Safe and just Earth system boundaries.

The stability and resilience of the Earth system and human well-being are inseparably linked1-3, yet their interdependencies are generally under-recognized; consequently, they are often treated independently4,5. Here, we use modelling and literature assessment to quantify safe and just Earth system boundaries (ESBs) for climate, the biosphere, water and nutrient cycles, and aerosols at global and subglobal scales. We propose ESBs for maintaining the resilience and stability of the Earth system (safe ESBs) and minimizing exposure to significant harm to humans from Earth system change (a necessary but not sufficient condition for justice)4. The stricter of the safe or just boundaries sets the integrated safe and just ESB. Our findings show that justice considerations constrain the integrated ESBs more than safety considerations for climate and atmospheric aerosol loading. Seven of eight globally quantified safe and just ESBs and at least two regional safe and just ESBs in over half of global land area are already exceeded. We propose that our assessment provides a quantitative foundation for safeguarding the global commons for all people now and into the future.

Trajectories of the Earth System in the Anthropocene.

We explore the risk that self-reinforcing feedbacks could push the Earth System toward a planetary threshold that, if crossed, could prevent stabilization of the climate at intermediate temperature rises and cause continued warming on a "Hothouse Earth" pathway even as human emissions are reduced. Crossing the threshold would lead to a much higher global average temperature than any interglacial in the past 1.2 million years and to sea levels significantly higher than at any time in the Holocene. We examine the evidence that such a threshold might exist and where it might be. If the threshold is crossed, the resulting trajectory would likely cause serious disruptions to ecosystems, society, and economies. Collective human action is required to steer the Earth System away from a potential threshold and stabilize it in a habitable interglacial-like state. Such action entails stewardship of the entire Earth System-biosphere, climate, and societies-and could include decarbonization of the global economy, enhancement of biosphere carbon sinks, behavioral changes, technological innovations, new governance arrangements, and transformed social values.

Climate for women in climate science: Women scientists and the Intergovernmental Panel on Climate Change.

The Intergovernmental Panel on Climate Change (IPCC) is an authoritative and influential source of reports on climate change. The lead authors of IPCC reports include scientists from around the world, but questions have been raised about the dominance of specific disciplines in the report and the disproportionate number of scholars from the Global North. In this paper, we analyze the as-yet-unexamined issue of gender and IPCC authorship, looking at changes in gender balance over time and analyzing women's views about their experience and barriers to full participation, not only as women but also at the intersection of nationality, race, command of English, and discipline. Over time, we show that the proportion of female IPCC authors has seen a modest increase from less than 5% in 1990 to more than 20% in the most recent assessment reports. Based on responses from over 100 women IPCC authors, we find that many women report a positive experience in the way in which they are treated and in their ability to influence the report, although others report that some women were poorly represented and heard. We suggest that an intersectional lens is important: not all women experience the same obstacles: they face multiple and diverse barriers associated with social identifiers such as race, nationality, command of English, and disciplinary affiliation. The scientific community benefits from including all scientists, including women and those from the Global South. This paper documents barriers to participation and identifies opportunities to diversify climate science.

Flooding, resettlement, and change in livelihoods: evidence from rural Mozambique.

Post-disaster development policies, such as resettlement, can have major impacts on communities. This paper examines how and why people's livelihoods change as a result of resettlement, and relocated people's views of such changes, in the context of natural disasters. It presents two historically-grounded, comparative case studies of post-flood resettlement in rural Mozambique. The studies demonstrate a movement away from rain-fed subsistence agriculture towards commercial agriculture and non-agricultural activities. The ability to secure a viable livelihood was a key determinant of whether resettlers remained in their new locations or returned to the river valleys despite the risks posed by floods. The findings suggest that more research is required to understand i) why resettlers choose to stay in or abandon designated resettlement areas, ii) what is meant by 'voluntary' and 'involuntary' resettlement in the realm of post-disaster reconstruction, and iii) the policy drivers of resettlement in developing countries.

Four degrees and beyond: the potential for a global temperature increase of four degrees and its implications.

The 1992 UN Framework Convention on Climate Change commits signatories to preventing 'dangerous anthropogenic interference with the climate system', leaving unspecified the level of global warming that is dangerous. In the late 1990s, a limit of 2°C global warming above preindustrial temperature was proposed as a 'guard rail' below which most of the dangerous climate impacts could be avoided. The 2009 Copenhagen Accord recognized the scientific view 'that the increase in global temperature should be below 2 degrees Celsius' despite growing views that this might be too high. At the same time, the continued rise in greenhouse gas emissions in the past decade and the delays in a comprehensive global emissions reduction agreement have made achieving this target extremely difficult, arguably impossible, raising the likelihood of global temperature rises of 3°C or 4°C within this century. Yet, there are few studies that assess the potential impacts and consequences of a warming of 4°C or greater in a systematic manner. Papers in this themed issue provide an initial picture of the challenges facing a world that warms by 4°C or more, and the difficulties ahead if warming is to be limited to 2°C with any reasonable certainty. Across many sectors--coastal cities, agriculture, water stress, ecosystems, migration--the impacts and adaptation challenges at 4°C will be larger than at 2°C. In some cases, such as farming in sub-Saharan Africa, a +4°C warming could result in the collapse of systems or require transformational adaptation out of systems, as we understand them today. The potential severity of impacts and the behavioural, institutional, societal and economic challenges involved in coping with these impacts argue for renewed efforts to reduce emissions, using all available mechanisms, to minimize the chances of high-end climate change. Yet at the same time, there is a need for accelerated and focused research that improves understanding of how the climate system might behave under a +4°C warming, what the impacts of such changes might be and how best to adapt to what would be unprecedented changes in the world we live in.