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Non-technical abstractLimiting dangerous climate change is widely believed to require negative emissions. This prospect has sparked concerns about whether negative emissions could be scaled up quickly enough, along with concerns about their likely ethical costs. Building upon scenario modelling, this paper examines ethical concerns with negative emissions via the comparison of three alternate climate futures. This paper shows that the severity of concerns depends upon implementation conditions, and especially the extent of deferred mitigation. Negative emissions can be a valuable means of limiting dangerous climate change, or an unjust gamble against the future.

Putting the recently adopted global Sustainable Development Goals or the Paris Agreement on international climate policy into action will require careful policy choices. Appropriately informing decision-makers about longer-term, wicked policy issues remains a considerable challenge for the scientific community. Typically, these vital policy issues are highly uncertain, value-laden and disputed, and affect multiple temporal and spatial scales, governance levels, policy fields, and socioeconomic contexts simultaneously. In light of this, science-policy interfaces should help facilitate learning processes and open deliberation among all actors involved about potentially acceptable policy pathways. For this purpose, science-policy interfaces must strive to foster some enabling conditions: (1) "representation" in terms of engaging with diverse stakeholders (including experts) and acknowledging divergent viewpoints; (2) "empowerment" of underrepresented societal groups by codeveloping and integrating policy scenarios that reflect their specific knowledge systems and worldviews; (3) "capacity building" regarding methods and skills for integration and synthesis, as well as through the provision of knowledge synthesis about the policy solution space; and (4) "spaces for deliberation", facilitating direct interaction between different stakeholders, including governments and scientists. We argue that integrated, multi-stakeholder, scientific assessment processes-particularly the collaborative assessments of policy alternatives and their various implications-offer potential advantages in this regard, compared with alternatives for bridging scientific expertise and public policy. This article is part of a collection on scientific advice to governments.

NATURAL CAPITAL Guidelines, respect and time can reconcile diverse views p.309 BIODIVERSITY Stakeholders in international panel rise up and respond p.309 Weigh the ethics of plans to mop up carbon dioxide Pinning climate hopes on negative emissions technologies is dangerous and demands reflection on the social aspects, warn Dominic Lenzi and colleagues. A pilot project in Spremberg, Germany, aims to capture carbon dioxide released from power stations.

Urban street space is increasingly contested. However, it is unclear what a fair street space allocation would look like. We develop a framework of ten ethical principles and three normative perspectives on street space -streets for transport, streets for sustainability, and streets as place -and discuss 14 derived street space allocation mechanisms. We contrast these ethically grounded allocation mechanisms with real-world allocation in 18 streets in Berlin. We find that car users, on average, had 3.5 times more space available than non-car users. While some allocation mechanisms are more plausible than others, none is without normative implications. Without exception, all principles suggest that on-street parking for cars is difficult to justify, and that more space should be allocated to cycling. We argue that street space fairness principles should be systematically integrated into urban and transport planning.