African forests are now emitting more carbon dioxide than they absorb, a fundamental shift that will make it more difficult for the world to cut its net emissions to zero.

Forests and shrubby woodlands on the continent have previously been one of the world’s biggest carbon sinks, accounting for 20 per cent of all the CO2 taken up by plants. The lion’s share of this is in the Congo rainforest, the second largest in the world after the Amazon. Sometimes called the “lungs of Africa”, it absorbs an estimated 600 million tonnes of CO2 per year. However, that number has been falling as the rainforest is destroyed by logging and mining.

Now, researchers have found that after gaining biomass from 2007 to 2010, African forests lost 106 million tonnes of biomass per year from 2011 to 2017. That is equivalent to roughly 200 million tonnes of CO2 emissions per year. This was driven by deforestation in the Congo rainforest, says at the University of Leicester, UK.

“If we are losing the tropical forests as one of the means of mitigating climate change, then we basically have to reduce our emissions of greenhouse gasses from fossil fuel burning even faster to get to near-zero emissions,” he says.

Balzter and his colleagues estimated the amount of biomass with satellite measurements of the colour and moisture content of the forest canopy, as well as its height at certain points. They compared this with measurements taken on the ground, although these are sparse in Africa.

But at University College London says satellite data can’t detect the type of trees in a forest and isn’t reliable for estimating the carbon absorbed by high-biomass intact forests or emitted in forests degraded by selective logging. A dense hardwood like mahogany might hold more carbon than a light balsa wood of the same size, for instance.

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“Deforestation in Democratic Republic of Congo… is higher than it was in the 2000s. And we all know that,” he says. “But whether that is enough to tip the whole carbon balance of the entire continent is unknown.”

The study also didn’t include the wet peatlands found underneath much of the Congo rainforest, which absorb a small amount of CO2 each year and hold about 30 billion tonnes of ancient carbon.

The Amazon rainforest, which was also once a major carbon sink, emitted more CO2 than it absorbed in several recent years. But whereas deforestation in the Amazon has fallen under a government crackdown, it has been growing in the Congo.

In the Democratic Republic of the Congo, impoverished farmers often destroy rainforest for slash-and-burn agriculture. Companies, many of them foreign-owned, illegally log colourful hardwoods like African teak and coralwood.

At the COP30 climate summit in the Amazon this month, Brazil announced the Tropical Forests Forever Facility, a fund that will pay its investment returns to tropical countries at a rate of $4 per hectare of forest left standing. But so far countries have donated only $6.6 billion to the fund, far short of the $25 billion goal.

Balzter says this mechanism could be more effective than carbon credits, which reward “avoided” emissions and have in many cases been found to be worthless.

“It’s really important to make this Tropical Forest Forever Facility work, and make it work quite quickly, to try and reverse this trend of the African tree biomass actually releasing carbon into the atmosphere,” he says.

Many forests are losing their winter snowpack as global temperatures rise, and that could substantially slow their growth – and reduce the amount of carbon they remove from the atmosphere.

Current projections “are not incorporating that complexity of winter climate change, so they are likely overestimating what the future carbon storage will be”, says at Boston University in Massachusetts.

Warming temperatures are generally expected to boost growth in temperate forests, mainly by spurring decomposition and making more nutrients available during the warm growing season. However, models largely don’t account for changes during winter – especially the loss of snow.

“The loss of deep, insulating snowpack cannot be understated,” says at the University of New Hampshire. Her research has shown deep snow days will disappear across most of the US by the end of the century, with consequences for water storage and ecosystem health.

To get a better handle on these cold-weather changes, Conrad-Rooney and their colleagues simulated how a global temperature increase of 5°C would affect the growth of red maple trees (Acer rubrum) in an experimental forest in New Hampshire. In some plots, they used buried cables to warm the soil during the growing season. In others, they also removed snow during winter and warmed the soil to induce cycles of freeze and thaw.

Measured over 10 years, the trees in both plots grew more than trees that were left alone. However, the plots where the snow was removed grew much more slowly, adding about half as much growth. The researchers attribute this difference to root damage caused by the snowless soil being more exposed to changing temperatures.

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“The snow typically acts as an insulating blanket to keep soils from freezing,” says Conrad-Rooney. “With less snow, there are more freeze-thaw cycles.”

Extrapolating to similar forests across the Northeast US, the researchers estimate the loss of snowpack expected by the end of the century would reduce carbon storage by a little over one million tonnes per year, compared with models that don’t account for disappearing snow.

“Snowpacks that come and go throughout the winter diminish the stable soil conditions our north-east ecosystems require for long-term storage of carbon,” says Burakowski.

However, not all snow-covered forests will respond to snowpack loss in the same way as the deciduous forests of the Northeast, says at the University of Utah. He points out accurately modelling various ecological responses remains a big challenge. “There’s so many things that are changing,” he says.