This Perspective describes the various dimensions of Carbon Dioxide Removal (CDR) durability and interprets them in the context of current policy making. Durability - together with scalability and sustainability - is an essential condition of CDR. It depends on (i) the duration of CO2 storage and (ii) the risk of reversing such storage. The risk profile of durability varies widely across CDR methods. Because engineered, novel CDR methods involve more stable forms of CO2 storage than nature-based CDR, these methods are often promoted as a priority for CDR mitigation investments. However, shorter-term CDR plays an essential role in balancing sources and sinks of greenhouse gases in the second half of this century. Decision makers must also consider CDR policies in a larger context that takes into account readiness and feasibility, policy alignment and co-benefits of different CDR methods. They must also address durability in CDR policies and contracts, which tend to span much shorter timeframes than those contemplated by science when discussing durability. We argue that nature-based conventional CDR and novel engineered CDR that show complementary timing and risk profiles can be deployed in synergistic CDR portfoliosto balance the conditions of durability, feasibility and social and environmental sustainability.

Large-scale reforestation, afforestation, and forest restoration schemes have gained global support as climate change mitigation strategies due to their significant carbon dioxide removal (CDR) potential. However, there has been limited research into the unintended consequences of forestation from a biophysical perspective. In the Community Earth System Model version 2 (CESM2), we apply a global forestation scenario, within a Paris Agreement-compatible warming scenario, to investigate the land surface and hydroclimate response. Compared to a control scenario where land use is fixed to present-day levels, the forestation scenario is up to 2 °C cooler at low latitudes by 2100, driven by a 10 % increase in evaporative cooling in forested areas. However, afforested areas where grassland or shrubland are replaced lead to a doubling of plant water demand in some tropical regions, causing significant decreases in soil moisture (∼ 5 % globally, 5 %–10 % regionally) and water availability (∼ 10 % globally, 10 %–15 % regionally) in regions with increased forest cover. While there are some increases in low cloud and seasonal precipitation over the expanded tropical forests, with enhanced negative cloud radiative forcing, the impacts on large-scale precipitation and atmospheric circulation are limited. This contrasts with the precipitation response to simulated large-scale deforestation found in previous studies. The forestation scenario demonstrates local cooling benefits without major disruption to global hydrodynamics beyond those already projected to result from climate change, in addition to the cooling associated with CDR. However, the water demands of extensive forestation, especially afforestation, have implications for its viability, given the uncertainty in future precipitation changes.

Forestation is widely proposed for carbon dioxide (CO2) removal, but its impact on climate through changes to atmospheric composition and surface albedo remains relatively unexplored. We assessed these responses using two Earth system models by comparing a scenario with extensive global forest expansion in suitable regions to other plausible futures. We found that forestation increased aerosol scattering and the greenhouse gases methane and ozone following increased biogenic organic emissions. Additionally, forestation decreased surface albedo, which yielded a positive radiative forcing (i.e., warming). This offset up to a third of the negative forcing from the additional CO2 removal under a 4°C warming scenario. However, when forestation was pursued alongside other strategies that achieve the 2°C Paris Agreement target, the offsetting positive forcing was smaller, highlighting the urgency for simultaneous emission reductions.

Integrated assessment models that incorporate biodiversity and ecosystem services could be an important tool for improving our understanding of interconnected social-economic-ecological systems, and for analyzing how policy alternatives can shift future trajectories towards more sustainable development. Despite recent scientific and technological advances, key gaps remain in the scientific community’s ability to deliver information to decision-makers at the pace and scale needed to address sustainability challenges. We identify five research frontiers for integrated social-economic-ecological modeling (primarily focused on terrestrial systems) to incorporate biodiversity and ecosystem services: 1) downscaling impacts of direct and indirect drivers on ecosystems; 2) incorporating feedbacks in ecosystems; 3) linking ecological impacts to human well-being, 4) disaggregating outcomes for distributional equity considerations, and 5) incorporating dynamic feedbacks of ecosystem services on the social-economic system. We discuss progress and challenges along each of these five frontiers and the science-policy linkages needed to move new research and information into action.

Forestation is widely proposed for carbon dioxide (CO 2 ) removal, but its impact on climate through changes to atmospheric composition and surface albedo remains relatively unexplored. We assessed these responses using two Earth system models by comparing a scenario with extensive global forest expansion in suitable regions to other plausible futures. We found that forestation increased aerosol scattering and the greenhouse gases methane and ozone following increased biogenic organic emissions. Additionally, forestation decreased surface albedo, which yielded a positive radiative forcing (i.e., warming). This offset up to a third of the negative forcing from the additional CO 2 removal under a 4°C warming scenario. However, when forestation was pursued alongside other strategies that achieve the 2°C Paris Agreement target, the offsetting positive forcing was smaller, highlighting the urgency for simultaneous emission reductions.

Integrated assessment models that incorporate biodiversity and ecosystem services could be an important tool for improving our understanding of interconnected social-economic-ecological systems, and for analyzing how policy alternatives can shift future trajectories towards more sustainable development. Despite recent scientific and technological advances, key gaps remain in the scientific community's ability to deliver information to decision-makers at the pace and scale needed to address sustainability challenges. We identify five research frontiers for integrated social-economic-ecological modeling (primarily focused on terrestrial systems) to incorporate biodiversity and ecosystem services: 1) downscaling impacts of direct and indirect drivers on ecosystems; 2) incorporating feedbacks in ecosystems; 3) linking ecological impacts to human well-being, 4) disaggregating outcomes for distributional equity considerations, and 5) incorporating dynamic feedbacks of ecosystem services on the social-economic system. We discuss progress and challenges along each of these five frontiers and the science-policy linkages needed to move new research and information into action.

Societal Impact StatementForest ecosystems absorb and store about 25% of global carbon dioxide emissions annually and are increasingly shaped by human land use and management. Climate change interacts with land use and forest dynamics to influence observed carbon stocks and the strength of the land carbon sink. We show that climate change effects on modeled forest land carbon stocks are strongest in tropical wildlands that have limited human influence. Global forest carbon stocks and carbon sink strength may decline as climate change and anthropogenic influences intensify, with wildland tropical forests, especially in Amazonia, likely being especially vulnerable.Summary Human effects on ecosystems date back thousands of years, and anthropogenic biomes—anthromes—broadly incorporate the effects of human population density and land use on ecosystems. Forests are integral to the global carbon cycle, containing large biomass carbon stocks, yet their responses to land use and climate change are uncertain but critical to informing climate change mitigation strategies, ecosystem management, and Earth system modeling. Using an anthromes perspective and the site locations from the Global Forest Carbon (ForC) Database, we compare intensively used, cultured, and wildland forest lands in tropical and extratropical regions. We summarize recent past (1900‐present) patterns of land use intensification, and we use a feedback analysis of Earth system models from the Coupled Model Intercomparison Project Phase 6 to estimate the sensitivity of forest carbon stocks to CO2 and temperature change for different anthromes among regions. Modeled global forest carbon stock responses are positive for CO2 increase but neutral to negative for temperature increase. Across anthromes (intensively used, cultured, and wildland forest areas), modeled forest carbon stock responses of temperate and boreal forests are less variable than those of tropical forests. Tropical wildland forest areas appear especially sensitive to CO2 and temperature change, with the negative temperature response highlighting the potential vulnerability of the globally significant carbon stock in tropical forests. The net effect of anthropogenic activities—including land‐use intensification and environmental change and their interactions with natural forest dynamics—will shape future forest carbon stock changes. These interactive effects will likely be strongest in tropical wildlands.

UNEP’s Emissions Gap Report 2024: No more hot air … please! is the 15th edition in a series that brings together many of the world’s top climate scientists to look at future trends in greenhouse gas emissions and provide potential solutions to the challenge of global warming. As climate impacts intensify globally, the report finds that nations must deliver dramatically stronger ambition and action in the next round of Nationally Determined Contributions or the Paris Agreement’s 1.5°C goal will be gone within a few years.

9;23) Au orstung, Wiederau orstung (2;5) Waldwiederherstellung (Torfgebiete) (2;5) Holzernte (vergrößert) (1;1) Holzernte (verringert) (6;13) andere aktive Bewirtschaftung (7;14) Vermeidung von Entwaldung (1;1)Waldschutz (2;3)

4.00 – 4.20: What is the mitigation potential of improved land-use, and how will it respond to future climate change? , Stephanie Roe 4.20 – 4.40: Agroforestry as a natural climate solution, Susan Cook-Patton 4.40 – 5.00: Global Carbon Dioxide Removal Potential of Trees in Agriculture , Vivian Griffey 5.00 – 5.20: Effect of land use and land cover change and CO2 fertilization on the future carbon sink for the conterminous U.S, Benjamin Felzer 5.20 – 5.40: What will happen to the terrestrial carbon sink once we reach net zero?, Charlie Koven What is the mitigation potential of improved land-use, and how will it respond to future climate change? Improved stewardship of land to reduce GHG emissions, enhance carbon removals and protect the residual sink have gained significant attention and importance in delivering on the Paris Agreement goal of limiting warming to 1.5°C and 2°C. To better understand the land sector’s role in mitigation pathways, this presentation will examine mitigation potentials for 20 land-based measures across >200 countries, and compare methods used in the IPCC reports including technical and economic estimates using “bottom-up” sectoral and integrated assessment model approaches. To address one of the main gaps in mitigation potential literature identified by the IPCC, this presentation will also explore the impact of future climate change on land-based sequestration potential. Agroforestry as a natural climate solution Restoring tree cover is a prominent climate solution, with the potential to remove gigatonnes of carbon dioxide out of the air. However, achieving this potential will depend on many human decisions about how and where to restore tree cover. In particular, agroforestry is a promising option, given its potential to simultaneously store additional carbon, enhance livelihoods, and support biodiversity. However, substantial uncertainty remains around how much carbon can be captured. One of the central challenges is the sheer diversity of agroforestry practices employed across the globe. Species identity, planting density, and management practices, as well as many other factors, will influence the overall climate mitigation potential of an individual agroforestry system. Although recent reviews have begun to compile carbon sequestration rates and stocks within agroforestry systems, the current evidence base is not fully comprehensive. Individual reviews examine only a subset of the existing literature and typically partition agroforestry systems into coarse categories that do not reflect the diversity of actual on-the-ground practices. As individuals, corporations, and governments decide whether and how to deploy agroforestry as a climate solution during this climate critical decade, there is a strong need for a readily available and comprehensive dataset to better predict climate outcomes across diverse agroforestry systems. We are therefore conducting a systematic review across over 25,000 published studies to find empirical estimates of carbon sequestration rates and stocks in agroforestry systems. To date, we have compiled data from over 1000 papers into a consistent data structure. Our goal is to create a publicly available dataset that can help to accelerate our scientific understanding of the climate mitigation potential of these human-natural ecosystems. Although agroforestry offers high potential as a climate solution, delivering on that promise requires a more precise understanding of how much carbon can be captured, based on the best available data. Global Carbon Dioxide Removal Potential of Trees in Agriculture Trees in agricultural systems have been identified as a high-potential Natural Climate Solution with benefits to biodiversity and climate resilience. However, efforts to quantify their CO2e mitigation potential at a global scale have been based on standing biomass, without carefully addressing potential impacts on agricultural production. We estimated the enhanced carbon sequestration potential of increasing tree cover without compromising long-term agricultural yields, starting with a Delphi-style expert elicitation on ideal tree types and densities in major cropping and grazing systems, disaggregated by climatic zones (biomes). Comparing expert-recommended tree density values to current levels, we applied the spatially explicit Cook-Patton (2020) aboveground carbon accumulation rates to create a 30x30m resolution global map of the additional carbon sequestration potential of trees in agriculture over the next 30 years, analyzing the results by biome and country. From the Delphi-style elicitation, we found a suggested pattern of recommended tree cover related to aridity, with the lowest recommended tree cover values in deserts. Carbon sequestration potential per hectare for both crop and grazing lands was generally higher in small tropical countries, such as Laos and Malaysia, but also high in small wet, temperate countries, such as Ireland and Belgium. Effect of land use and land cover change and CO2 fertilization on the future carbon sink for the conterminous U.S Historic land use and land cover change (LULCC) has been responsible for the largest release of CO2 from terrestrial ecosystems, while climate change, rising atmospheric CO2 levels, nitrogen deposition, and surface ozone have all contributed to changing carbon dynamics since the 1860s. My current work with the Terrestrial Ecosystems Model (TEM) highlights the importance of land-use legacy on the future carbon sink, even without further LULCC, due to soil nutrients and forest stand-age legacy. The new experiments will start from initial conditions that account for land-use legacy from 1750. This study will involve using the SSP3rcp7 climate from NCAR CESM2 from 2015 – 2099 for the conterminous U.S., downscaled and bias correct to CRU4.04 historical data. The land use transitions are developed from Land Use Harmonization (LUH2) data into land cover cohorts, starting from fractional land cover at 2015. Future U.S. runs with TEM will involve experiments with and without LULCC, and with and without future changes in CO2, to evaluate the effect of LULCC and CO2 fertilization. Current results for the conterminous U.S. without LULCC for the RCP8.5 scenario show increasing vegetation carbon, little change in soil carbon, with increasing cumulative Net Ecosystem Productivity and carbon storage. What will happen to the terrestrial carbon sink once we reach net zero? Currently about half of anthropogenic CO2 emissions are taken up by land and ocean sinks, which on land has been driven largely by CO2 fertilization. While a large amount of research has focused on how these sinks may weaken under further unmitigated CO2 emission scenarios, less focus has been on the behavior of these sinks under highly mitigated future scenarios. Here we ask how we expect carbon sinks to behave if we are able to achieve net zero or net negative carbon dioxide emissions, what are some of the key uncertainties governing these coupled carbon-climate responses to net zero and net negative emissions, and how these dynamics may inform both climate policy and the potential for using biospheric sinks as a form of carbon dioxide removal.

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The majority of scenarios that meet the goals of the Paris agreements exceed sustainability and precautionary thresholds in land, biodiversity and BECCS potentials. Risks may be best avoided by demand-side driven rapid decarbonization and less land-intensive carbon dioxide removal technologies.

Land‐based climate mitigation measures have gained significant attention and importance in public and private sector climate policies. Building on previous studies, we refine and update the mitigation potentials for 20 land‐based measures in >200 countries and five regions, comparing “bottom‐up” sectoral estimates with integrated assessment models (IAMs). We also assess implementation feasibility at the country level. Cost‐effective (available up to $100/tCO 2 eq) land‐based mitigation is 8–13.8 GtCO 2 eq yr −1 between 2020 and 2050, with the bottom end of this range representing the IAM median and the upper end representing the sectoral estimate. The cost‐effective sectoral estimate is about 40% of available technical potential and is in line with achieving a 1.5°C pathway in 2050. Compared to technical potentials, cost‐effective estimates represent a more realistic and actionable target for policy. The cost‐effective potential is approximately 50% from forests and other ecosystems, 35% from agriculture, and 15% from demand‐side measures. The potential varies sixfold across the five regions assessed (0.75–4.8 GtCO2eq yr −1 ) and the top 15 countries account for about 60% of the global potential. Protection of forests and other ecosystems and demand‐side measures present particularly high mitigation efficiency, high provision of co‐benefits, and relatively lower costs. The feasibility assessment suggests that governance, economic investment, and socio‐cultural conditions influence the likelihood that land‐based mitigation potentials are realized. A substantial portion of potential (80%) is in developing countries and LDCs, where feasibility barriers are of greatest concern. Assisting countries to overcome barriers may result in significant quantities of near‐term, low‐cost mitigation while locally achieving important climate adaptation and development benefits. Opportunities among countries vary widely depending on types of land‐based measures available, their potential co‐benefits and risks, and their feasibility. Enhanced investments and country‐specific plans that accommodate this complexity are urgently needed to realize the large global potential from improved land stewardship.

Better land stewardship is needed to achieve the Paris Agreement's temperature goal, particularly in the tropics, where greenhouse gas emissions from the destruction of ecosystems are largest, and where the potential for additional land carbon storage is greatest. As countries enhance their nationally determined contributions (NDCs) to the Paris Agreement, confusion persists about the potential contribution of better land stewardship to meeting the Agreement's goal to hold global warming below 2°C. We assess cost-effective tropical country-level potential of natural climate solutions (NCS)—protection, improved management and restoration of ecosystems—to deliver climate mitigation linked with sustainable development goals (SDGs). We identify groups of countries with distinctive NCS portfolios, and we explore factors (governance, financial capacity) influencing the feasibility of unlocking national NCS potential. Cost-effective tropical NCS offers globally significant climate mitigation in the coming decades (6.56 Pg CO 2 e yr −1 at less than 100 US$ per Mg CO 2 e). In half of the tropical countries, cost-effective NCS could mitigate over half of national emissions. In more than a quarter of tropical countries, cost-effective NCS potential is greater than national emissions. We identify countries where, with international financing and political will, NCS can cost-effectively deliver the majority of enhanced NDCs while transforming national economies and contributing to SDGs. This article is part of the theme issue ‘Climate change and ecosystems: threats, opportunities and solutions’.

There is a clear need for transformative change in the land management and food production sectors to address the global land challenges of climate change mitigation, climate change adaptation, combatting land degradation and desertification, and delivering food security (referred to hereafter as "land challenges"). We assess the potential for 40 practices to address these land challenges and find that: Nine options deliver medium to large benefits for all four land challenges. A further two options have no global estimates for adaptation, but have medium to large benefits for all other land challenges. Five options have large mitigation potential (>3 Gt CO2 eq/year) without adverse impacts on the other land challenges. Five options have moderate mitigation potential, with no adverse impacts on the other land challenges. Sixteen practices have large adaptation potential (>25 million people benefit), without adverse side effects on other land challenges. Most practices can be applied without competing for available land. However, seven options could result in competition for land. A large number of practices do not require dedicated land, including several land management options, all value chain options, and all risk management options. Four options could greatly increase competition for land if applied at a large scale, though the impact is scale and context specific, highlighting the need for safeguards to ensure that expansion of land for mitigation does not impact natural systems and food security. A number of practices, such as increased food productivity, dietary change and reduced food loss and waste, can reduce demand for land conversion, thereby potentially freeing-up land and creating opportunities for enhanced implementation of other practices, making them important components of portfolios of practices to address the combined land challenges.