Forests, grasslands, and oceans serve as vital carbon sinks, absorbing and storing carbon dioxide (CO2) to help regulate its levels in the atmosphere. Historically, these natural systems have absorbed nearly half of the CO2 generated by human activities, acting as a critical buffer against rapid climate warming. However, their effectiveness is highly dependent on environmental conditions, including temperature, water availability, and nutrient levels.
Land carbon storage declined sharply in 2023
Land carbon sinks, which store carbon in plants and soils, have traditionally absorbed about one-third of the carbon dioxide emitted by human activities. Recently published research in National Science Review found a sharp decline in carbon storage levels in 2023.
Researchers calculated carbon storage numbers using a combination of advanced models, satellite data, and direct measurements. Dynamic global vegetation models (DGVMs) simulated how land ecosystems absorbed and released carbon, while satellite observations, like those from NASA’s OCO-2 mission, tracked atmospheric CO2 levels globally.
In 2023, the global net land CO2 sink reached a low of 0.44 ± 0.21 gigatonnes of carbon (GtC) per year, the weakest value since 2003. The 2023 storage capacity of terrestrial ecosystems to absorb CO2 was about one-fifth of measured levels over the past two decades.
Factors contributing to the decline in 2023
In this study, researchers identified the followed main contributors to the sharp decline:
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1. Amazon drought and carbon loss
The Amazon rainforest experienced an extreme drought from June to November 2023. This event resulted in a substantial carbon loss of 0.31 ± 0.19 GtC per year. Drought conditions stressed vegetation and reduced the rainforest’s ability to sequester carbon, reversing years of carbon storage gains.
2. Boreal wildfires in Canada
Canada faced unprecedented wildfires in 2023, burning over 184,961 square kilometers of forest, more than 2.5 times the previous peak. These fires emitted an estimated 0.58 ± 0.10 GtC, significantly contributing to the decline in global carbon sinks.
Boreal forests, which are typically carbon storage hotspots, were instead a major source of emissions due to the fires. A 2010 study found that in temperate and boreal regions, warming temperatures are reducing soil moisture, making water a critical constraint. These limitations weaken the feedback loop where plants absorb CO2 to mitigate climate change.
3. Southeast Asia and tropical dynamics
South-East Asia saw a carbon loss of 0.13 ± 0.12 GtC per year, while the tropics, which had recovered from the 2015-16 El Niño event, switched to a net carbon loss during the 2023 El Niño. This reversal highlights the sensitivity of tropical ecosystems to climatic oscillations and extreme weather events.
4. Northern hemisphere decline
Land CO2 uptake in regions north of 20°N declined by half since 2015, dropping to 1.13 ± 0.24 GtC per year in 2023. This trend underscores the compounding effects of warming, reduced soil moisture, and other stressors on northern ecosystems.
Ocean carbon sinks: A mixed picture
While the ocean carbon sink showed a slight increase in 2023, reaching 2.60 ± 0.72 GtC per year, this improvement was regionally uneven. The equatorial eastern Pacific saw enhanced carbon uptake due to reduced upwelling during the transition from La Niña to El Niño conditions. However, high sea surface temperatures (SSTs) in the Northeastern Atlantic reduced the sink’s efficiency in that region.
The impact of extreme heat on carbon sinks
Globally, land regions exposed to record-breaking temperatures in 2023 contributed a gross carbon loss of 1.73 GtC per year. Areas experiencing extreme heat, such as those in the 95th percentile of temperature anomalies, showed a complete negation of carbon uptake, even though they accounted for only 8.61% of the global land area. This finding illustrates the disproportionate impact of extreme temperatures on ecosystem functionality and carbon dynamics.
The decoupling of greenness and carbon sinks
Interestingly, 2023 saw a record-high global greening level, yet this did not translate into stronger carbon sinks. While vegetation greenness, as measured by satellite-derived indices, generally correlates with carbon uptake, this relationship appears to have weakened. Factors such as post-fire regrowth, which increases greenness without restoring carbon stocks, and regional drought stress likely contributed to this decoupling.
A 2010 study published in Science found that 86% of land ecosystems worldwide are becoming less efficient at absorbing CO2 as its atmospheric levels rise. This decline affects the CO2 fertilization effect (CFE), the process by which elevated CO2 boosts photosynthesis and plant growth. While CFE has been a key mechanism for mitigating climate change, its effectiveness is steadily decreasing.
Researchers analyzed decades of data from field experiments, satellite observations, and land-surface models to investigate this trend. Researchers found that the global average CFE dropped from 21% to 12% per 100 ppm of atmospheric CO2 between 1982 and 2020. This decline suggests that land ecosystems are becoming less reliable as a buffer against climate change, raising concerns about the role plants will play in offsetting future emissions.
Implications for climate mitigation
The weakening of natural carbon sinks in 2023 signals a troubling shift in the Earth’s ability to regulate atmospheric CO2 levels. With land ecosystems absorbing less carbon, more CO2 remains in the atmosphere, accelerating climate change. This trend challenges the reliability of natural sinks as a mitigation strategy and underscores the urgency of reducing fossil fuel emissions.
Moreover, the events of 2023 highlight the need to reassess the global carbon budget. Current climate models may underestimate the impacts of extreme weather, droughts, and fires on carbon sinks, leading to overly optimistic projections of future CO2 uptake.
Strategies to Enhance Carbon Sequestration
To bolster the capacity of natural carbon sinks, several strategies can be implemented:
- Reforestation and Afforestation: Planting new forests and restoring degraded ones can increase CO₂ absorption. These efforts not only sequester carbon but also provide additional environmental benefits, such as biodiversity conservation and soil stabilization.
- Sustainable Land Management: Adopting practices that maintain soil health and prevent deforestation can enhance the land’s ability to absorb CO₂. Techniques such as agroforestry, conservation tillage, and cover cropping contribute to increased carbon storage in soils.
- Protecting Existing Forests: Preventing deforestation and forest degradation is crucial, as mature forests store significant amounts of carbon. Implementing policies that curb illegal logging and promote sustainable forestry practices can help preserve these vital carbon sinks.
- Wetland Restoration: Wetlands are highly efficient carbon sinks. Restoring degraded wetlands can enhance their capacity to sequester CO₂ and provide additional ecological benefits, including water filtration and flood control.
The significant decline in land carbon sink efficiency in 2023 highlights the vulnerability of these essential systems to stressors such as drought, extreme heat, and wildfires. These findings underscore the importance of reducing fossil fuel emissions while prioritizing strategies like reforestation and sustainable land management to strengthen carbon sinks. As natural systems face increasing challenges, ongoing research and improved data will play a critical role in shaping effective and informed climate solutions.
References
Ke, P., Ciais, P., Sitch, S., Li, W., Bastos, A., Liu, Z., … & Chevallier, F. (2024). Low latency carbon budget analysis reveals a large decline of the land carbon sink in 2023. National Science Review, nwae367. DOI: 10.1093/nsr/nwae367.
Wang, S., Zhang, Y., Ju, W., Chen, J. M., Ciais, P., Cescatti, A., … & Peñuelas, J. (2020). Recent global decline of CO2 fertilization effects on vegetation photosynthesis. Science, 370(6522), 1295-1300. DOI: 10.1126/science.abb7772
