has now become a global network of children activists fighting to slow down Earth’s warming by reforesting the planet.

N0w 19, Finkbeiner’s Plant-for-the-Planet, along with the UN’s Billion Tree campaign, has planted more than 14 billion trees in more than 130 nations. And the group has even urged for the planting goal to increase to one trillion trees, or 150 trees for every person on Earth.

This environmentalist urgency was sparked in Finkbeiner during a fourth grade school assignment, when he was researching how climate change threatened polar bears, his favourite animal. When browsing Google, he came across information on the Kenyan activist Wangari Maathai who, over 30 years, planted 30 million trees in Africa, and ultimately won the Nobel Prize in 2004. Felix then wrote a report about trees, expressing his desire to plant one million of them in Germany.

The movement took off from there, with Finkbeiner speaking to the UN and the European Parliament about his goal. He kept planting trees, and his goals kept getting bigger. His organization gained so much support, it even caused the first scientific, full-scale global tree count, which is giving NASA more insight for its ongoing study of forests’ abilities to store carbon dioxide and protect the Earth.

“Felix is a combination of inspirational and articulate,” notes Thomas Crowther, an ecologist who managed the tree count.  “A lot of people are good at one of those things. Felix is really good at both.”

The tree study commenced as Plant-for-the-Planet’s ambitions expanded. And as bigger projects came to be, more questions surfaced. Were the 14 billion trees planted making any difference? Would the reforestation on the Yucatan Peninsula in Mexico — which aims to plant 10 million trees — make a difference? There were a lot of questions, especially whether or not all that planting can keep up with the continuing deforestation around the globe.

Crowther and his team at Yale wanted answers, and so the two-year study, published in Nature in 2015, came to be. The study found that Earth has 3 trillion trees, which is seven times the number of previous estimates, and that since the dawn of agriculture dating back 12,000 years ago, this number has fallen by almost half. About 10 billion trees are lost every year, so planting a billion may be a positive thing, but according to such findings, it’s still not nearly enough.