Recent Analyses Indicate a Weakening of Terrestrial and Marine Carbon Sinks

Methane carbon sinks

A synthesis of recent reports, culminating in the Global Carbon Budget 2025, indicates a significant and concerning trend: the capacity of the Earth's natural carbon sinks to absorb anthropogenic CO2 is diminishing. This weakening, observed in both terrestrial and marine systems, coincides with record-high global CO2 emissions from fossil fuels in 2025.

This development is critical because these planetary sinks—primarily the oceans and terrestrial biosphere—have historically absorbed more than half of all anthropogenic carbon emissions. A reduction in this "uptake efficiency" effectively accelerates the growth rate of atmospheric CO2 concentration, compounding the challenge of climate change mitigation. The data suggests this is no longer a distant projection but an observed phenomenon, creating a positive feedback loop where warming itself impairs the planet's absorptive capacity.

The Threshold of the Amazon: From Carbon Sink to Source

The Amazon rainforest, long considered the planet's most significant terrestrial carbon sink, is now under intense scrutiny as it approaches a potential biome-scale tipping point. Recent studies indicate the system's resilience is degrading. This is driven not only by deforestation but also by climate-induced stressors, such as intensifying droughts and anomalous heat, which reduce photosynthetic activity and increase widespread tree mortality.

Consequently, large portions of the Amazon, particularly in the heavily degraded southeastern region, are now believed to be a net carbon source, where emissions from decomposition and fires exceed the carbon sequestered through photosynthesis. In response to this urgent knowledge gap, a major joint campaign between the European Space Agency (ESA) and Brazil's National Institute for Space Research (INPE) was launched in November 2025. This mission is utilizing research aircraft equipped with advanced sensors to conduct large-scale atmospheric measurements, quantifying the fluxes of CO2 and CH4 across the basin to definitively assess its net carbon balance and proximity to an irreversible tipping point.

Empirical Evidence of Saturation in Global Forest Ecosystems

The Amazon's instability is part of a broader, global trend of terrestrial sink degradation. An October 2025 study provided a stark empirical confirmation of this process, finding that Australia's tropical forests have already transitioned to a net carbon source. This "flip" is a critical finding, demonstrating that regional forest ecosystems can cross this threshold, whereby stress-induced mortality and respiration permanently outpace carbon sequestration.

This phenomenon is not confined to the tropics. The vast boreal forests of the high northern latitudes, which store more carbon in their soils and permafrost than all the planet's vegetation, are also showing signs of distress. A January 2025 report from the Finnish Natural Resources Institute found that Finland's extensive forestlands have registered as a net carbon source. This alarming shift is attributed to a combination of declining tree growth and logging, but most significantly, to accelerated soil respiration. As the climate warms, microbial decomposition of ancient organic matter in the soil accelerates, releasing vast stores of CO2 and CH4 that were locked away for millennia. This feedback is a long-feared mechanism that is now being observed, with similar declines in sink capacity noted across the boreal region, exacerbated by increased wildfire frequency, pest outbreaks, and drought.

Impaired Uptake in the Global Ocean Sink

The ocean, the planet's largest carbon sink, is also exhibiting a measurable decline in its uptake efficiency. This process is driven by complex changes to its fundamental physical and biological pumps.

The primary mechanism for carbon absorption is the thermophysical solubility pump. CO2 dissolves more readily in cold, alkaline seawater. A September 2025 study from ETH Zurich, analyzing the "unprecedented" 2023 marine heatwave, found that the ocean absorbed "significantly less" CO2 during this period. The anomalous heat reduced the solubility gradient between the atmosphere and the sea surface, thereby slowing the rate of CO2 diffusion into the water.

This warming also weakens the biological pump. Rising sea surface temperatures lead to stronger stratification of the water column. This layering prevents the upwelling of cold, nutrient-rich deep water, which in turn limits the growth of phytoplankton. These microorganisms are the base of the marine food web and are responsible for sequestering carbon, which sinks to the deep ocean when they die. A more stratified, less productive ocean has a weaker biological pump.

These findings are particularly significant in the context of the recent 2023-2024 El Niño. While El Niño events typically increase the net ocean sink (by suppressing CO2 outgassing in the Pacific), the 2025 analyses show that anomalous global warming, especially in the North Atlantic, was strong enough to completely counteract this effect, resulting in a net weakening of the ocean sink.

A Global Synthesis and Systemic Implications

The Global Carbon Budget 2025 integrates these disparate findings into a single, coherent picture. The report confirms that the "airborne fraction" of CO2 is increasing because the fraction absorbed by land and ocean sinks is declining. It quantifies this feedback, estimating that 8% of the total rise in atmospheric CO2 since 1960 can now be attributed to this climate-driven weakening of the planet's natural buffers.

This has profound implications. Climate models must now be updated to treat carbon sinks not as a static or reliable "discount" on emissions, but as a dynamic and vulnerable variable that is coupled to warming. This research presents a dual challenge: global decarbonization efforts must accelerate not only to reduce the primary source of emissions but also to protect the remaining integrity of the planet's failing natural sinks.