Planting trees could make a huge dent in our growing climate crisis. But we'd need to cover an area the size of the US, starting right now.

• A new study found that planting trees could help suck a sizeable amount of carbon dioxide out of the atmosphere.
• The researchers found that trees could feasibly be planted across nearly 1 billion hectares of land - an area larger than the United States - without impinging on urban or agricultural land.
• That mass reforestation could remove two-thirds of all atmospheric carbon emissions caused by human activity, the study suggests. But the world would have to start planting trees now.
• However, given forecasts of increasing greenhouse-gas emissions and the ongoing deforestation in the Amazon rainforest, environmental activists point out that addressing the climate crisis will require action far beyond planting trees.
• Visit Business Insider's homepage for more stories.

You reap what you sow, so it may be time for humans to start planting more seeds.

A recent study found that planting trees could be an effective way to address climate change - perhaps more than previously thought. If an area the size of the US were covered in new trees, the research says, that greenery could remove two-thirds of all human-made carbon dioxide from the atmosphere.

For the study, published today in the journal Science, a team of researchers at Crowther Lab in Switzerland mapped out all the land on Earth that could feasibly support a forest, then subtracted current forests along with agricultural and urban areas from the total.

According to their calculations, nearly 1 billion hectares of land on the planet could hypothetically get covered with new trees. That vast reforestation would increase the Earth's forest land by a third, and because trees naturally absorb carbon dioxide from the air, those additional forests could lower the concentration of carbon in the atmosphere.

"If we act now, this could cut carbon dioxide in the atmosphere by up to 25%, to levels last seen almost a century ago," Tom Crowther, a senior author of the study, said in a press release.

The study's authors see their research as groundwork for a global action plan.

An achievable goal

According to the study authors, 50% of the available land for new forests is in Russia, the United States, Canada, Australia, Brazil, and China.

The researchers analyzed different types of forest ecosystems to determine their natural tree cover and the amount of carbon they absorb. Overall, the study's authors estimated that the potential reforestation areas (highlighted on the map above) could store 205 metric gigatons of carbon if they get restored to the status of existing forests.

That's a tall order, though, since it takes years for new trees to grow, restore soil, encourage further plant growth, and attract the diversity of species that make up a thriving forest ecosystem.

"It will take decades for new forests to mature and achieve this potential. It is vitally important that we protect the forests that exist today, pursue other climate solutions, and continue to phase out fossil fuels from our economies in order to avoid dangerous climate change," Crowther said.

Still, his team's research reaffirms an existing goal proposed by the UN's Intergovernmental Panel on Climate Change (IPCC). In a report published in October 2018, the IPCC suggested that adding 1 billion hectares of forest globally by 2050 could help limit global warming to 1.5 degrees Celsius - the ambitious goal set in the Paris climate agreement.

Read More: Stunning photos reveal the fragility and resilience of the Earth and its animals

The new study reports that this is "undoubtedly achievable under the current climate."

The study highlights existing initiatives to plant trees across the globe like the Bonn Challenge, in which 48 participating countries are trying to restore 350 million hectares of forest by 2030. But the researchers said the Bonn Challenge isn't quite on track, given that 43% of participating countries have only committed to restore less than half of the land that they could reforest.

Planting trees could be tougher than it sounds

Planting new trees isn't all rainbows and butterflies, though.

Because forests' carbon-sequestering capacity varies significantly between regions, environments, and climates, some researchers have critiqued the new study, saying its estimate that 1 billion hectares of trees would remove 25% of atmospheric carbon is too high.

Plus, the world isn't generally heading in a forest-friendly direction. Deforestation in the Amazon soared 60% last month, with 1.5 soccer fields getting cut down every minute. About 15 million trees are cut down globally each year, and the world has lost 46% of all trees since the beginning of human civilization, according to a 2015 study.

Meanwhile, atmospheric carbon dioxide levels are the highest they've been since before the dawn of humanity. So planting trees should not be seen as the singular cure for our climate woes, as climate activist Greta Thunberg pointed out on Twitter.

"Yes, of course we need to plant as many trees as possible," she wrote. "But there's absolutely no way around stopping our emissions of greenhouse gases and leaving the fossil fuels in the ground."