It begins each day at nightfall. As the light disappears, billions of zooplankton, crustaceans and other marine organisms rise to the ocean surface to feed on microscopic algae, returning to the depths at sunrise. The waste from this frenzy – Earth’s largest migration of creatures – sinks to the ocean floor, removing millions of tonnes of carbon from the atmosphere each year.
This activity is one of thousands of natural processes that regulate the Earth’s climate. Together, the planet’s oceans, forests, soils and other natural carbon sinks absorb about half of all human emissions.
But as the Earth heats up, scientists are increasingly concerned that those crucial processes are breaking down.
In 2023, the hottest year ever recorded, preliminary findings by an international team of researchers show the amount of carbon absorbed by land has temporarily collapsed. The final result was that forest, plants and soil – as a net category – absorbed almost no carbon.
There are warning signs at sea, too. Greenland’s glaciers and Arctic ice sheets are melting faster than expected, which is disrupting the Gulf Stream ocean current and slows the rate at which oceans absorb carbon. For the algae-eating zooplankton, melting sea ice is exposing them to more sunlight – a shift scientists say could keep them in the depths for longer, disrupting the vertical migration that stores carbon on the ocean floor.
At least 118 countries are relying on the land to meet national climate targets. But rising temperatures, increased extreme weather and droughts are pushing the ecosystems into uncharted territory.
The kind of rapid land sink collapse seen in 2023 has not been factored into most climate models. If it continues, it raises the prospect of rapid global heating beyond what those models have predicted.
For the past 12,000 years, the Earth’s climate has existed in a fragile equilibrium. Its stable weather patterns allowed the development of modern agriculture, which now supports a population of more than 8 billion people.
As human emissions rose, the amount absorbed by nature increased too: higher carbon dioxide can mean plants grow faster, storing more carbon. But this balance is beginning to shift, driven by rising heat.
“This stressed planet has been silently helping us and allowing us to shove our debt under the carpet thanks to biodiversity,” says Rockström. “We are lulled into a comfort zone – we cannot really see the crisis.”
Only one major tropical rainforest – the Congo basin – remains a strong carbon sink that removes more than it releases into the atmosphere. Exacerbated by El Niño weather patterns, deforestation and global heating, the Amazon basin is experiencing a record-breaking drought, with rivers at an all-time low. Expansion of agriculture has turned tropical rainforests in south-east Asia into a net source of emissions in recent years.
“In 2023 the accumulation of CO2 in the atmosphere is very high and this translates into a very, very low absorption by the terrestrial biosphere,” says Philippe Ciais, a researcher at the French Laboratory of Climate and Environmental Sciences, who was an author of the most recent paper.
“In the northern hemisphere, where you have more than half of CO2 uptake, we have seen a decline trend in absorption for eight years,” he says. “There is no good reason to believe it will bounce back.”
“None of these models have factored in losses like extreme factors which have been observed, such as the wildfires in Canada last year that amounted to six months of US fossil emissions. Two years before, we wrote a paper that found that Siberia also lost the same amount of carbon,” says Ciais.
“Another process which is absent from the climate models is the basic fact that trees die from drought. This is observed and none of the models have drought-induced mortality in their representation of the land sink,” he says. “The fact that the models are lacking these factors probably makes them too optimistic.”
Even a modest weakening of nature’s ability to absorb carbon would mean the world would have to make much deeper cuts to greenhouse gas emissions to achieve net zero. The weakening of land sinks – which has so far been regional – also has the effect of cancelling out nations’ progress on decarbonisation and progress towards climate goals, something that is proving a struggle for many countries.
In Australia, huge soil carbon losses from extreme heat and drought in the vast interior – known as rangelands – are likely to push its climate target out of reach if emissions continue to rise, a study this year found. In Europe, France, Germany, the Czech Republic and Sweden have all experienced significant declines in the amount of carbon absorbed by land, driven by climate-related bark beetle outbreaks, drought and increased tree mortality.
Finland, which has the most ambitious carbon neutrality target in the developed world, has seen its once huge land sink vanish in recent years – meaning that despite reducing its emissions across all industries by 43%, the country’s total emissions have stayed unchanged.
So far, these changes are regional. Some countries, such as China and the US, are not yet experiencing such declines.
“We shouldn’t rely on natural forests to do the job. We really, really have to tackle the big issue: fossil fuel emissions across all sectors,” says Prof Pierre Friedlingstein of Exeter University, who oversees the annual Global Carbon Budget calculations.
“We can’t just assume that we have forests and the forest will remove some CO2, because it’s not going to work in the long term.”