Mitigating climate change requires clean energy and removing atmospheric carbon. Building soil carbon is an appealing way to increase carbon sinks and reduce emissions due to the associated benefits to agriculture. However, practical implementation of soil carbon climate strategies lag behind the potential, partly because we lack clarity around the magnitude of opportunity and how to capitalize on it. Here we quantify the role of soil carbon in natural (landbased) climate solutions (NCS), and review some of the project design mechanisms available to tap into the potential. We show that soil carbon represents 25% of the 23.8 GtCO2eyr-1 NCS potential of which 40% is protection of existing soil carbon and 60% is rebuilding depleted stocks. Soil carbon comprises 9% of the mitigation potential of forests, 72% for wetlands, and 47% for agriculture and grasslands. Soil carbon is important to land-based efforts to prevent carbon emissions, remove atmospheric carbon dioxide and deliver ecosystem services in addition to climate mitigation. Protecting and restoring soil organic matter delivers many benefits to people and nature 1,2. Globally, soils hold three times more carbon than the atmosphere 3 , and the role of soil organic matter as a regulator of climate has been recognized by scientists for decades 4. Recent work has highlighted the historical loss of carbon from this pool 3 , and the threat of future accelerated loss under warming scenarios 4,5. Soil organic carbon as a natural climate solution (NCS) thus has a role both through restoring a carbon sink and protecting against further CO 2 emissions in response to predicted land use change and climate change. This dual role for soil in the global carbon budget suggests climate benefits can be achieved through strategies that both conserve existing soil organic carbon stocks (avoid loss), and restore stocks in carbon-depleted soils 6. There are important additional benefits. Protecting and increasing soil carbon storage can (i) protect or increase soil fertility, (ii) maintain or increase resilience to climate change, (iii) reduce soil erosion, and where implemented through conservation of natural ecosystems iv) reduce habitat conversion, all in line with the United Nations Sustainable Development Goals (SDG's) 7 , the goals of the United Nationals Framework Convention on Climate Change (UNFCCC) and the United Nations Convention on Combating Desertification (UNCCD). As such, soil carbon is promoted as a common denominator amongst a variety of global and national initiatives 7. Although recent academic comment and perspective pieces point the way towards accelerated action on soils 8,9 , there remains much uncertainty around actionable pathways for achieving the global opportunity. Here we examine the scientific and policy context surrounding soil carbon projects, to aid prioritization and decision making.

a b s t r a c tTo meet the challenge of proactive ecosystem-based climate mitigation and adaptation, new sources of funding are needed. Peatlands provide the most efficient global store of terrestrial carbon. Degraded peatlands, however, contribute disproportionally to global greenhouse gas (GHG) emissions, with approximately 25% of all CO 2 emissions from the land use sector, while restoration can be cost-effective. Peatland restoration therefore provides a newopportunity for investing in ecosystem-based mitigation through the development of carbon markets. Set in the international policy and carbon market context, this paper demonstrates the necessary scientific evidence and policy frameworks needed to develop ecosystem service markets for peatland restoration. Using the UK and NE Germany as case studies, we outline the climate change mitigation potential of peatlands and how changes in GHG emissions after restoration may be measured. We report on market demand research in carbon market investments that provide sponsors with quantification and officially certified recognition of the climate and other co-benefits. Building on this, we develop the necessary requirements for developing regional carbon markets to fund peatland restoration.

Abstract1. Coastal blue carbon activities are being implemented by a variety of countries, using different approaches. Existing regulatory regimes, including on coastal protection, are still very useful tools to protect and conserve mangroves, seagrasses and saltmarshes, and preserve their carbon value and role. These approaches suffer, however, from 'traditional' issues such as lack of enforcement, human and financial constraints as well as unclear or misguiding government mandates.2. Successes are witnessed using a community-based carbon project approach, ensuring high stakeholder participation via direct or indirect incentive programmes. Comprehensive coastal zone management approaches seem very promising, but success overall, and regarding carbon specifically, are yet to be reported.3. The Paris Agreement has introduced new tools which could serve as means to trigger more and better coastal adaptation and mitigation efforts. Their implementation details are, however, still under negotiation and their impacts can only be expected in a few years.

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The adoption of the Paris Agreement is a milestone in international climate politics and brings years of near deadlock negotiations to a conclusion. The Agreement creates a global process of engagement, follow-up, regular stock-take exercises and cooperative action. On the one hand, it represents a step forward, overcoming the many divisions that had marked the Kyoto area: between developed and developing countries, between industrialized nations inside the Protocol and those outside, and between those supportive of market mechanisms and those that vehemently opposed them. On the other hand, individual country contributions fall short of the overall climate goal, and the risk is that the Paris Agreement remains a shell without sufficient action and support. It thus remains to be seen whether the Paris Agreement is the right framework through which to address the collective action problem of climate change.