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Last year, trees and land absorbed very little CO2. Is nature’s carbon sink breaking down?

The sudden breakdown of carbon sinks was not included in climate models, which could lead to a faster rise in global temperatures. Each night, billions of zooplankton and other marine organisms rise to the ocean's surface to feed, then return to the depths by dawn. Their waste sinks to the ocean floor, removing vast amounts of carbon from the atmosphere annually. This is just one of many natural processes that help regulate Earth’s climate. Together, natural carbon sinks like oceans, forests, and soil absorb about half of human carbon emissions.

However, as the planet heats up, scientists are increasingly worried that these processes are failing. In 2023, the hottest year on record, preliminary research found that land's ability to absorb carbon temporarily collapsed, with forests, plants, and soils absorbing almost no carbon. Oceans, too, are showing warning signs, with Greenland's glaciers and Arctic ice melting faster than expected, affecting ocean currents and slowing carbon absorption. The breakdown of vertical migration in marine ecosystems due to melting ice could further disrupt carbon storage.

Johan Rockström, from the Potsdam Institute for Climate Impact Research, stated at New York Climate Week that Earth's natural systems are showing "massive cracks," signaling that nature's ability to balance human impacts is faltering. The 2023 land carbon sink breakdown may be temporary, but it highlights the fragility of ecosystems and poses major challenges for the climate crisis.

Without natural systems, achieving net zero emissions is impossible. With human carbon pollution reaching a record 37.4 billion tonnes in 2023, and many countries relying on land sinks to meet climate targets, the stakes are high. Rising temperatures and extreme weather are pushing ecosystems into unknown territory, and the rapid land sink collapse in 2023 wasn't accounted for in most climate models. If this continues, it could lead to faster global warming than previously predicted.

Tim Lenton, a climate scientist from Exeter University, says surprising changes in the biosphere are raising questions about how much we can rely on carbon sinks. A recent study found regional variations in carbon absorption by forests, with boreal forests, which store a significant portion of land carbon, seeing a steep decline in absorption due to beetle infestations and climate-related fires. Combined with weakening Amazon rainforests and drought in the tropics, 2023 saw a spike in atmospheric carbon.

Oceans, which absorb the majority of warming caused by fossil fuels, are also showing signs of weakening in their ability to absorb CO2. This suggests the carbon sink system might degrade faster than models predict. Some factors, like wildfires and tree mortality due to drought, are not included in models, making them overly optimistic. A weakened carbon sink means nations will need to cut emissions even more deeply to meet climate goals.

In some regions, the collapse of natural carbon sinks is already undermining climate progress. Countries like Australia and Finland have seen drastic changes in their carbon sinks, which could make it impossible to meet climate targets. These issues are regional for now, but the global risks are significant. Experts warn that we must protect existing carbon sinks through measures like halting deforestation and reducing emissions, rather than relying on forests to offset pollution.