The global potential for increased storage of carbon on land

Constraining the climate crisis requires urgent action to reduce anthropogenic emissions while simultaneously removing carbon dioxide from the atmosphere. Improved information about the maximum magnitude and spatial distribution of opportunities for additional land-based removals of CO₂ is needed to guide on-the-ground decision-making about where to implement climate change mitigation strategies. Here, we present a globally consistent spatial dataset (approximately 500-m resolution) of current, potential, and unrealized potential carbon storage in woody plant biomass and soil organic matter. We also provide a framework for prioritizing actions related to the restoration, management, and maintenance of woody carbon stocks and associated soils. By comparing current to potential carbon storage, while excluding areas critical to food production and human habitation, we find 287 petagrams (PgC) of unrealized potential storage opportunity, of which 78% (224 PgC) is in biomass and 22% (63 PgC) is in soil. Improved management of existing forests may offer nearly three-fourths (206 PgC) of the total unrealized potential, with the majority (71%) concentrated in tropical ecosystems. However, climate change is a source of considerable uncertainty. While additional research is needed to understand the impact of natural disturbances and biophysical feedbacks, we project that the potential for additional carbon storage in woody biomass will increase (+17%) by 2050 despite projected decreases (−12%) in the tropics. Our results establish an absolute reference point and conceptual framework for national and jurisdictional prioritization of locations and actions to increase land-based carbon storage.

Emissions of carbon to the atmosphere must remain below ∼250 petagrams (PgC) (918 PgCO₂) from 2021 onward to achieve the Paris Agreement’s goal of limiting global temperature rise to well below 2 °C (1–3). At present rates, that amount of carbon will be emitted by 2045. It follows that even necessary and drastic cuts in emissions (i.e., a rapid transition from fossil fuels to renewable energy sources) must be accompanied by carbon dioxide removal (CDR) or negative emissions strategies (4). Promising options for large-scale CDR include improved land stewardship (5), commonly referred to as natural climate solutions (NCS) (6–8). In particular, increasing carbon storage in woody biomass (e.g., forest ecosystems) is widely recognized as having high climate mitigation potential while also affording an array of environmental and socio-economic cobenefits (6–9). While a growing body of research has estimated the near-term potential for land-based climate mitigation (6, 8, 10), these studies emphasize the climate benefit over short, 10- to 30-y planning horizons. They do not include estimates of the upper limit for additional land-based carbon storage or its spatial distribution. This information is essential for landscape-level planning and targeted implementation of NCS, given that the potential for additional carbon storage is necessarily defined by both the rate at which carbon can be sequestered and the magnitude of the available reservoir. Therefore, we provide 500-m-resolution global maps to quantify the maximum potential for additional carbon storage in ecosystems dominated by woody vegetation (i.e., trees and shrubs), under baseline (1960 to 1990) and future (representative concentration pathway scenario 8.5 [RCP8.5]) climate conditions. This information can be used to help direct NCS toward areas with the greatest maximum opportunity, inform when NCS will saturate, and identify the types of NCS actions that are best suited to a given location.One approach to estimating maximum additional carbon storage—or the difference between current and potential carbon, which we term “unrealized potential” carbon—is a bookkeeping approach that tracks carbon fluxes through time. Under this approach, net land-based emissions since 1850 are estimated to have been 108 to 188 PgC, including both biomass (above and below ground) and soil organic matter (13–17). Estimates that account for preindustrial (i.e., pre-1850) land use are more varied and increase post-1850 estimates by as much as 325 to 357 PgC (18) or as little as 48 to 153 PgC (11–13, 15). This high uncertainty limits the practical utility of this approach.Other investigators have sought instead to quantify unrealized potential by comparing estimates of current and potential land carbon storage. Sanderman et al. (6), considering only soil organic carbon (SOC), estimated net losses in the upper 2 m of soil from agricultural land use to be 116 PgC since 10,000 BC. Erb et al. (19), focusing on changes in vegetation biomass, found losses in carbon due to human land use to be significantly larger (447 PgC) than the studies cited above that consider only the postindustrial period, but generally consistent with some of those that account for preindustrial human disturbance (18). Bastin et al. (20), in a study focused on the restoration of global tree cover, identified an additional reservoir of 206 PgC when considering all carbon pools (aboveground and belowground biomass, soil, litter, and dead wood) after excluding cropland and urban areas.However, all of these global analyses fall short in delivering the robust spatially explicit information needed for targeted planning and implementation of landscape-level NCS. While the global dataset produced by Bastin et al. (20) has a reasonably high spatial resolution (30 arc seconds; approximately 900 m) and considers all land carbon pools, the product is limited to the storage potential afforded by the expansion of tree cover. Moreover, the result is subject to the uncertainty inherent in indirect estimates of carbon stock from area-based metrics of tree/forest cover (21). In comparison, the data product created by Erb et al. (19), which is based on several disparate yet direct estimates of terrestrial carbon storage, is limited by its treatment of only the biomass carbon pool and coarse spatial resolution (5 arc min; approximately 9.3 km). The authors themselves remark that “the uncertainty range could be narrowed if a single robust, validated method would be applied continuously in the stocktaking efforts” (19).Here, we apply a consistent suite of methods to generate spatially explicit global estimates of current (ca. 2016) and climate-constrained potential land carbon storage in aboveground woody biomass (AGB), belowground woody biomass (BGB), and SOC pools at a spatial resolution of approximately 500 m. The difference between current and potential land carbon storage represents the unrealized potential for additional carbon accumulation in global woody biomass and soils. We then disaggregate this global estimate of unrealized potential carbon storage using a conceptual framework we term the NCS opportunity space: seven discrete, internally consistent, and spatially explicit categories of broad NCS action (Fig. 1). Categories are defined quantitatively in terms of woody carbon density, thereby avoiding the uncertainty associated with derivative approximations of potential carbon storage based on forest area or canopy cover. After applying safeguards to lands currently utilized for food production, human habitation (e.g., urban areas), and sensitive biodiversity (nonwoody grasslands), we demonstrate the utility of the opportunity space framework for landscape-level NCS planning by analyzing the global, regional, and national potential for additional land carbon storage attributable to restoration (e.g., reforestation), management (e.g., improved natural forest stewardship), and maintenance (i.e., the sequestration benefit accrued through avoided forest conversion) of woody carbon stocks and associated soils. Finally, we evaluate the uncertainty that climate change poses to the magnitude and spatial distribution of the unrealized potential for additional carbon storage through 2050.Open in a separate windowFig. 1.The NCS opportunity space, consisting of seven categories defined by the ratio of current (x axis) to potential (y axis) carbon storage as well as carbon-based thresholds delineating NCS-relevant systems. Categories include: Restore/High suitability for forestry-based NCS (R/H; red), Maintain and manage/High suitability for forestry-based NCS (MM/H; dark green), Maintain/High suitability for forestry-based NCS (M/H; dark blue), Restore/Low suitability for forestry-based NCS (R/L; orange), Maintain and manage/Low suitability for forestry-based NCS (MM/L; light green), Maintain/Low suitability for forestry-based NCS (M/L; light blue), and Nonwoody (yellow). † denotes associated grassland/savanna biodiversity considerations.