ABSTRACT. Anthropogenic pressures on the Earth System have reached a scale where abrupt global environmental change can no longer be excluded. We propose a new approach to global sustainability in which we define planetary boundaries within which we expect that humanity can operate safely.Transgressing one or more planetary boundaries may be deleterious or even catastrophic due to the risk of crossing thresholds that will trigger non-linear, abrupt environmental change within continental-to planetary-scale systems. We have identified nine planetary boundaries and, drawing upon current scientific understanding, we propose quantifications for seven of them. These seven are climate change (CO 2 concentration in the atmosphere <350 ppm and/or a maximum change of +1 W m -2 in radiative forcing); ocean acidification (mean surface seawater saturation state with respect to aragonite ≥ 80% of pre-industrial levels); stratospheric ozone (<5% reduction in O 3 concentration from pre-industrial level of 290 Dobson Units); biogeochemical nitrogen (N) cycle (limit industrial and agricultural fixation of N 2 to 35 Tg N yr -1 ) and phosphorus (P) cycle (annual P inflow to oceans not to exceed 10 times the natural background weathering of P); global freshwater use (<4000 km 3 yr -1 of consumptive use of runoff resources); land system change (<15% of the ice-free land surface under cropland); and the rate at which biological diversity is lost (annual rate of <10 extinctions per million species). The two additional planetary boundaries for which we have not yet been able to determine a boundary level are chemical pollution and atmospheric aerosol loading. We estimate that humanity has already transgressed three planetary boundaries: for climate change, rate of biodiversity loss, and changes to the global nitrogen cycle. Planetary boundaries are interdependent, because transgressing one may both shift the position of other boundaries or cause them to be transgressed. The social impacts of transgressing boundaries will be a function of the social-ecological resilience of the affected societies. Our proposed boundaries are rough, first estimates only, surrounded by large uncertainties and knowledge gaps. Filling these gaps will require major advancements in Earth System and resilience science. The proposed concept of "planetary boundaries" lays the groundwork for shifting our approach to governance and management, away from the essentially sectoral analyses of limits to growth aimed at minimizing negative externalities, toward the estimation of the safe space for human development. Planetary boundaries define, as it were, the boundaries of the "planetary playing field" for humanity if we want to be sure of avoiding major human-induced environmental change on a global scale.

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Adaptation to climate change has steadily risen on global policy agendas and entered a new era with the 2015 Paris Agreement, which established a global goal on adaptation. While this goal responds to calls to strengthen global governance of adaptation, it has not yet been operationalized. Further, few studies take stock of current global adaptation governance to inform the implementation of the goal. Against this background this review asks: To what extent is there global governance of climate change adaptation? Can it be characterized as a strong domain of global governance? In what ways is it contested? Global adaptation governance is defined here as occurring when state and non‐state actors in the global (including transnational) sphere authoritatively and intentionally shape the actions of constituents towards climate change adaptation as a public goal. Although empirical evidence is scant, it is proposed here that global adaptation governance is indeed emerging. Yet, its further strengthening appears contested. First, measurement of progress towards adaptation as a public goal at the global level is severely challenged by the ambiguity of adaptation and the lack of distinct metrics. Second, the lack of a clear global‐level problem‐framing, or recognition of adaptation as a global public good, has meant limited legitimacy of global governance initiatives. A consequence of contestation is that governance forms and functions used so far have not been authoritative in how they seek to shape actions. The review concludes by identifying research needs for advancing science and policy on adaptation. This article is categorized under: Policy and Governance > Multilevel and Transnational Climate Change Governance