Description: If solar geoengineering were to be deployed so as to mask a high level of global warming, and then stopped suddenly, there would be a rapid and damaging rise in temperatures. This effect is often referred to as termination shock, and it is an influential concept. Based on studies of its potential impacts, commentators often cite termination shock as one of the greatest risks of solar geoengineering. However, there has been little consideration of the likelihood of termination shock, so that conclusions about its risk are premature. This paper explores the physical characteristics of termination shock, then uses simple scenario analysis to plot out the pathways by which different driver events (such as terrorist attacks, natural disasters, or political action) could lead to termination. It then considers where timely policies could intervene to avert termination shock. We conclude that some relatively simple policies could protect a solar geoengineering system against most of the plausible drivers. If backup deployment hardware were maintained and if solar geoengineering were implemented by agreement among just a few powerful countries, then the system should be resilient against all but the most extreme catastrophes. If this analysis is correct, then termination shock should be much less likely, and therefore much less of a risk, than has previously been assumed. Much more sophisticated scenario analysis—going beyond simulations purely of worst-case scenarios—will be needed to allow for more insightful policy conclusions.

Description: To avoid the most dangerous consequences of anthropogenic climate change, the Paris Agreement provides a clear and agreed climate mitigation target of stabilizing global surface warming to under 2.0 °C above preindustrial, and preferably closer to 1.5 °C. However, policy makers do not currently know exactly what carbon emissions pathways to follow to stabilize warming below these agreed targets, because there is large uncertainty in future temperature rise for any given pathway. This large uncertainty makes it difficult for a cautious policy maker to avoid either: (1) allowing warming to exceed the agreed target; or (2) cutting global emissions more than is required to satisfy the agreed target, and their associated societal costs. This study presents a novel Adjusting Mitigation Pathway (AMP) approach to restrict future warming to policy-driven targets, in which future emissions reductions are not fully determined now but respond to future surface warming each decade in a self-adjusting manner. A large ensemble of Earth system model simulations, constrained by geological and historical observations of past climate change, demonstrates our self-adjusting mitigation approach for a range of climate stabilization targets ranging from 1.5 to 4.5 °C, and generates AMP scenarios up to year 2300 for surface warming, carbon emissions, atmospheric CO 2 , global mean sea level, and surface ocean acidification. We find that lower 21 st century warming targets will significantly reduce ocean acidification this century, and will avoid up to 4m of sea-level rise by year 2300 relative to a high-end scenario.

Description: Despite the societal relevance of sea-level research, a knowledge-to-action gap remains between researchers and coastal communities. In the agricultural and water-management sectors, intermediaries such as consultants and extension agencies have a long and well-documented history of helping to facilitate the application of scientific knowledge on the ground. However, the role of such intermediaries in adaptation to sea-level rise, though potentially of vital importance, has been less thoroughly explored. In this commentary, we describe three styles of science intermediation that can connect researchers working on sea-level projections with decision-makers relying on those projections. We illustrate these styles with examples of recent and ongoing contexts for the application of sea-level research, at different spatial scales and political levels ranging from urban development projects to international organizations. Our examples highlight opportunities and drawbacks for the researchers involved and communities adapting to rising seas.

Description: We use multiple synthetic mitigation sea-level scenarios, together with a non-mitigation sea-level scenario from the Warming Acidification and Sea-level Projector model. We find sea-level rise continues to accelerate post 2100 for all but the most aggressive mitigation scenarios indicative of 1.5°C and 2.0°C. Using the Dynamic Interactive Vulnerability Assessment modelling framework, we project land and population exposed in the 1 in 100 year coastal flood plain under sea-level rise and population change. In 2000, the flood plain is estimated at 540 x10 3 km 2 . By 2100, under the mitigation scenarios, it ranges between 610 x10 3 km 2 and 640 x10 3 km 2 [580 x10 3 km 2 and 700 x10 3 km 2 for the 5 th and 95 th percentiles]. Thus differences between the mitigation scenarios are small in 2100. However, in 2300, flood plains are projected to increase to between 700 x10 3 km 2 and 960 x10 3 km 2 in 2300 [610 x10 3 km 2 and 1,290 x10 3 km 2 ] for the mitigation scenarios, but 1,630 x10 3 km 2 [1,190 x10 3 km 2 and 2,220 x10 3 km 2 ] for the non-mitigation scenario. The proportion of global population exposed to sea-level rise in 2300 is projected to be between 1.5% and 5.4% [1.2% to 7.6%] (assuming no population growth after 2100) for the aggressive mitigation and the non-mitigation scenario, respectively. Hence over centennial timescales there are significant benefits to climate change mitigation and temperature stabilization. However, sea-levels will continue to rise albeit at lower rates. Thus potential impacts will keep increasing necessitating adaptation to existing coastal infrastructure and the careful planning of new coastal developments.

Description: Increasing demands for water, driven by population growth and socio-economic development, environmental regulations and future climate uncertainty, are highlighting limitations on water supplies. This water-energy-food-environment nexus is not confined to semi-arid regions but is emerging as a key business, societal and economic risk in humid and temperate countries, where abundant water supplies and regulation have historically coped with fluctuating demands between industry, power generation, agriculture, domestic supply and the environment. In the UK, irrigation is supplemental to rainfall, consumptive in use and concentrated in the driest years and most resource-stressed catchments. This paper describes an empirical application of a mixed methods approach to integrate agriculture into a robust decision-making framework, focusing on a water-stressed region in England. The approach shows that competing demands between sectors can be reconciled and that potential options or portfolios compatible with multi-sectoral collaboration and investment can be identified. By combining model outputs to forecast the impacts of climate and socio-economic change on agricultural demand within a regional water resource simulator, future spatial estimates of demand were derived. A set of search and tracked metrics were used to drive multi-criteria searches to identify preferred supply and demand management orientated portfolios. The methodological challenges in forecasting agricultural demand, defining acceptable ‘trade-offs’, managing scale and uncertainty issues and the importance of engendering open dialogue between stakeholders is described. The study provides valuable insights for countries where similar emergent issues regarding conflicts over water demand exist.

Description: To maintain the chance of keeping the average global temperature increase below 2°C and to limit long-term climate change, removing carbon dioxide from the atmosphere (Carbon Dioxide Removal, CDR) is becoming increasingly necessary. We analyze optimal and cost-effective climate policies in the Dynamic Integrated Assessment Model of Climate and the Economy (DICE2016R) and investigate i) the utilization of (ocean) CDR under different climate objectives, ii) the sensitivity of policies with respect to carbon cycle feedbacks, and iii) how well carbon cycle feedbacks are captured in the carbon-cycle models used in state-of-the-art integrated assessment models. Overall, the carbon cycle model in DICE2016R shows clear improvements compared to its predecessor, DICE2013R, capturing much better long-term dynamics and also oceanic carbon outgassing due to excess oceanic storage of carbon from CDR. However, this comes at the cost of a (too) tight short-term remaining emission budget, limiting the model suitability to analyze low emission scenarios accurately. With DICE2016R, the compliance with the 2°C goal is no longer feasible without negative emissions via CDR. Overall, the optimal amount of CDR has to take into account i) the emission substitution effect and ii) compensation for carbon cycle feedbacks.

Description: Bioenergy Carbon Capture and Storage (BECCS) has been proposed to reduce atmospheric CO 2 concentrations, but concerns remain about competition for arable land and freshwater. The synergistic integration of algae production, which does not require arable land or freshwater, with BECCS (called “ABECCS”) can reduce CO 2 emissions without competing with agriculture. This study presents a techno-economic and life-cycle assessment for co-locating a 121-ha algae facility with a 2,680-ha eucalyptus forest for BECCS. The eucalyptus biomass fuels combined heat and power generation (CHP) with subsequent amine based carbon capture and storage (CCS). A portion of the captured CO 2 is used for growing algae and the remainder is sequestered. Biomass combustion supplies CO 2 , heat, and electricity, thus increasing the range of sites suitable for algae cultivation. Economic, energetic, and environmental impacts are considered. The system yields as much protein as soybeans while generating 61.5 TJ of electricity and sequestering 29,600 t of CO 2 per year. More energy is generated than consumed and the freshwater footprint is roughly equal to that for soybeans. Financial break-even is achieved for product value combinations ranging from 1) algal biomass sold for $1,780/t without a carbon credit to 2) algal biomass sold for $100/t with a carbon credit of $396/t. Sensitivity analysis shows significant reductions to the cost of carbon sequestration are possible. The ABECCS system represents a unique technology for negative emissions without reducing protein production or increasing water demand, and should therefore be included in the suite of technologies being considered to address global sustainability.

Description: Extreme events are of interest worldwide given their potential for substantial impacts on social, ecological, and technical systems. Many climate-related extreme events are increasing in frequency and/or magnitude due to anthropogenic climate change, and there is increased potential for impacts due to the location of urbanization and the expansion of urban centers and infrastructures. Many disciplines are engaged in research and management of these events. However, a lack of coherence exists in what constitutes and defines an extreme event across these fields, which impedes our ability to holistically understand and manage these events. Here, we review 10 years of academic literature and use text analysis to elucidate how six major disciplines--climatology, earth sciences, ecology, engineering, hydrology, and social sciences--define and communicate extreme events. Our results highlight critical disciplinary differences in the language used to communicate extreme events. Additionally, we found a wide range in definitions and thresholds, with more than half of examined papers not providing an explicit definition, and disagreement over whether impacts are included in the definition. We urge distinction between extreme events and their impacts, so that we can better assess when responses to extreme events have actually enhanced resilience. Additionally, we suggest that all researchers and managers of extreme events be more explicit in their definition of such events as well as be more cognizant of how they are communicating extreme events. We believe clearer and more consistent definitions and communication can support transdisciplinary understanding and management of extreme events.

Description: Climate change, globalization, urbanization, social isolation, and increased interconnectedness between physical, human, and technological systems (Cutter et al., 2015) pose major challenges to disaster risk reduction (DRR). Subsequently, economic losses caused by natural hazards are increasing in many regions of the world (Figure 1), despite scientific progress, persistent policy action and international cooperation (United Nations, 2015).