Experimental design
We examined the effect of upper-bound estimates of marine plastic–carbon leaching and incineration emissions on the global climate system via five experiments in which various fluxes of carbon, based on projected marine plastic–carbon leaching and projected plastic incineration emissions, are added to the climate system (
Table 1).
The model was initialized with a 10 000 year spin up at the year 850. From 850 to the year 2005, historical forcing (land cover change, solar, volcanic, aerosol, and other GHGs) was used to force the model. In addition, over the period 2005–2100, changing CO
2, land surface, aerosol, and other greenhouse gas (GHG) forcing followed representative concentration pathway (RCP) 8.5 (
Moss et al. 2008). From 2000–2100, the equivalent CO
2 emissions were diagnosed (
Zickfeld et al. 2013) and then used in subsequent sensitivity experiments (GW, GWPL—see
Table 1). This was done to ensure that our plastic–carbon leaching and incineration experiments allowed atmospheric CO
2 to freely evolve away from prescribed RCP 8.5 values. The experiments listed in
Table 1 all started from the year 2000 initial state. This initial condition was used to be consistent with the RCPs developed as part of the Intergovernmental Panel on Climate Change (IPCC) 5th assessment whose emissions trajectories all start from the same base year (year 2000). Our conclusions are insensitive to this initial starting date.
In the
Control scenario (
Table 1), emissions of anthropogenic CO
2 were ceased after the year 2000, thereby providing an estimate of the committed atmospheric CO
2 adjustment to historical forcing. The
Global Warming (GW) scenario includes fossil fuel emissions (diagnosed from RCP 8.5) throughout the entirety of the 100 year integration period (to year 2100). This scenario represents a continuing increase in our dependence on fossil fuels throughout the 21st century, including their use in the manufacturing of plastics. The
Global Warming + Plastic Leaching (GWPL) scenario is the same as GW but with the addition of plastic–carbon leaching (see
Table 1). It is designed to estimate the additional climatic impact of a lurking and previously not considered source of fossil carbon (plastics) in the ocean in an already dramatically altering climate state.
The plastic–carbon flux was derived from marine plastic debris values projected for the year 2040 (29 million metric tonnes per year;
Lau et al. 2020). Although research has determined an ∼90% carbon content in plastics (
Zhu et al. 2020), we assumed a 100% conversion to test the upper-bound effect of this leaching. As noted in the introduction, plastics leach DOC into the marine environment contributing to the inorganic carbon pool via microbial degradation (
Hansell 2013). In fact, the majority of DOC is labile, meaning that it can be rapidly broken down (within hours or days) by microbes and bacteria for biomass production and respiration, ultimately producing dissolved inorganic carbon (DIC). The small fraction of DOC that is resistant to microbial degradation and breaks down on time scales of thousands to tens of thousands of years is known as recalcitrant DOC. Through the process of respiration or decomposition, DOC is broken down into DIC, which is composed of HCO
3−, CO
2, and CO
32−, with HCO
3− making up the largest fraction of DIC. Given the rapid breakdown of DOC into DIC in ambient seawater, we assume that all carbon being leached from the plastic debris is in the form of DIC. This is an acceptable substitute, as natural labile DOC breaks down rapidly and contributes to the ocean DIC pool rather than the organic carbon pool (
Hansell et al. 2009). By assuming the complete conversion from DOC to DIC, we can estimate the upper-bound effects of plastic-derived carbon leaching on the global climate system. This plastic–carbon leaching flux was added to the global ocean surface at a constant rate throughout the 100 year time period, beginning in the year 2000.
The PL scenario assumes the cessation of anthropogenic fossil fuel combustion in the year 2000 and only includes emissions from marine plastic–carbon leaching beginning in 2000. This scenario represents a global shift to carbon neutrality that fails to address ocean plastic pollution. The plastic–carbon flux value is the same as in GWPL. The
Incineration of Projected Plastic (IPP) scenario assumes the cessation of anthropogenic fossil fuel emissions in the year 2000, and zero marine plastic–carbon leaching. Instead, this experiment uses an estimate of global plastic production in 2050 (1.1 billion metric tonnes;
Geyer 2020) and assumes, as an upper-bound case, that this amount of plastic is produced and incinerated each year from 2000 to 2100. The resulting emissions are added directly to the atmosphere. In this scenario, a carbon-neutral world has widely adopted incineration through waste-to-energy systems, into their new, circular economy. Incineration produces greater emissions than recycling (
Zheng and Suh 2019;
Stegmann et al. 2022); therefore, this scenario represents a high-end potential emission estimate resulting from continuing plastics gasification and pyrolysis in an otherwise carbon-neutral world. These incineration emissions were calculated based on the conversion of 1 kg of low-density polyethylene (LDPE) to 2.9 kg of CO
2e (
Benavides et al. 2020) and were added at a constant rate throughout the 100 year time period, beginning in the year 2000.
The Incineration of Projected Marine Plastic (IPMP) scenario also assumes the cessation of anthropogenic fossil fuel emissions in the year 2000. Instead of adding the carbon flux (from plastic–carbon leaching) into the ocean surface, it is assumed that all marine plastic would be removed and incinerated through gasification and pyrolysis, and the subsequent CO2 emissions would therefore enter the atmosphere directly. In other words, the same amount of carbon that was added to the ocean throughout the PL scenario is now being added directly into the atmosphere to compare the relative effects of plastic–carbon injection in the surface ocean versus the atmosphere. Note that the incineration carbon flux is lower in IPMP relative to PL only because the earth's surface area (5.10 × 108 km2) is greater than the ocean's surface area (3.57 × 108 km2).
In the GWPL and PL experiments, the calculated carbon flux was evenly distributed as a surface flux throughout the global ocean. This allowed for the focus to be on the effect of plastic–carbon on climate, without the need for incorporating the leaching process from plastic to seawater in the model. The projected plastic–carbon leaching value was calculated from the annual rate reported by
Lau et al. (2020) (29 million metric tonnes), the molar mass of carbon (12.022 g mol
−1), and the surface area of the ocean in the model (3.57 × 10
8 km
2). The IPP emission value was calculated from the projected annual production of plastic (1.1 billion tonnes per year by 2050;
Geyer 2020), the conversion of 1 kg of LDPE to 2.9 kg of CO
2e (
Benavides et al. 2020), the 12:44 ratio between C and CO
2, and the earth's surface area (5.10 × 10
8 km
2). The calculation of emissions for IPMP was based on the carbon emissions from plastic–carbon leaching in GW and GWPL, but instead of dividing these emissions by the surface area of the ocean, they were divided by the earth's surface area. All fluxes and additional emissions were added into the ocean or atmosphere at a constant rate from 2000–2100, and all model output was annually averaged.
A series of other experiments (not shown) were also conducted wherein all global emissions from marine plastic–carbon leaching were released exclusively into either the North Pacific, North Atlantic, South Pacific, or South Atlantic subtropical gyre (where surface convergence tends to concentrate marine plastics) as well as evenly along all global coastlines (where the majority of plastic enters the marine environment). Our analysis and conclusions below are insensitive to this regional distribution of marine plastic–carbon leaching.