Although decades of research suggest that higher species richness improves ecosystem functioning and stability, planted forests are predominantly monocultures. To determine whether diversification of plantations would enhance aboveground carbon storage, we systematically reviewed over 11,360 publications, and acquired data from a global network of tree diversity experiments. We compiled a maximum dataset of 79 monoculture to mixed comparisons from 21 sites with all variables needed for a meta-analysis. We assessed aboveground carbon stocks in mixed-species planted forests vs. (a) the average of monocultures, (b) the best monoculture, and (c) commercial species monocultures, and examined potential mechanisms driving differences in carbon stocks between mixtures and monocultures. On average, we found that aboveground carbon stocks in mixed planted forests were 70% higher than the average monoculture, 77% higher than commercial monocultures, and 25% higher than the best performing monocultures, although the latter was not statistically significant. Overyielding was highest in four-species mixtures (richness range 2–6 species), but otherwise none of the potential mechanisms we examined (nitrogen-fixer present vs. absent; native vs. non-native/mixed origin; tree diversity experiment vs. forestry plantation) consistently explained variation in the diversity effects. Our results, predominantly from young stands, thus suggest that diversification could be a very promising solution for increasing the carbon sequestration of planted forests and represent a call to action for more data to increase confidence in these results and elucidate methods to overcome any operational challenges and costs associated with diversification.
NbS Target: Climate Change Mitigation
Climate change mitigation
Natural ecosystems store large amounts of carbon globally, as organisms absorb carbon from the atmosphere to build large, long-lasting, or slow-decaying structures such as tree bark or root systems. An ecosystem’s carbon sequestration potential is tightly linked to its biological diversity. Yet when considering future projections, many carbon sequestration models fail to account for the role biodiversity plays in carbon storage. Here, we assess the consequences of plant biodiversity loss for carbon storage under multiple climate and land-use change scenarios. We link a macroecological model projecting changes in vascular plant richness under different scenarios with empirical data on relationships between biodiversity and biomass. We find that biodiversity declines from climate and land use change could lead to a global loss of between 7.44-103.14 PgC (global sustainability scenario) and 10.87-145.95 PgC (fossil-fueled development scenario). This indicates a self-reinforcing feedback loop, where higher levels of climate change lead to greater biodiversity loss, which in turn leads to greater carbon emissions and ultimately more climate change. Conversely, biodiversity conservation and restoration can help achieve climate change mitigation goals.
This independent economic study, commissioned by the RSPB, National Trust and The Wildlife Trusts, is the best evidence yet for how much investment is needed and how it needs to be spent. The analysis shows an annual investment of up to £5.9 billion will be required for at least the next ten years, demonstrating a costed pathway to deliver legally binding nature and climate targets, and ultimately help secure the future of British farming.
Scaling funding to effective nature-based solutions (NbS) for adaptation is key to tackle climate change and support sustainable development. NbS can play a crucial role in adaptation and investments deliver multidimensional benefits for climate mitigation, resilience, people and livelihoods as well as the protection, maintenance, or enhancement of biodiversity. UNEP estimates that approximately $11tn of investment in NbS is required between 2022 to 2050, equating to over $500bn of annual investment by 2030. This means that investment into NbS needs to be quickly and drastically scaled from its current levels of around $200bn per year.
Pathways to achieving net zero carbon emissions commonly involve deploying reforestation, afforestation, and bioenergy crops across millions of hectares of land. It is often assumed that by helping to mitigate climate change, these strategies indirectly benefit biodiversity. Here, we modeled the climate and habitat requirements of 14,234 vertebrate species and show that the impact of these strategies on species’ habitat area tends not to arise through climate mitigation, but rather through habitat conversion. Across locations, reforestation tends to provide species more habitat through both land-cover change and climate mitigation, whereas habitat loss from afforestation and bioenergy cropping typically outweighs the climate mitigation benefits. This work shows how and where land-based mitigation strategies can be deployed without inadvertently reducing the area of habitat for global biodiversity.
International commitments advocate large-scale forest restoration as a nature-based solution to climate change mitigation through carbon (C) sequestration. Mounting evidence suggests that mixed compared to monospecific planted forests may sequester more C, exhibit lower susceptibility to climate extremes and offer a broader range of ecosystem services. However, experimental studies comprehensively examining the control of tree diversity on multiple C stocks and fluxes above- and belowground are lacking. To address this gap, we leverage data from the Sardinilla experiment in Panama, the oldest tropical tree diversity experiment, which features a gradient of one-, two-, three- and five-species mixtures of native tree species. Over 16 years, we measured multiple above- and belowground C stocks and fluxes, ranging from tree aboveground C, over leaf litter C production, to soil organic carbon (SOC). We show that tree diversity significantly increased aboveground C stocks and fluxes, with a 57% higher gain in aboveground tree C in five-species mixtures compared to monocultures (35.7 ± 1.8 vs. 22.8 ± 3.4 Mg C ha−1) 16 years after planting. In contrast, we observed a net reduction in SOC (on average −11.2 ± 1.1 Mg C ha−1 across diversity levels) and no significant difference in SOC3 stocks (the predominantly tree-derived, i.e., C3 plant-derived SOC fraction) between five-species mixtures and monocultures (13.0 ± 0.9 vs. 15.1 ± 1.3 Mg C ha−1). Positive tree diversity effects persisted despite repeated climate extremes and strengthened over time for aboveground tree growth. Structural equation models showed that higher tree growth in mixtures enhanced leaf litter and coarse woody debris C fluxes to the soil, resulting in a tightly linked C cycle aboveground. However, we did not observe significant links between above- and belowground C stocks and fluxes. Our study elucidates the mechanisms through which higher tree diversity bolsters the climate mitigation potential of tropical forest restoration. Restoration schemes should prioritize mixed over monospecific planted forests.
Many governments and industries are relying on future large-scale, land-based carbon dioxide (CO2) removal (CDR) to avoid making necessary steep greenhouse gas (GHG) emission cuts today. Not only does this risk locking us into a high overshoot above 1.5°C, but it will also increase biodiversity loss, imperiling the Kunming-Montreal Global Biodiversity Framework (KMGBF) goals (4). Such CDR deployments also pose major economic, technological, and social feasibility challenges; threaten food security and human rights; and risk overstepping multiple planetary boundaries, with potentially irreversible consequences. We propose three ways to build on the Intergovernmental Panel on Climate Change (IPCC) analyses of CDR mitigation potential by assessing sustainability risks associated with land-use change and biodiversity loss: estimate the sustainable CDR budget based on socioecological thresholds; identify viable mitigation pathways that do not overstep these thresholds; and reframe governance around allocating limited CDR supply to the most legitimate uses.
This research was conducted over the last year as part of the Hitachi-Imperial Centre for Decarbonisation and Natural Climate Solutions, with the aim of better understanding biodiversity and ecosystem function relationships. While we know that biodiversity often improves ecosystem functioning and so the provision of nature-based solutions, our goal was to synthesise disparate studies to build a complete, quantitative picture of how different kinds of ecosystem functions respond to biodiversity, since the shape of the relationship will be key for designing future interventions.
This small-scale study identifies the best practice current and emerging NBS in Ireland, through literature review and consultation with experts in the field, which have exhibited the potential to mitigate climate change and its impacts or enable adaptation to the impacts of climate change. Some of the case studies identified in this review may not have been implemented as NBS, but they tackle specific climate and societal problems while also providing ecosystem services, as well as benefits to human well-being and biodiversity.
The voluntary carbon market has come under intense scrutiny over concerns around the environmental and social integrity of the carbon credits being sold. Even certification bodies are seemingly incapable of delivering on their promise to certify “high-integrity” credits. The certification of jurisdictional REDD+ carbon credits by the Architecture for REDD+ Transactions (ART) to the Government of Guyana (GoG) in December 2022 provides an important case study on the challenges facing the high-integrity carbon market.
The Cerrado is the second largest biome in Latin America and the most biodiverse savanna in the world. Yet it has lost half its native vegetation to agriculture – and conversion is accelerating. Urgent action is needed to balance production with protection, in a way that delivers gains for climate, nature and people.
Nature-based solutions (NBS) can deliver many benefits to human wellbeing, including some crucial to climate adaptation. We quantitatively assess the global potential of NBS strategies of protection, restoration, and agroforestry by modeling global climate change mitigation and local ecosystem services (water availability, sediment retention, runoff, pollination, nitrogen retention, green water storage, and coastal protection). The strategies with the most potential to help people do not necessarily deliver the most climate change mitigation: per area of conservation action, agroforestry provides substantial benefits (>20% increase in at least one local ecosystem service) to three times more people on average than reforestation while providing less than one tenth the carbon sequestration per unit area. Each strategy delivers a different suite of ecosystem service benefits; for instance, avoided forest conversion provides a strong increase in nitrogen retention (100% increase to 72 million people if fully implemented globally) while agroforestry increases pollination services (100% increase to 3.0 billion people if fully implemented globally). One common disservice shared by all the NBS strategies modeled here is that increased woody biomass increases transpiration, reducing annual runoff and in some watersheds negatively impacting local water availability. In addition, the places with the greatest potential for climate change mitigation are not necessarily the ones with the most people. For instance, reforestation in Latin America has the greatest climate change mitigation potential, but the greatest ecosystem service benefits are in Africa. Focusing on nations with high climate mitigation potential as well as high local ecosystem service potential, such as Nigeria in the case of reforestation, India for agroforestry, and the Republic of Congo for avoided forest conversion, can help identify win-win sites for implementation. We find that concentrating implementation of these three conservation strategies in critical places, covering 5.8 million km2, could benefit 2.0 billion people with increased local ecosystem services provision. These critical places cover only 35% of the possible area of implementation but would provide 80% of the benefits that are possible globally for the selected set of ecosystem services under the NBS scenarios examined here. We conclude that targeting these critical places for protection, restoration, and agroforestry interventions will be key to achieving adaptation and human wellbeing goals while also increasing nature-based carbon mitigation.
Established in 2002, the Amazon Protected Areas Program (ARPA) supports 120 Conservation Units (CUs) in the Brazilian Amazon, covering 62 Mha. Here, we quantified the impact of ARPA support on reducing deforestation and CO2 emissions between 2008 and 2020. We started by examining critical methodological choices, often brushed over in the impact evaluation studies on protected areas (PAs). We then applied a covariate balancing method to control for variation in covariates so as to compare differences in deforestation between Strictly Protected (SP) and Sustainable Use (SU) CUs with and without ARPA support as well as to assess the influence of ARPA investment mechanism on the differential reductions. Next, we estimated total reductions in deforestation and CO2 emissions by using the Adjusted Odds Ratio. We found that ARPA support accounts for additional deforestation reductions of 9 % in SP CUs and 39 % in SU CUs in relation to non-supported CUs. The effects of ARPA investment mechanism were statistically significant for both categories of CUs. CUs plus Indigenous Lands (i.e., PAs) reduced by 21 % (2.0 ± 0.3 Mha) Amazon deforestation between 2008 and 2020. Of this total, ARPA CUs accounts for 264 ± 25 thousand ha, the equivalent of 104 ± 10 Mtons of CO2 emissions. If deforestation continues unabated, PAs will become the last citadels of the Amazon. However, protecting the Amazon only with PAs does not suffice. Additional investments in a comprehensive conservation policy mix are needed along with a monitoring and evaluation strategy to provide evidence on what works for effective and socially equitable forest conservation.
Forest regeneration is a crucial strategy for mitigating and adapting to global warming. Yet its precise impact on local climate remains uncertain, a factor that complicates decision-making when it comes to prioritizing investments. Here, we developed global maps illustrating how natural forest regeneration influences key local climate drivers—land surface temperature (LST), albedo, and evapotranspiration—using models fitted at a 1-km spatial resolution with a random forest classifier. We found that natural forest regeneration can alter annual mean LST by 0.01 °C, −0.59 °C, −0.50 °C, and −2.03 °C in Boreal, Mediterranean, Temperate, and Tropical regions, respectively. These variations underscore the region-specific effects of forest regeneration. Importantly, natural forest regeneration reduces LST across 64% of 1 billion hectares and 75% of 148 million hectares of potentially restorable land under different scenarios. These findings improve understanding of how forest regeneration can help regulate local climate, supporting climate adaptation efforts.
Extensive forest restoration is a key strategy to meet nature-based sustainable development goals and provide multiple social and environmental benefits1. Yet achieving forest restoration at scale requires cost-effective methods2. Tree planting in degraded landscapes is a popular but costly forest restoration method that often results in less biodiverse forests when compared to natural regeneration techniques under similar conditions3. Here we assess the current spatial distribution of pantropical natural forest (from 2000 to 2016) and use this to present a model of the potential for natural regeneration across tropical forested countries and biomes at a spatial resolution of 30 m. We estimate that an area of 215 million hectares—an area greater than the entire country of Mexico—has potential for natural forest regeneration, representing an above-ground carbon sequestration potential of 23.4 Gt C (range, 21.1–25.7 Gt) over 30 years. Five countries (Brazil, Indonesia, China, Mexico and Colombia) account for 52% of this estimated potential, showcasing the need for targeting restoration initiatives that leverage natural regeneration potential. Our results facilitate broader equitable decision-making processes that capitalize on the widespread opportunity for natural regeneration to help achieve national and global environmental agendas.
Achieving net-zero climate targets requires some level of carbon dioxide removal. Current assessments focus on tonnes of CO2 removed, without specifying what form these removals will take. Here, we show that countries’ climate pledges require approximately 1 (0.9–1.1) billion ha of land for removals. For over 40% of this area, the pledges envisage the conversion of existing land uses to forests, while the remaining area restores existing ecosystems and land uses. We analyse how this demand for land is distributed geographically and over time. The results are concerning, both in terms of the aggregate area of land, but also the rate and extent of land use change. Our findings demonstrate a gap between governments’ expected reliance on land and the role that land can realistically play in climate mitigation. This adds another layer to the observed shortcomings of national climate pledges and indicates a need for more transparency around the role of land in national climate mitigation plans.
Natural climate solutions (NCS) play a critical role in climate change mitigation. NCS can generate win–win co-benefits for biodiversity and human well-being, but they can also involve trade-offs (co-impacts). However, the massive evidence base on NCS co-benefits and possible trade-offs is poorly understood. We employ large language models to assess over 2 million published journal articles, primarily written in English, finding 257,266 relevant studies on NCS co-impacts. Using machine learning methods to extract data (for example, study location, species and other key variables), we create a global evidence map on NCS co-impacts. We find that global evidence on NCS co-impacts has grown approximately tenfold in three decades, and some of the most abundant evidence relates to NCS that have lower mitigation potential. Studies often examine multiple NCS, indicating some natural complementarities. Finally, we identify countries with high carbon mitigation potential but a relatively weak body of evidence on NCS co-impacts. Through effective methods and systematic and representative data on NCS co-impacts, we provide timely insights to inform NCS-related research and action globally.
Multi-functional urban green infrastructure (GI) can deliver nature-based solutions that help address climate change, while providing wider benefits for human health and biodiversity. However, this will only be achieved effectively, sustainably and equitably if GI is carefully planned, implemented and maintained to a high standard, in partnership with stakeholders. This paper draws on original research into the design of a menu of GI standards for England, commissioned by Natural England—a United Kingdom Government agency. It describes the evolution of the standards within the context of United Kingdom government policy initiatives for nature and climate. We show how existing standards and guidelines were curated into a comprehensive framework consisting of a Core Menu and five Headline Standards. This moved beyond simplistic metrics such as total green space, to deliver GI that meets five key ‘descriptive principles’: accessible, connected, locally distinctive, multi-functional and varied, and thus delivers 5 ‘benefits principles’: places that are nature rich and beautiful, active and healthy, thriving and prosperous, resilient and climate positive, and with improved water management. It also builds in process guidance, bringing together stakeholders to co-ordinate GI development strategically across different sectors. Drawing on stakeholder feedback, we evaluate the strengths and weaknesses of the standards and discuss how they provide clarity and consistency while balancing tensions between top-down targets and the need for flexibility to meet local needs. A crucial factor is the delivery of the standards within a framework of supporting tools, advice and guidance, to help planners with limited resources deliver more effective and robust green infrastructure with multiple benefits.
The term carbon (C) sequestration has not just become a buzzword but is something of a siren’s call to scientific communicators and media outlets. Carbon sequestration is the removal of C from the atmosphere and the storage, for example, in soil. It has the potential to partially compensate for anthropogenic greenhouse gas emissions and is, therefore, an important piece in the global climate change mitigation puzzle. However, the term C sequestration is often used misleadingly and, while likely unintentional, can lead to the perpetuation of biased conclusions and exaggerated expectations about its contribution to climate change mitigation efforts. Soils have considerable potential to take up C but many are also in a state of continuous loss. In such soils, measures to build up soil C may only lead to a reduction in C losses (C loss mitigation) rather than result in real C sequestration and negative emissions. In an examination of 100 recent peer-reviewed papers on topics surrounding soil C, only 4% were found to have used the term C sequestration correctly. Furthermore, 13% of the papers equated C sequestration with C stocks. The review, further, revealed that measures leading to C sequestration will not always result in climate change mitigation when non-CO2 greenhouse gases and leakage are taken into consideration. This paper highlights potential pitfalls when using the term C sequestration incorrectly and calls for accurate usage of this term going forward. Revised and new terms are suggested to distinguish clearly between C sequestration in soils, SOC loss mitigation, negative emissions, climate change mitigation, SOC storage, and SOC accrual to avoid miscommunication among scientists and stakeholder groups in future.
Improved knowledge of glacial-to-interglacial global temperature change yields Charney (fast-feedback) equilibrium climate sensitivity 1.2 ± 0.3°C (2σ) per W/m2, which is 4.8°C ± 1.2°C for doubled CO2. Consistent analysis of temperature over the full Cenozoic era—including ‘slow’ feedbacks by ice sheets and trace gases—supports this sensitivity and implies that CO2 was 300–350 ppm in the Pliocene and about 450 ppm at transition to a nearly ice-free planet, exposing unrealistic lethargy of ice sheet models. Equilibrium global warming for today’s GHG amount is 10°C, which is reduced to 8°C by today’s human-made aerosols. Equilibrium warming is not ‘committed’ warming; rapid phaseout of GHG emissions would prevent most equilibrium warming from occurring. However, decline of aerosol emissions since 2010 should increase the 1970–2010 global warming rate of 0.18°C per decade to a post-2010 rate of at least 0.27°C per decade. Thus, under the present geopolitical approach to GHG emissions, global warming will exceed 1.5°C in the 2020s and 2°C before 2050. Impacts on people and nature will accelerate as global warming increases hydrologic (weather) extremes. The enormity of consequences demands a return to Holocene-level global temperature. Required actions include: (1) a global increasing price on GHG emissions accompanied by development of abundant, affordable, dispatchable clean energy, (2) East-West cooperation in a way that accommodates developing world needs, and (3) intervention with Earth’s radiation imbalance to phase down today’s massive human-made ‘geo-transformation’ of Earth’s climate. Current political crises present an opportunity for reset, especially if young people can grasp their situation.
Efforts to avert dangerous climate change by conserving and restoring natural habitats are hampered by concerns over the credibility of methods used to quantify their long-term impacts. Here we develop a flexible framework for estimating the net social benefit of impermanent nature-based interventions that integrates three substantial advances: (1) conceptualizing the permanence of a project’s impact as its additionality over time; (2) risk-averse estimation of the social cost of future reversals of carbon gains; and (3) post-credit monitoring to correct errors in deliberately pessimistic release forecasts. Our framework generates incentives for safeguarding already credited carbon while enabling would-be investors to make like-for-like comparisons of diverse carbon projects. Preliminary analyses suggest nature-derived credits may be competitively priced even after adjusting for impermanence.
The remaining carbon budget (RCB), the net amount of CO2 humans can still emit without exceeding a chosen global warming limit, is often used to evaluate political action against the goals of the Paris Agreement. RCB estimates for 1.5 °C are small, and minor changes in their calculation can therefore result in large relative adjustments. Here we evaluate recent RCB assessments by the IPCC and present more recent data, calculation refinements and robustness checks that increase confidence in them. We conclude that the RCB for a 50% chance of keeping warming to 1.5 °C is around 250 GtCO2 as of January 2023, equal to around six years of current CO2 emissions. For a 50% chance of 2 °C the RCB is around 1,200 GtCO2. Key uncertainties affecting RCB estimates are the contribution of non-CO2 emissions, which depends on socioeconomic projections as much as on geophysical uncertainty, and potential warming after net zero CO2.
Land-based carbon sequestration projects, such as tree planting, are a prominent strategy to offset carbon emissions. However, we risk reducing natural ecosystems to one metric – carbon. Emphasis on restoring ecosystems to balance ecosystem services, biodiversity conservation, and carbon sequestration is a more appropriate strategy to protect their functioning.
In light of the critical role of tropical forests in stabilizing the global climate system through both carbon and noncarbon pathways, maintaining and increasing incentives for large-scale forest conservation is an essential component of climate action. Demand for carbon credits, one of the most promising mechanisms for funding large-scale forest conservation, has grown rapidly in recent years, with the voluntary carbon market seeing transactions worth almost US$2 billion in 2021. In 2022, however, the volume of transactions leveled off, at least in part due to concerns about reputational risk from corporate buyers afraid of greenwashing accusations.
Early in August this year, a high-profile summit was held in Belém, Brazil, where the eight Amazonian countries discussed the future of the Amazon. The nations recognized that the Amazon is very close to reaching a tipping point for turning into a degraded ecosystem. The result of their discussions was the Belém Declaration, an ambitious plan to protect and conserve the Amazon forests and to support Indigenous Peoples and local communities. Concern arose, however, because they failed to agree on attaining zero deforestation by 2030 and on avoiding new explorations in the Amazon for fossil fuel. The Declaration also lacks specific and measurable indicators. The ministers of Foreign Affairs therefore have a very important role in further refining the agenda and deadlines so that the Belém Declaration can be implemented.
For over three decades, science has pointed to the risks of the Amazon reaching a tipping point. Several recent studies now demonstrate how close it is: The dry season over southern Amazon has lengthened by 4 to 5 weeks over the past 40 years, the mortality of wet-loving tree species has increased, and the loss of trees is turning the forests into a carbon source rather than a carbon sink.