A recent tree-planting campaign started by a YouTube personality set an ambitious goal: raise $20 million to plant 20 million trees by Jan. 1, 2020.

The project, known as #TeamTrees, offered the kind of internet-savvy effort that tends to achieve some virality. Silicon Valley heavyweights including YouTube CEO Susan Wojcicki, Twitter CEO Jack Dorsey and Tesla CEO Elon Musk, have already pushed the fundraiser past $14 million.

But while reforestation efforts have long been held up as a key way to help mitigate the effects of climate change, new research is showing that the scientific benefits of widespread tree-planting campaigns may be murkier than scientists originally thought.

“This notion challenges conventional wisdom and can be a difficult truth, because many of us — myself included — have an affinity for forests and think of forests as healthy landscapes,” said Christopher Williams, a professor of geography at Clark University in Worcester, Massachusetts.

The fundraiser is also a mixed blessing to environmentalists. It’s heartening to see the general public and notable figures get behind climate efforts, but broader changes and policies need to be enacted in order to stop climate change, said Peter Ellis, a forest ecologist at The Nature Conservancy, an environmental nonprofit based in Arlington, Virginia.

“Reforestation needs to be part of the solution if we’re going to succeed, but we need to understand that trees everywhere isn’t always a good thing,” said Ellis, who co-authored a key 2017 study published in the Proceedings of the National Academy of Sciences about the benefits of reforestation and other natural climate solutions.

Nature’s filter

Forests have been likened to the planet’s lungs, because similar to how the vital organs absorb oxygen and exhale carbon dioxide, trees absorb carbon dioxide from the atmosphere and breathe out oxygen. As such, forests play a crucial role in removing carbon dioxide, a harmful greenhouse gas that drives global warming, from the air.

This natural process is also why tree-planting and other reforestation efforts have long been heralded as important — and natural — ways to offset rising carbon emissions and fight climate change. And these campaigns have proven to be a popular way to motivate the public to take action.

But emerging research suggests that the effectiveness of tree-planting campaigns can vary, and that the impacts of forests on the climate system are complex, ranging from how changing landscapes can alter the delicate balances that exist in many ecosystems to greenhouse gases, such as methane, that can be emitted from trees themselves. In many cases, the long-term implications of these effects are still unknown.

“The key word is uncertainty,” said Kristofer Covey, an ecologist at Skidmore College in Saratoga Springs, New York, who has conducted extensive research on methane emissions from forests in the northeastern United States. “It’s hard to tease out what we should be doing right now, and I would be very hesitant to upset the apple cart based on what we have. That said, we need to start accounting for these things if we’re going to lead successful campaigns with land management.”

In 2013, a British ecologist named Sunitha Pangala journeyed to the Amazon armed with sensors that she attached to more than 2,000 trees to measure emissions of methane. She found that these trees — particularly in parts of the forest that seasonally flood and become waterlogged — were to blame for approximately half of the Amazon’s total methane emissions, with every 100,000 square feet of tropical wetland releasing several pounds of methane each day.

“The methane was moving from soil through trees, and the trees were acting like straws,” Covey said. “That’s a huge effect, and it’s not accounted for.”

Pangala’s findings were published in 2017, and though much more research is needed, Covey said there is evidence that in some areas, the warming effects from methane emissions could offset a forest’s ability to store carbon dioxide. This is especially true, he added, for tropical wetlands.

Potential drawbacks

In addition to methane, trees can emit what are known as volatile organic compounds, which are themselves not greenhouse gases, but can interact with other gases in the atmosphere to cause ozone and photochemical smog.

These compounds, which evolved as a stress response in plants and trees, are still being actively studied, but some research has suggested that they are more common with certain species — namely, pine trees. Though the vast majority of greenhouse gas emissions come from burning fossil fuels, the impact of volatile organic compounds on global climate change is still being actively debated in the scientific community.

“There’s still a lot that we don’t know, which is why there’s a certain level of inherent risk with reforestation,” said Benjamin Poulter, a research scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

Poulter co-authored a commentary published Oct. 18 in the journal Science about a recent study on the potential of global tree restoration as a solution for climate change. That study claimed that new forests could remove 205 gigatonnes of carbon dioxide from the atmosphere, making it “our most effective climate change solution to date.”

Poulter and his colleagues argued that the study overestimated that potential and overlooked some negative consequences that reforestation efforts can have. For one, adding trees in certain regions can change how that land absorbs or reflects energy from the sun. At high latitudes, such as in parts of Canada and Siberia, snow-covered ground is more reflective than darker, tree-covered areas.

“The concern is if you start planting trees where you have snow, you’re changing the color of the land surface and making it darker,” Poulter said. “Dark surfaces absorb more energy than lighter surfaces, so you’re actually going to warm the environment.”

An arsenal of climate solutions

Still, scientists say there is no question that trees harbor enormous potential for storing carbon, and that preventing deforestation has demonstrated benefits for the environment. Protecting and restoring forests could reduce global emissions by 18 percent by 2030, according to a 2018 report released by the United Nation’s Intergovernmental Panel on Climate Change.

But Chris Field, director of the Woods Institute for the Environment at Stanford University, said reforestation efforts can be complicated, and sometimes areas with the greatest potential for such natural climate solutions are also places where there are weak institutions or governments in place to enact such policies.

“You can’t just go into an area where they are having a civil war and plant a bunch of trees,” Field said. “When you see these really optimistic numbers about what natural climate solutions can contribute, it’s important to recognize that that’s a theoretical potential. A lot of complicated, hard-to-change things would need to occur before we get close to realizing that potential.”

There is scientific consensus, however, that the surest way to fight climate change is by tackling the root of the problem: reducing the amount of carbon dioxide and other greenhouse gases that are released into the atmosphere.

“No matter how large our reforestation efforts may be, what’s more important is that we wean ourselves off fossil fuel use,” Williams, of Clark University, said. “We shouldn’t see this, or any other nature-based climate change solution, as a silver bullet.”