A preliminary analysis has revealed that the extreme heat and rainfall of 2024—the hottest and wettest year recorded—now emits as much carbon dioxide as land ecosystems can mitigate. This marks the second consecutive year where land carbon sinks have nearly vanished due to climate stresses, potentially explaining the unprecedented surge in atmospheric CO₂ levels in 2024.

The implications of these findings are serious: land carbon sinks, which typically absorb billions of tons of CO₂ annually, are weakening more rapidly than anticipated, threatening our climate goals. However, it’s uncertain whether this trend over the past two years indicates a permanent shift.

“Everyone involved in this research was taken aback,” noted Guido van der Werf from Wageningen University, Netherlands. “We anticipated a minor sink, but the current situation reveals very few sinks exist.”

Land forests, grasslands, and other ecosystems work to absorb CO₂ from the atmosphere as they grow. However, when these ecosystems decay or burn, the stored carbon is released back into the atmosphere. Ideally, these processes should remain balanced. Yet, over the last fifty years, ecosystems have been absorbing more carbon than they are releasing, forming fluctuating carbon sinks.

This imbalance is thought to stem primarily from rising CO₂ levels benefiting plant growth, as well as other aspects like nutrient pollution and afforestation. Nonetheless, the effects of increased CO₂ are predicted to be finite as ecosystems may eventually lose their fertilization benefits. “We understand that land carbon sinks will decline, but the pace remains uncertain,” said van der Werf.

In 2023, researchers grew increasingly alarmed as land carbon sinks diminished significantly due to extreme wildfires, heat, and drought, all exacerbated by rising greenhouse gas levels and emerging El Niño patterns in the Pacific, which are generally linked to weakened sinks.

In 2024, it was expected that the carbon sink would strengthen with the decline of El Niño and fewer wildfires. Yet, international teams, including van der Werf, found that it remained incredibly weak.

To evaluate carbon cycling, researchers utilized satellite data to track land greenness corresponding to plant growth and compute the productivity of Earth’s terrestrial ecosystems. They then subtracted the CO₂ released through wildfires and decomposition, leveraging global atmospheric CO₂ measurements for their estimates.

The analysis revealed that the land carbon sink for 2024 was nearly nonexistent, with approximately 2.6 billion tonnes less than expected. This was less than the struggling sink observed between the 2015 and 2016 El Niño events, marking the worst land carbon sink in over a decade.

However, this weakening, unlike in 2023, was not predominantly driven by wildfires and drought. Instead, the study indicated an escalated rate of organic matter decomposition. While ecosystems did show increased productivity (with greenness reaching record highs), it was outpaced by a measure referred to as total ecosystem respiration, reflecting the overall CO₂ output.

“The substantial drop in land carbon sinks stems from respiration,” explained Guanyu Dong, from Nanjing University, who led the study. “The mechanisms at play are completely different.” While most regions have experienced a weakened sink, this trend is especially pronounced in tropical grasslands and shrublands, he added.

Van der Werf indicated that these figures are preliminary estimations and other research teams analyzing the carbon sink could yield varying results. Yet, this decline could largely elucidate the record spike in CO₂ concentrations witnessed in 2024, which is too substantial to be solely attributed to fossil fuel emissions.

The recurring disappearance of sinks for two consecutive years may foreshadow a more rapid decline than previously anticipated. “This suggests that a more pessimistic model might hold true,” noted van der Werf. Early losses in carbon sinks lead to faster increases in airborne CO₂ levels, potentially escalating temperatures even after emissions reach zero.

“That is certainly a possibility, and underscores the alarming nature of this phenomenon,” stated Scott Denning from Colorado State University, who was not involved in the research. He cautioned that the past two years could be aberrations, making it challenging to generalize about accelerated rates of decomposition. “One must interpret even these two years of data cautiously to avoid concluding it represents a permanent collapse,” he advised.