Stratospheric aerosol injection (SAI) has been proposed as a potential method for mitigating risks and impacts associated with anthropogenic climate change. One such risk is widespread permafrost thaw and associated carbon release. While permafrost has been shown to stabilize under different SAI scenarios, natural variability may mask this forced response and make it difficult to detect if and when SAI is stabilizing permafrost. Here we use the 10-member ensemble from the ARISE-SAI-1.5 simulations to assess the spread in projected active layer depth and permafrost temperature across boreal permafrost soils and specifically in four peatland and Yedoma regions. The forced response in active layer depth and permafrost temperature quickly diverges between an SAI and non-SAI world, but individual ensemble members overlap for several years following SAI deployment. We find that, due to projected permafrost variability, it may take more than a decade of SAI deployment to detect the effects of SAI on permafrost temperature and almost 30 years to detect its effects on active layer depth. Not only does natural variability make it more difficult to detect SAI's influence, it could also affect the likelihood of reaching a permafrost tipping point. In some realizations, SAI fails to prevent a local tipping point that is also reached in a non-SAI world. Our results underscore the importance of accounting for natural variability in assessments of SAI's potential influence on the climate system. Injecting highly reflective particles into the upper atmosphere, or stratospheric aerosol injection (SAI), is a proposed climate intervention method for deliberately stabilizing or cooling the Earth's temperature and preventing undesirable impacts of human-caused climate change, such as thawing permafrost. Permafrost can potentially release stored carbon into the atmosphere as carbon dioxide and methane that contributes to the greenhouse effect. Climate model simulations show that SAI could stabilize permafrost and prevent it from thawing, but that natural fluctuations in the Earth's climate may cause a wide range of outcomes for future permafrost thaw depth and soil temperature. We show that, due to these natural climate fluctuations, it may take 10-30 years of SAI to clearly see its influence on permafrost thaw depth and temperature. Certain conditions that lead to runaway thaw and soil carbon release (i.e., tipping points) may also occur even if SAI successfully stabilizes the Earth's globally averaged temperature. When weighing possible outcomes of proposed climate intervention strategies, it is important to consider the effects of natural climate fluctuations in assessing the pros and cons of different strategies. Projected natural variability in permafrost fields in peatland and Yedoma regions can mask forced response to stratospheric aerosol injection (SAI) Effect of SAI on active layer and soil temperature is only detectable after more than a decade of aerosol deployment Natural variability affects likelihood of reaching precursor to permafrost tipping point despite surface cooling effect of SAI

Agriculture occupies more than a third of the world's land with many, large-scale impacts on the environment and human health. This article investigates the failure of policy to manage these impacts, asks whether private law can fill the gap, and what this means for policymakers. The investigation takes the form of a case study of synthetic nitrogen fertilizer (SNF) in English policy and law. The SNF industry has been chosen because, by its own account, it underpins the modern food system, which is recognized as needing urgent transformation. The article first assesses the damage caused by SNF to health and the environment and the potential legal remedies. It then assesses industry claims that SNF (a) provides food security, (b) is beneficial to soil and water, and (c) reduces greenhouse gas emissions. If misleading, these representations could amount to unlawful 'greenwashing'. While private law can never replace good policy and regulation, the article concludes that there is evidence to enable private law to supplement policy, and that this role is made possible as well as necessary by the absence of effective regulation and enforcement. Private litigation could catalyze policymakers to implement the robust regulatory regime that agriculture demands. As the law must focus on scientific evidence and causation, it can also help elucidate and publicize the science on which policy is based. Finally, because of the strict constraints within which private litigation must operate, it can direct policymakers towards strategic interventions (or tipping points) that could catalyze systemic change.

The Paris Agreement calls for emissions reductions to limit climate change, but how will the carbon cycle change if it is successful? The land and oceans currently absorb roughly half of anthropogenic emissions, but this fraction will decline in the future. The amount of carbon that can be released before climate is mitigated depends on the amount of carbon the ocean and terrestrial ecosystems can absorb. Policy is based on model projections, but observations and theory suggest that climate effects emerging in today's climate will increase and carbon cycle tipping points may be crossed. Warming temperatures, drought, and a slowing growth rate of CO2 itself will reduce land and ocean sinks and create new sources, making carbon sequestration in forests, soils, and other land and aquatic vegetation more difficult. Observations, data-assimilative models, and prediction systems are needed for managing ongoing long-term changes to land and ocean systems after achieving net-zero emissions. International agreements call for stabilizing climate at 1.5 degrees above preindustrial, while the world is already seeing damaging extremes below that. If climate is stabilized near the 1.5 degrees target, the driving force for most sinks will slow, while feedbacks from the warmer climate will continue to cause sources. Once emissions are reduced to net zero, carbon cycle-climate feedbacks will require observations to support ongoing active management to maintain storage.