Since humans came out of the trees and started chopping them down to make fire and houses, we’ve removed about 46 percent of the world’s forests. That’s a tragedy for bird habitats and aesthetics alike, but it turns out it’s also a tragedy for humankind.

Forests — along with other plants, soil and oceans — serve as carbon sinks, places where the planet-warming carbon emitted by humans is absorbed and neutralized. In 2018, it was estimated that humans produced about 37 billion tons of carbon dioxide from burning fossil fuels, an all-time high … and an increase of about 60 percent since 1990. Historically, about half of those emissions stayed in the atmosphere while the other half was absorbed into forests and oceans. And it turns out that despite humanity’s skyrocketing emissions …

The Northern Hemisphere’s natural carbon sinks have kept pace with humanity’s rising emissions by absorbing increasing amounts of carbon dioxide.

That’s according to measurements published in Nature in April, which called such sinks “a 50 percent discount on climate change.” The Northern Hemisphere, home to two-thirds of the Earth’s land and vegetation, is where most of the developed countries that lead in carbon emissions, like the U.S. and China, are located. By comparison, the surface of the Southern Hemisphere is more than 80 percent water; thus its carbon sinks are mostly oceans rather than forests. It’s not that the forests have gotten bigger, it’s that they’ve gotten better at absorbing carbon … and they’re keeping pace with humanity’s emissions hadn’t been studied in depth before now.

Lead study author Philippe Ciais, of the Laboratory for Sciences of Climate and Environment, in France, explains that the research has filled in gaps on how carbon uptake has evolved over the past few decades, as carbon emissions have risen sharply. It turns out, he says, that forests may be the key.

Higher temperatures could mean longer growing seasons but also longer summers, which tend to bring an uptick in tree-killing phenomena like droughts and fires.

“Forests have more absorption capacity than grasslands,” Ciais explains. “Once forests absorb carbon dioxide by way of photosynthesis, they can store it in their trunk or branches, in addition to storing it in the soil. But grasslands can only store carbon in the soil.” Ocean sinks, which dominate the carbon sink ecosystem in the Southern Hemisphere, have further drawbacks: As they absorb more carbon dioxide, ocean waters undergo acidification processes that are often deadly to marine life.

Patrick Megonigal, a soil and carbon dioxide scientist at the Smithsonian Environmental Research Center, has conducted studies on small parcels of land over the past three decades. He found that when carbon dioxide levels are elevated, vegetation sees higher levels of growth. And that’s stepped up, faster than the less efficient soil sink: “Over the years, soil sink has reduced, and CO2 emissions have increased,” he says. “So this increase in land sink has been possible because plants have made up for the soil portion and have kept up with CO2 emissions.”

But just because forests have currently upped their levels of absorption out of necessity, scientists caution, doesn’t mean they’ll do it forever. Forests, after all, are also vulnerable to climate change.

“We don’t know if the trend will continue, because the sink processes that have led to an increase in uptake can also lead to other disruptions,” says Sean McMahon, a forest ecologist at SERC. Higher temperatures could mean longer growing seasons but also longer summers, which tend to bring an uptick in tree-killing phenomena like droughts and fires. And if trees start dying, all the carbon they’ve absorbed gets released into the atmosphere. Changes like that, Ciais admits, could swiftly reduce the Northern Hemisphere’s carbon sink capacity. It’s also difficult to disentangle complex processes like nitrogen fertilization and carbon sequestration, and thus difficult to say how they might evolve under shifting climate conditions. But studies like this could underscore the importance of preserving the world’s forests, not just for the 80 percent of animals that live in them, but also for those of us who depend on them to keep climate change relatively under control.

In 2017, Conservation International launched the largest-ever tropical reforestation effort with an aim to restore 73 million trees in the Brazilian Amazon by 2023. The Amazon, which lies near the equator, is a critical carbon sink area. But over the years, the sink capacity has suffered drastic setbacks due to logging, and carbon released from dying trees has made deforestation a major contributor to Brazil’s greenhouse gas emissions, which are the sixth-highest in the world.

“We know that the climate is changing, that CO2 in the atmosphere is increasing and also that there is a mitigating sink in the Northern Hemisphere,” McMahon says. “But we also know that the loss of this sink could accelerate everything that much more rapidly.”