While there is a clear picture of how much fossil-derived carbon is pumped into the atmosphere, the carbon stored in human-made products remains less understood. Ecological experts estimate that humans annually incorporate roughly 400 million tons of fossil carbon into durable materials like plastics, buildings, and infrastructure.

These products, often viewed as a potential "carbon sink," come with a critical caveat: without proper waste management, their environmental benefits could quickly transform into significant hazards. A recent study, published in Cell Reports’ Sustainability sheds light on this overlooked aspect.

"We have accumulated more carbon in human-made stuff on the planet than there is carbon in the natural world, but we completely overlook it, and those stocks get bigger and bigger," says eco economist Klaus Hubacek of the University of Groningen. "The message is to look at stocks rather than just flows."

Fossil carbon in global annual CO2 emissions vs annual addition of fossil carbon in durables for 1995–2019. Image Source: Cell Reports "Sustainability".

The hidden reservoirs of fossil carbon in the "technosphere" — the extensive collection of all human-made materials, both functional and discarded — remain a mystery. To unravel this puzzle, researchers turned to the only available dataset of material inputs/outputs from different economic sectors, focusing on 2011. In one year alone, a rare glimpse into the technosphere's inner workings provided the foundation for estimating these carbon stocks and their annual changes worldwide.

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The researchers assessed how much carbon moves in and out of various sectors by estimating the average carbon content of products — for instance, plastics typically contain about 74% fossil carbon. The analysis included not only finished products like durable plastics, but also transitional materials made from fossil carbon used in various industries.

In 2011, about 9% of all extracted fossil carbon found new life in long-lasting products within the technosphere. To put it in perspective, if this carbon had been emitted as carbon dioxide (CO₂), it would have nearly matched the European Union's total annual emissions.

Centuries to breakdown in landfills

The fate of these products is concerning. A large amount will end up in landfills, incinerated, or simply become litter. Between 1995 and 2019, it was estimated that 3.7 billion tons of fossil carbon were discarded: 1.2 billion tons went to landfills, another 1.2 billion were incinerated, 1.1 billion were recycled, and the rest ended up as litter. This highlights the urgent need for sustainable waste management to prevent long-term harm.

"On the one hand, you can consider it as a form of carbon sequestration if this fossil carbon ends up sequestered in landfill, but on the other hand, it poses an environmental hazard, and if you burn it, you increase carbon emissions," says ecological economist Franco Ruzzenenti of the University of Groningen.

Lengthening product lifespans and upping recycling rates are some ways to reduce the amount of fossil carbon entering the dead zone. Ecological experts also emphasize the importance of having policies in place that act to reduce the waste entering the landfills.

Looking to the future, researchers plan to conduct a similar analysis of biogenic carbons (plant-derived) to see how wood and similar materials can be used to reduce products entering landfills.