Matthew N. Hayek, Helen Harwatt, William J. Ripple, and Nathaniel D. Mueller, September 2020.
Extracts from the above titled Research Paper by James O’Donovan.
Extensive land uses to meet dietary preferences incur a ‘carbon opportunity cost’ given the potential for carbon sequestration through ecosystem restoration. Here we map the magnitude of this opportunity, finding that shifts in global food production to plant-based diets by 2050 could lead to sequestration of 332–547 GtCO2, equivalent to 99–163% of the CO2 emissions budget consistent with a 66% chance of limiting warming to 1.5 °C. Restoration of native ecosystems, including forests, is a land-based option for atmospheric carbon dioxide (CO2) removal.
Ecosystem restoration is constrained largely by land requirements of food production, the largest human use of land globally. Food production therefore incurs a ‘carbon opportunity cost’, that is, the potential for natural CO2 removal via ecosystem restoration on land. This cost can vary greatly depending on the ‘potential’ or ‘native’ vegetation of a given region and types of food produced.
Animal-sourced foods such as meat and dairy have large land footprints because animals typically consume more food macronutrients than they produce. Quantifying the spatial distribution of agriculture’s cumulative carbon opportunity cost within this century can inform efforts to limit global warming to 1.5 °C. Ongoing agricultural emissions can be abated by shifts to less-resource-intensive, plant-based diets but the potential for cumulative CO2 removal from native vegetation regrowth in areas occupied by animal agriculture has not previously been calculated in a spatially explicit manner.
Here we quantify the total carbon opportunity cost of animal agricultural production to be 152.5 (94.2–207.1) gigatons of carbon (GtC) in living plant biomass across all continents and biomes (Fig. 1). We approximated the potential for CO2 removal in soil and litter as an additional 63 GtC. This estimate is associated with large but unknown uncertainty because of a deficit of data and the complexity of dynamics of non-living carbon pools in restored ecosystems. Pastures for ruminant meat and dairy production represent the majority of the total carbon opportunity cost—72%—compared with animal feed croplands, which suppress the remaining 28% of native vegetation carbon. Potential productivity on remaining cropland is sufficient to supply the current global population with 78g per capita per day of protein (after factoring losses from both storage and consumer waste), an amount exceeding dietary recommendations, accounting for variation in nutritional requirements among demographic groups and for disparities in food availability. The cumulative potential of CO2 removal on land currently occupied by animal agriculture is comparable in order of magnitude to the past decade of global fossil fuel emissions.
To understand the potential future consequences of animal-sourced food consumption on global CO2 budgets, we modelled land use of three global dietary scenarios to the year 2050 relative to the present day (base year 2015). The net CO2 balance was calculated for a business-as-usual (BAU) diet following economic trends, a healthier diet with approximately 70% meat reduction globally relative to BAU 13 (the EAT-Lancet Commission or ELC diet) and a vegan (VGN) diet with no animal-sourced foods.
The BAU diet results in land clearing, with land-use-change emissions of 86 (68–105) GtCO2 (Fig. 3) because optimistic future improvements in yields are insufficient to meet expected animal feed demands. The ELC and VGN diets result in 332 (210 to 459) and 547 (358 to 743) total GtCO2 removal, respectively, approximately equal to the past 9 and 16 years of fossil fuel emissions. Ecosystem soil and litter could remove an additional 135 and 225 GtCO2 for ELC and VGN, respectively, but this estimate is highly uncertain. Smaller future increases in crop yields would result in less land sparing and CO2 removal from ELC and VGN diets compared with present day: 199 and 424 GtCO2 , respectively. However, plant-rich diets would permit even greater mitigation compared with BAU; lower yields result in greater land-clearing emissions of 247 GtCO2 . Ceasing fossil fuel use is necessary to limit global warming, but CO2 removal following plant-rich dietary shifts could substantially contribute to international greenhouse gas reduction targets.
Cumulative CO2 emissions (anthropogenic emissions minus removal) must remain below 335 GtCO2 after 2019 to limit warming to 1.5 °C at a 66% likelihood level. CO2 removal from terrestrial vegetation following ELC or VGN dietary shifts would increase permissible CO2 emissions by 99% (63%–137%) or 163% (107%– 222%), respectively. Adding net CO2 uptake by native ecosystem soil and litter to this total increases the 1.5°C budget by 139% or 230%, respectively. By contrast, most future scenarios of 1.5°C warming rely on nascent bioenergy carbon capture and storage technology to remove 151 to 1,191 GtCO2 from the atmosphere – an amount of CO2 comparable to plant-rich diets.
Carbon uptake saturates after around 25 years for tropical forests and around 30 years for temperate forests. Changes in diets and agricultural land use within the next two decades could contribute substantially toward carbon neutrality by 2050. Overshooting 1.5°C warming poses substantial risks to human and natural systems, including a weakened terrestrial ecosystem carbon sink. However, even in high-emission pathways, terrestrial ecosystems are expected to act as a net carbon sink through 2100, although the precise magnitude is subject to on-going investigation.
Changes to global agricultural production would be economically disruptive and could incur sociocultural costs, which must be compared with the costs of climate warming from unabated agricultural emissions. Restoration efforts could minimize trade-offs by targeting the highest-carbon areas. Financial incentives to restore high-carbon forests may come from higher-income, higher-emitting nations, providing investments to protect livelihoods, strengthen food security and improve agricultural productivity. Our analysis also reveals substantial opportunities for CO2 removal in high-income countries and temperate ecoregions that are often neglected in scientific and policy conversations.
This analysis uses the most up-to-date and high-resolution data to map ecosystem carbon trade-offs associated with animal-sourced food production. Our results demonstrate substantial carbon opportunity costs incurred by resource-intensive diets, comparable to the remaining carbon budget to 1.5°C. Animal agriculture across all continents and income categories represents a profound trade-off when compared with potential GHG mitigation. If future dietary shifts do not occur, carbon trade-offs are expected to grow, even with large improvements in yields and optimized cropland distribution. Our carbon accounting approach illuminates areas where policies could prioritize ecosystem restoration and CO2 removal, including but not limited to tropical Latin American forests outside of the Amazon basin and temperate forests in Western Europe and East Asia, where carbon trade-offs are largest.