Plastic production is a bigger climate threat than carbon leaching from plastic pollution
Plastic pollution leaches carbon and erodes into microplastic fragments, creating a raft of environmental issues, but new research into the climate impacts of carbon leaching indicates a negligible impact on the CO₂ in our atmosphere.
Most plastics are made from fossil fuels, so understanding the release of CO₂ throughout the plastics ‘life cycle’ from extraction through to its various end-of-life scenarios (e.g. recycling, landfill, incineration) is important for better management and mitigation of the climate warming impacts from plastics.
There are concerns that carbon leaching from plastic pollution could disrupt the ocean's carbon cycle and the important role the ocean plays in mitigating greenhouse gas emissions. GNS Science carbon cycle modeller Karin Kvale and researchers from the University of Victoria, Canada have investigated the climate impacts of plastic pollution, and found that it contributes an insignificant amount of CO₂ to the atmosphere.
Impact of plastic leaching on the ocean’s carbon cycle
To understand the minor potential for carbon from plastics to influence climate, it’s important to consider the different climate impacts of organic and inorganic carbon.
In the ocean carbon cycle, dissolved inorganic carbon at the ocean’s surface is converted into organic carbon through photosynthesis, by microscopic marine plants called phytoplankton. Most of this naturally-produced organic carbon is used within hours to days by bacteria, and converted back into dissolved inorganic carbon and the process starts again. A small amount of the dissolved inorganic carbon may exchange with the atmosphere at the sea surface, becoming atmospheric CO₂. The organic carbon remaining after bacterial processing slowly builds up into a natural reservoir and does not directly interact with the atmosphere.
Plastics leach organic carbon, which is also consumed by bacteria and converted to dissolved inorganic carbon. In some marine environments the amount of organic carbon leaching from plastics pollution may be equivalent to or greater than natural sources of organic carbon.
It’s been suggested that this new source of organic carbon could act as a substitute for naturally produced organic carbon, potentially disrupting global carbon cycling and increasing the production of dissolved inorganic carbon, which could outgas to the atmosphere as additional CO₂.
Kvale says that a key assumption in these concerns is that organic carbon is a major influence on atmospheric carbon dioxide over human-relevant timescales.
“It is true that dissolved organic carbon is an important part of global carbon cycling over millennia because it is a large natural store for carbon in the ocean. But organic carbon does not directly interact with the atmosphere."
Over short timescales, the bacterial processing of organic carbon into inorganic carbon is tiny compared to ocean-atmosphere cycling and the continuing fossil fuel emissions of CO₂ directly to the atmosphere. Plastics pollution cannot be expected to increase enough to change this minor role of organic carbon in setting atmospheric CO₂.
Modelling the climate effects of plastic-carbon leaching and burning plastics
To examine the climate effects of marine plastic carbon leaching, the researchers used an Earth system model – a tool that simulates the interactions between the atmosphere, ocean, land, biosphere and other processes, and estimates where our climate might be heading under different conditions.
They simulated the impacts of adding plastics carbon (as dissolved inorganic carbon) to the surface ocean for 100 years, at a rate that likely overestimates the amount of plastics pollution. This was modelled with both strong climate warming (to see if plastics carbon might produce unexpected climate feedbacks) and without (to see if it could alter the climate by itself).
In both cases, plastics carbon only increased atmospheric CO₂ concentrations by 1 part per million (ppm) over a century – a very small increase, considering that current burning of fossil fuels is raising atmospheric CO₂ by more than 2ppm each year.
Credit: Gurgacz et. al, FACETS October 2023
The study also examined the impact of plastics incineration, using a pessimistic scenario that all plastic projected to be produced in the year 2050 would be burned and directly converted into atmospheric CO₂ for 100 years. Under these conditions, atmospheric CO₂ increased a little over 21ppm by the year 2100, equivalent to the impact of fewer than nine years of current fossil fuels emissions.
Plastics production a significant and growing source of greenhouse gas emissions
Kvale says that the results show, relative to the current continued widespread burning of fossil fuels for energy, carbon emitted from plastic waste will not have significant direct impacts on atmospheric CO₂ levels.
Of course, environmental plastics pollution has many negative impacts beyond possible climate effects. While our work does not take away from the importance of cleaning up plastic pollution and implementing stringent measures to prevent it, the justification for doing so is not grounded in a mitigative response to climate change.
She also notes the contribution plastics production makes to climate change – it currently represents about 4.5% of our total global greenhouse gas emissions.
“As fossil fuel consumption is reduced in other sectors, emissions from the production of plastics are expected to increase in both proportional footprint and absolute amount,” says Kvale.
She says the development of an international legally binding UN plastics pollution treaty is an excellent opportunity to recognise the increasing contribution of plastics production to climate change and to seek regulatory measures to address these emissions.
Plastic pollution has potential to impact the ocean’s ‘biological carbon pump’
Direct leaching of organic carbon and greenhouse gases from plastics pollution may be of minimal concern, but research from Kvale shows there are other reasons to be cautious about the impact of plastics on global carbon cycling, such as the potential of microplastics to interact with the tiny organisms at the base of the ocean food web (phytoplankton and zooplankton) and alter the ‘biological carbon pump’.
When phytoplankton take up atmospheric carbon through photosynthesis, they integrate carbon into their tissues and skeletons. This carbon eventually makes its way to the deeper ocean, when the phytoplankton die and sink, or when they’re eaten by zooplankton, whose faeces carries the carbon to the deep ocean. The carbon is released through decomposition or buried in seafloor sediments.
This is the biological pump, the biologically-mediated transport of carbon out of the surface ocean, where it has no contact with the atmosphere and is effectively stored. Research shows that microplastics are altering how the zooplankton faeces sink and break down in the ocean.
Plastics are thought to remain in the ocean for up to hundreds of years or more, fragmenting into buoyant and neutrally buoyant microplastics.
Zooplankton eat microplastics but gain no energy from it, making their foraging less efficient. This can impact food web dynamics and nutrient cycling, which might change the ‘winners’ and ‘losers’ in ocean phytoplankton communities (potentially altering the dynamics and patterns of organic particle sinking).
It has also been shown that when zooplankton eat microplastics, the plastic makes its way into their faeces, and its buoyancy causes the sinking rates of their faeces to slow. Microplastics easily incorporate into marine snows (aggregates of biological material), which could also slow the sinking rate of organic material to the deep sea.
Slower sinking of biological particles means they rot higher in the water column, leaving their carbon behind to be more quickly recycled back to the atmosphere.
While the biological carbon pump has a minor role in setting atmospheric CO₂ over human policy timescales, the persistence of microplastics in the ocean and their now ubiquitous distribution suggests more study is needed to determine their potential climate impact.
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Inorganic carbon
Inorganic carbon is carbon compounds without hydrogen, organic carbon is carbon-hydrogen compounds.
The study used an Earth system climate model simulate what would happen if dissolved inorganic carbon was added to the surface ocean for 100 years at a rate of 29 million metric tonnes per year - the amount of carbon leached from plastics projected to enter the ocean in the year 2040. This is a pessimistic estimate of the amount of plastics pollution expected, as currently pollution rates are well below this level, and the international treaty to limit plastic pollution is underway.