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inform and support global sustainability goals
<br />and pathways. This evolution is needed more
<br />than ever before; there are severe implementa-
<br />tion gaps in many global environmental policies
<br />relating to the PB issues, where problematic
<br />trends are not being halted or reversed despite
<br />international consensus about the urgency of the
<br />problems. The prospect of tighter resource con-
<br />straints and rising environmental hazards is also
<br />unavoidably turning the focus onto global social
<br />equity and the planetary stewardship of Earth's
<br />life- support system. There is a need for a truly
<br />global evidence base, with much greater integra-
<br />tion among issues, in order to respond to these
<br />global challenges. New research initiatives [e.g.,
<br />Future Earth (www.futureearth.org)] provide evi-
<br />dence that science can respond to this need by
<br />applying Earth - system research to advance a new
<br />generation of integrated global analyses and to
<br />explore options for transformations toward sus -
<br />tainability. This is a clear sign that, as the risks
<br />of the Anthropocene to human well-being be-
<br />come clearer, research is maturing to a point
<br />where a systemic step - change is possible -and
<br />necessary -in exploring and defining a safe and
<br />just planetary operating space for the further
<br />development of human societies.
<br />Methods summary
<br />Our approach to building the planetary bound-
<br />aries framework is described above. We have
<br />implemented the framework through an ex-
<br />pert assessment and synthesis of the scientific
<br />knowledge of intrinsic biophysical processes that
<br />regulate the stability of the Earth system. Our
<br />precautionary approach is based on the main-
<br />tenance of a Holocene -like state of the Earth
<br />system and on an assessment of the level of
<br />human - driven change that would risk destabi-
<br />lizing this state. For the climate change PB, there
<br />is already much literature on which to base
<br />such an assessment. For others, such as strato-
<br />spheric ozone, ocean acidification, extinction
<br />rates, and P and N cycles, we have used estimates
<br />of preindustrial values of the control variable
<br />as a Holocene baseline. Where large, undesira-
<br />ble thresholds exist and have been studied (e.g.,
<br />polar ice sheets, Amazon rainforest, aragonite
<br />dissolution, atmospheric aerosols, and the south
<br />Asian monsoon), quantitative boundaries can be
<br />readily proposed. For others, where the focus is
<br />on erosion of Earth - system resilience, the bound-
<br />aries are more difficult (but not impossible) to
<br />quantify, as reflected in larger uncertainty zones.
<br />We used large -scale assessments of the impacts
<br />of human activities on Earth - system functioning
<br />[e.g., Intergovernmental Panel on Climate Change
<br />(17 18), the International Geosphere- Biosphere
<br />Programme synthesis (16), and chemicals (75, SO)]
<br />as sources of community -level understanding
<br />on which to propose PBs. Our update has also
<br />relied on post -2009 assessments of individual
<br />boundaries by the relevant expert research com-
<br />munities; examples include phosphorus (3), ni-
<br />trogen (5), biosphere integrity (7), freshwater use
<br />(5, 63), and novel entities [with a focus on chem-
<br />icals (28, 73)]. Finally, some new analyses have
<br />been undertaken specifically for this paper: (i) a
<br />freshwater -use PB based on the EWF approach
<br />(33, 63); (ii) the linkage of the phosphorus and
<br />nitrogen boundaries via the N:P ratio in grow-
<br />ing crop tissue (33); and (iii) the use of major
<br />forest biomes as the basis for the land - system
<br />change PB (33).
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