Global climate change mitigation necessitates substantial reductions in greenhouse gas emissions and significant negative emissions. The Intergovernmental Panel on Climate Change (IPCC) highlights large-scale carbon dioxide removal (CDR) methods, such as afforestation and reforestation (collectively referred to as afforestation in this study for simplification) and bioenergy with carbon capture and storage (BECCS), as vital for achieving stringent climate goals. While BECCS involves capturing CO2 from bioenergy facilities, afforestation involves establishing forests on previously non-forested land. Both are projected to play significant roles in CO2 removal, with comparable annual mitigation volumes estimated in some scenarios. A key concern, however, is the potential for land-based CDR strategies to negatively impact food security and biodiversity. Inappropriate forest expansion can reduce habitats and harm biodiversity. To address these concerns, this research investigates the impact of forest type selection within afforestation strategies and their interaction with food-related measures to enhance carbon sequestration while minimizing negative effects on food and land sustainability. This study aims to quantitatively assess the effects of strategic forest type selection and food measures on carbon sequestration potential and their interaction with global food and land systems.

Existing literature extensively explores the potential of BECCS and afforestation for climate change mitigation, with estimates of their respective contributions to carbon sequestration. However, there is growing concern about the sustainability of land-based CDR strategies. Studies have highlighted the potential conflicts between large-scale CDR and food security, biodiversity, and other aspects of sustainability. Concerns about the ecological impacts of afforestation, particularly the conversion of natural habitats, are well documented. The potential for land-based CDR to negatively affect multiple sustainability objectives under stringent climate goals has been emphasized. While food-related measures, such as agricultural intensification and dietary changes, are proposed to enhance carbon sequestration through BECCS, few studies have specifically examined the optimal forest type selection for afforestation and the design of complementary food-related policies to minimize negative impacts on food and land systems. This paper directly addresses this gap.

This study employs an integrated assessment framework combining an economic model (AIM/Hub), a land-use allocation model (AIM/PLUM), and a terrestrial vegetation model (VISIT). AIM/Hub estimates regional land demand across 17 global regions based on socioeconomic factors, including food measures (agricultural intensification, dietary change, trade globalization, and improved food distribution). It incorporates emissions mitigation through a carbon budget and uniform carbon price across sectors. The regional land demand data are then input into AIM/PLUM, which allocates land for afforestation and other land uses at a half-degree grid resolution to maximize landowner profits based on biophysical productivity. VISIT is used to calculate the carbon sequestration potential of different forest types. Nine climate mitigation scenarios were analyzed, differing in the availability of afforestation and BECCS, forest type selection schemes (Aff-Cur: native forest; Aff-Div: selection from the same agro-ecological zone; Aff-Cmax: most carbon-intensive forest), and the implementation of food-related measures. Forest type selection was based on the growth rates of 12 different forest types, with Aff-Cmax maximizing carbon sequestration potential irrespective of ecological considerations. A tree growth function was used to account for carbon sequestration changes over time, parameterized using net primary production (NPP) data from VISIT. The food-related measures considered agricultural intensification, trade globalization, and improvements in food distribution equality, as well as dietary change. The study calculates the land intensity of carbon sequestration potential (LIC) by dividing the amount of carbon sequestration by the land area allocated. This allows for direct comparison of the efficiency of afforestation and BECCS in terms of carbon sequestration per unit area. Various economic, energy, food system and land-use change indicators are assessed to evaluate the impacts of different scenarios on sustainability.

Careful selection of forest types significantly impacts carbon sequestration potential. The study found that selecting carbon-intensive forest types from within the same agro-ecological zone (Aff-Div) increased global carbon sequestration by 2% compared to using native forest types (Aff-Cur). Selecting the most carbon-intensive forest types regardless of ecological considerations (Aff-Cmax) resulted in a 25% increase in global carbon sequestration compared to Aff-Cur. Regional differences in carbon sequestration are observed, with substantial increases in regions outside of the OECD and EU. For example, in Latin America, carbon sequestration increased by 37% in Aff-Cmax compared to Aff-Cur. The analysis also shows that the land intensity of carbon sequestration potential (LIC) for afforestation is lower than for BECCS, meaning that afforestation requires a larger land area to achieve equivalent carbon sequestration. Even in the Aff-Cmax scenario, global sequestration by afforestation remains lower than that achieved by BECCS in the BECCS-only scenario. Afforestation and BECCS impact land-use change differently. Afforestation leads to decreased cropland and pastureland, while BECCS results in increased cropland due to the land required for bioenergy production. Implementation of food-related measures significantly enhances carbon sequestration for both afforestation and BECCS scenarios, with the Aff-CmaxFodPol scenario yielding the highest sequestration. This is driven by reduced land demand for agriculture, particularly due to dietary changes. Economic analysis shows that afforestation-only scenarios lead to higher carbon prices and greater GDP losses compared to BECCS-only scenarios. Afforestation raises electricity prices and land/food prices more significantly than BECCS, increasing the risk of hunger. Food measures mitigate the negative economic and food-system impacts of both afforestation and BECCS but do not entirely eliminate the risk of hunger.

This study's findings highlight the importance of strategic forest type selection for optimizing carbon sequestration through afforestation. The 25% increase in carbon sequestration achieved with Aff-Cmax demonstrates the potential benefits of careful planning. However, the lower LIC of afforestation compared to BECCS necessitates considering the trade-offs. The increased risk of hunger associated with afforestation compared to BECCS underscores the need for complementary policies, including measures to improve agricultural productivity and food distribution. The significant improvements in carbon sequestration from the inclusion of food policies suggest that integrated approaches are crucial for maximizing climate change mitigation while maintaining sustainability. The results do not diminish the importance of future afforestation but rather emphasize the necessity for complementary policies that reduce negative impacts. The superiority of BECCS over afforestation in terms of carbon sequestration efficiency and lower negative impacts on food and land systems is also noteworthy. This suggests a potential role for a combination of both land-based CDR technologies, coupled with proactive food-related policies, to achieve ambitious climate targets. While this study shows that the combined approach involving carbon intensive forests and food measures yields the best results, the uncertainty of BECCS adoption is also a crucial factor for future climate policy.

This study demonstrates that careful forest type selection in afforestation can substantially increase carbon sequestration without compromising sustainability, particularly when coupled with food-related measures. However, the lower land-use efficiency of afforestation compared to BECCS highlights the importance of complementary policies and integrated approaches to mitigate negative impacts on food and land systems. Future research should explore other land-based CDR technologies and integrate more detailed biodiversity and socio-economic considerations within integrated assessment models. Achieving ambitious climate goals requires a balanced strategy encompassing optimized land-based CDR options and effective food policies.

This study has several limitations. First, it doesn't consider all available land-based CDR technologies, focusing primarily on afforestation and BECCS. Second, the absence of a biodiversity model limits the assessment of ecological impacts. Third, the costs of food-related measures are not explicitly included. Fourth, the study doesn't incorporate albedo effects or climate-forest interactions. Finally, the analysis relies on a single integrated assessment model, which may affect the generalizability of the findings. Future work incorporating these considerations would provide a more comprehensive understanding of the impacts of afforestation and BECCS on climate change mitigation and sustainability.