'Planetary Boundaries: Guiding Human Development on a Changing Planet'

Abstract

The planetary boundaries framework defines a safe operating space for humanity based on the intrinsic biophysical processes
that regulate the stability of the Earth system. Here, we revise and update the planetary boundary framework, with a focus
on the underpinning biophysical science, based on targeted input from expert research communities and on more general scientific
advances over the past 5 years. Several of the boundaries now have a two-tier approach, reflecting the importance of cross-scale
interactions and the regional-level heterogeneity of the processes that underpin the boundaries. Two core boundaries—climate
change and biosphere integrity—have been identified, each of which has the potential on its own to drive the Earth system
into a new state should they be substantially and persistently transgressed.

Full text

RESEARCH ARTICLE SUMMARY
◥
SUSTAINABILITY
Planetary boundaries: Guiding
human development on a
changing planet
Will Steffen,*Katherine Richardson, Johan Rockström, Sarah E. Cornell, Ingo Fetzer,
Elena M. Bennett, Reinette Biggs, Stephen R. Carpenter, Wim de Vries,
Cynthia A. de Wit, Carl Folke, Dieter Gerten, Jens Heinke, Georgina M. Mace,
Linn M. Persson, Veerabhadran Ramanathan, Belinda Reyers, Sverker Sörlin
INTRODUCTION: There is an urgent need for
a new paradigm that integrates the continued
development of human societies and the main-
tenance of the Earth system (ES) in a resilient
and accommodating state. The planetary bound-
ary (PB) framework contributes to such a
paradigm by providing a science-based analysis
of the risk that human perturbations will de-
stabilize the ES at the planetary scale. Here, the
scientific underpinnings of the PB framework
are updated and strengthened.
RATIONALE: The relatively stable, 11,700-year-
long Holocene epoch is the only state of the ES
that we know for certain can support contem-
porary human societies. There is increasing evi-
dence that human activities are affecting ES
functioning to a degree that threatens the re-
silience of the ES—its ability to persist in a
Holocene-like state in the face of increasing
human pressures and shocks. The PB frame-
work is based on critical processes that reg-
ulate ES functioning. By combining improved
scientific understanding of ES functioning with
the precautionary principle, the PB framework
identifies levels of anthropogenic perturbations
below which the risk of destabilization of the
ES is likely to remain low—a“safe operating
space”for global societal development. A zone
of uncertainty for each PB highlights the area
of increasing risk. The current level of anthro-
pogenic impact on the ES, and thus the risk to
the stability of the ES, is assessed by compar-
ison with the proposed PB (see the figure).
RESULTS: Three of the PBs (climate change,
stratospheric ozone depletion, and ocean acid-
ification) remain essentially unchanged from
the earlier analysis. Regional-level boundaries
as well as globally aggregated PBs have now
been developed for biosphere integrity (earlier
“biodiversity loss”), biogeochemical flows, land-
system change, and freshwater use. At present,
only one regional boundary (south Asian mon-
soon) can be established for atmospheric aerosol
loading. Although we cannot identify a single PB
for novel entities (here de-
fined as new substances,
new forms of existing sub-
stances, and modified life
forms that have the po-
tential for unwanted geo-
physical and/or biological
effects), they are included in the PB framework,
given their potential to change the state of the
ES. Two of the PBs—climate change and bio-
sphere integrity—are recognized as “core”PBs
based on their fundamental importance for the
ES. The climate system is a manifestation of the
amount, distribution, and net balance of energy
at Earth’s surface; the biosphere regulates ma-
terial and energy flows in the ES and increases
its resilience to abrupt and gradual change.
Anthropogenic perturbation levels of four of
the ES processes/features (climate change, bio-
sphere integrity, biogeochemical flows, and land-
system change) exceed the proposed PB (see the
figure).
CONCLUSIONS: PBs are scientifically based
levels of human perturbation of the ES beyond
whichESfunctioningmaybesubstantially
altered. Transgression of the PBs thus creates
substantial risk of destabilizing the Holocene
state of the ES in which modern societies have
evolved. The PB framework does not dictate
how societies should develop. These are po-
litical decisions that must include considera-
tion of the human dimensions, including equity,
not incorporated in the PB framework. Never-
theless, by identifying a safe operating space
for humanity on Earth, the PB framework
can make a valuable contribution to decision-
makers in charting desirable courses for socie-
tal development. ▪
RESEARCH
736 13 FEBRUARY 2015 •VOL 347 ISSUE 6223 sciencemag.org SCIENCE
Current status of the control variables for seven of the planetary boundaries. The green zone
is the safe operating space, the yellow represents the zone of uncertainty (increasing risk), and the
red is a high-risk zone.The planetary boundary itself lies at the intersection of the green and yellow
zones. The control variables have been normalized for the zone of uncertainty; the center of the
figure therefore does not represent values of 0 for the control variables. The control variable shown
for climate change is atmospheric CO
2
concentration. Processes for which global-level boundaries
cannot yet be quantified are represented by gray wedges; these are atmospheric aerosol loading,
novel entities, and the functional role of biosphere integrity.
The list of author affiliations is available in the full article online.
*Corresponding author. E-mail: will.steffen@anu.edu.au
Cite this article as W. Steffen et al., Science 347, 1259855
(2015). DOI: 10.1126/science.1259855
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RESEARCH ARTICLE
◥
SUSTAINABILITY
Planetary boundaries: Guiding
human development on a
changing planet
Will Steffen,
1,2
*Katherine Richardson,
3
Johan Rockström,
1
Sarah E. Cornell,
1
Ingo Fetzer,
1
Elena M. Bennett,
4
Reinette Biggs,
1,5
Stephen R. Carpenter,
6
Wim de Vries,
7,8
Cynthia A. de Wit,
9
Carl Folke,
1,10
Dieter Gerten,
11
Jens Heinke,
11,12,13
Georgina M. Mace,
14
Linn M. Persson,
15
Veerabhadran Ramanathan,
16,17
Belinda Reyers,
1,18
Sverker Sörlin
19
The planetary boundaries framework defines a safe operating space for humanity based
on the intrinsic biophysical processes that regulate the stability of the Earth system.
Here, we revise and update the planetary boundary framework, with a focus on the
underpinning biophysical science, based on targeted input from expert research
communities and on more general scientific advances over the past 5 years. Several of the
boundaries now have a two-tier approach, reflecting the importance of cross-scale
interactions and the regional-level heterogeneity of the processes that underpin the
boundaries. Two core boundaries—climate change and biosphere integrity—have been
identified, each of which has the potential on its own to drive the Earth system into a new
state should they be substantially and persistently transgressed.
The planetary boundary (PB) approach (1,2)
aims to define a safe operating space for
human societies to develop and thrive, based
on our evolving understanding of the func-
tioning and resilience of the Earth system.
Since its introduction, the framework has been
subject to scientific scrutiny [e.g., (3–7)] and has
attracted considerable interest and discussions
within the policy, governance, and business sec-
tors as an approach to inform efforts toward glob-
al sustainability (8–10).
In this analysis, we further develop the basic
PB framework by (i) introducing a two-tier ap-
proach for several of the boundaries to account
for regional-level heterogeneity; (ii) updating the
quantification of most of the PBs; (iii) identifying
two core boundaries; and (iv) proposing a regional-
level quantitative boundary for one of the two
that were not quantified earlier (1).
The basic framework: Defining
a safe operating space
Throughout history, humanity has faced environ-
mental constraints at local and regional levels,
with some societies dealing with these challenges
more effectively than others (11,12). More recent-
ly, early industrial societies often used local water-
ways and airsheds as dumping grounds for their
waste and effluent from industrial processes. This
eroded local and regional environmental quality
and stability, threatening to undermine the pro-
gress made through industrialization by damag-
ing human health and degrading ecosystems.
Eventually, this led to the introduction of local
or regional boundaries or constraints on what
could be emitted to and extracted from the en-
vironment (e.g., chemicals that pollute airsheds
or waterways) and on how much the environment
could be changed by direct human modification
(land-use/cover change in natural ecosystems)
(13).Theregulationofsomehumanimpactson
the environment—for example, the introduction
of chemical contaminants—is often framed in
the context of “safe limits”(14).
These issues remain, but in addition we now
face constraints at the planetary level, where the
magnitudeofthechallengeisvastlydifferent.
The human enterprise has grown so dramatically
since the mid-20th centur y (15) that the relatively
stable, 11,700-year-long Holocene epoch, the only
state of the planet that we know for certain can
support contemporary human societies, is now
beingdestabilized(figs.S1andS2)(16 –18). In
fact, a new geological epoch, the Anthropocene,
has been proposed (19).
The precautionary principle suggests that hu-
mansocietieswouldbeunwisetodrivetheEarth
system substantially away from a Holocene-like
condition. A continuing trajectory away from the
Holocene could lead, with an uncomfortably high
probability, to a very different state of the Earth
system, one that is likely to be much less hos-
pitable to the development of human societies
(17,18,20). The PB framework aims to help guide
human societies away from such a trajectory by
defining a “safe operating space”in which we can
continue to develop and thrive. It does this by
proposing boundaries for anthropogenic pertur-
bation of critical Earth-system processes. Respect-
ing these boundaries would greatly reduce the
risk that anthropogenic activities could inadver-
tently drive the Earth system to a much less hos-
pitable state.
Nine processes, each of which is clearly being
modified by human actions, were originally sug-
gested to form the basis of the PB framework (1).
Although these processes are fundamental to
Earth-system functioning, there are many other
ways that Earth-system functioning could be de-
scribed, including potentially valuable metrics
for quantifying the human imprint on it. These
alternative approaches [e.g., (4)] often represent
ways to explore and quantify interactions among
the boundaries. They can provide a valuable com-
plement to the original approach (1)andfurther
enrich the broader PB concept as it continues to
evolve.
The planetary boundary
framework: Thresholds, feedbacks,
resilience, uncertainties
A planetary boundary as originally defined (1)is
not equivalent to a global threshold or tipping
point. As Fig. 1 shows, even when a global- or
continental/ocean basin–level threshold in an
Earth-system process is likely to exist [e.g., (20,21)],
the proposed planetary boundary is not placed
at the position of the biophysical threshold but
rather upstream of it—i.e., well before reaching
the threshold. This buffer between the boundary
(the end of the safe operating space, the green
zone in Fig. 1) and the threshold not only ac-
counts for uncertainty in the precise position of
the threshold with respect to the control variable
RESEARCH
SCIENCE sciencemag.org 13 FEBRUARY 2015 •VOL 347 ISSUE 6223 1259855-1
1
Stockholm Resilience Centre, Stockholm University, 10691
Stockholm, Sweden.
2
Fenner School of Environment and
Society, The Australian National University, Canberra, ACT 2601,
Australia.
3
Center for Macroecology, Evolution, and Climate,
University of Copenhagen, Natural History Museum of Denmark,
Universitetsparken 15, Building 3, 2100 Copenhagen, Denmark.
4
Department of Natural Resource Sciences and McGill School of
Environment, McGill University, 21, 111 Lakeshore Road, Ste-
Anne-de-Bellevue, QC H9X 3V9, Canada.
5
Centre for Studies in
Complexity, Stellenbosch University, Private Bag X1,
Stellenbosch 7602, South Africa.
6
Center for Limnology,
University of Wisconsin, 680 North Park Street, Madison WI
53706 USA.
7
Alterra Wageningen University and Research
Centre, P.O. Box 47, 6700AA Wageningen, Netherlands.
8
Environmental Systems Analysis Group, Wageningen University,
P.O. Box 47, 6700 AA Wageningen, Netherlands.
9
Department
of Environmental Science and Analytical Chemistry, Stockholm
University, 10691 Stockholm, Sweden.
10
Beijer Institute of
Ecological Economics, Royal Swedish Academy of Sciences,
SE-10405 Stockholm, Sweden.
11
Research Domain Earth
System Analysis, Potsdam Institute for Climate Impact
Research (PIK), Telegraphenberg A62, 14473 Potsdam,
Germany.
12
International Livestock Research Institute, P.O.
Box 30709, Nairobi, 00100 Kenya.
13
CSIRO (Commonwealth
Scientific and Industrial Research Organization), St. Lucia,
QLD 4067, Australia.
14
Centre for Biodiversity and
Environment Research (CBER), Department of Genetics,
Evolution and Environment, University College London, Gower
Street, London WC1E 6BT, UK.
15
Stockholm Environment
Institute, Linnégatan 87D, SE-10451 Stockholm, Sweden.
16
Scripps Institution of Oceanography, University of California
at San Diego, 8622 Kennel Way, La Jolla, CA 92037 USA.
17
TERI (The Energy and Resources Institute) University, 10
Institutional Area, Vasant Kunj, New Delhi, Delhi 110070,
India.
18
Natural Resources and the Environment, CSIR, P.O.
Box 320, Stellenbosch 7599, South Africa.
19
Division of
History of Science, Technology and Environment, KTH Royal
Institute of Technology, SE-10044 Stockholm, Sweden.
*Corresponding author. E-mail: will.steffen@anu.edu.au
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but also allows society time to react to early warn-
ing signs that it may be approaching a thresh-
old and consequent abrupt or risky change.
The developing science of early-warning signs
canwarnofanapproachingthresholdorade-
crease in the capability of a system to persist
under changing conditions. Examples include
“critical slowing down”in a process (22), in-
creasing variance (23), and flickering between
states of the system (24–26). However, for such
science to be useful in a policy context, it must
provide enough time for society to respond in
order to steer away from an impending thresh-
old before it is cross ed (27,28). The problem of
system inertia—for example, in the climate sys-
tem (18)—needs to be taken into account in as-
sessing the time needed for society to react to
early-warning signs.
Not all Earth-system processes included in the
PB approach have singular thresholds at the global/
continental/ocean basin level (1). Nevertheless, it
is important that boundaries be established for
these processes. They affect the capacity of the
Earth system to persist in a Holocene-like state
under changing conditions (henceforth “resilience”)
by regulating biogeochemical flows (e.g., the ter-
restrial and marine biological carbon sinks) or by
providing the capacity for ecosystems to tolerate
perturbations and shocks and to continue func-
tioning under changing abiotic conditions (29,30).
Examples of such processes are land-system
change, freshwater use, change in biosphere in-
tegrity [rate of biodiversity loss in (1,2)], and
changes in other biogeochemical flows in addi-
tion to carbon (e.g., nitrogen and phosphorus).
Placing boundaries for these processes is more
difficult than for those with known large-scale
thresholds (21) but is nevertheless important for
maintaining the resilience of the Earth system as
a whole. As indicated in Fig. 1, these processes,
many of which show threshold behavior at local
and regional scales, can generate feedbacks to
the processes that do have large-scale thresholds.
The classic example is the possible weakening of
natural carbon sinks, which could further de-
stabilize the climate system and push it closer to
large thresholds [e.g, loss of the Greenland ice
sheet (18)]. An interesting research question of
relevance to the PB framework is how small-
scale regime shifts can propagate across scales
and possibly lead to global-level transitions (31,32).
A zone of uncertainty, sometimes large, is as-
sociated with each of the boundaries (yellow zone
inFig.1).Thiszoneencapsulatesbothgapsand
weaknesses in the scientific knowledge base and
intrinsic uncertainties in the functioning of the
Earth system. At the “safe”end of the zone of un-
certainty, current scientific knowledge suggests
that there is very low probability of crossing a
critical threshold or substantially eroding the re-
silience of the Earth system. Beyond the “danger”
end of the zone of uncertainty, current knowl-
edge suggests a much higher probability of a
change to the functioning of the Earth system
that could potentially be devastating for human
societies. Application of the precautionary prin-
ciple dictates that the planetary boundary is set
at the “safe”end of the zone of uncertainty. This
does not mean that transgressing a boundary will
instantly lead to an unwanted outcome but that
the farther the boundary is transgressed, the
higher the risk of regime shifts, destabilized sys-
tem processes, or erosion of resilience and the
fewer the opportunities to prepare for such
changes. Observations of the climate system show
this principle in action by the influence of in-
creasing atmospheric greenhouse gas concentra-
tions on the frequency and intensity of many
extreme weather events (17,18).
Linking global and regional scales
PB processes operate across scales, from ocean
basins/biomes or sources/sinks to the level of the
Earth system as a whole. Here, we address the
subglobal aspects of the PB framework. Rock-
ström et al.(1) estimated global boundaries on-
ly, acknowledging that the control variables for
many processes are spatially heterogeneous. That
is, changes in control variables at the subglobal
level can influence functioning at the Earth-
system level, which indicates the need to define
subglobal boundaries that are compatible with
the global-level boundary definition. Avoiding
the transgression of subglobal boundaries would
thus contribute to an aggregate outcome within
a planetary-level safe operating space.
WefocusonthefivePBsthathavestrongre-
gional operating scales: biosphere integrity, biogeo-
chemical flows [earlier termed “phosphorus (P)
and nitrogen (N) cycles”(1,2)], land-system change,
freshwater use, and atmospheric aerosol loading.
Table S1 describes how transgression of any of
the proposed boundaries at the subglobal level
affects the Earth system at the global level.
For those processes where subglobal dynamics
potentiallyplayacriticalroleinglobaldynamics,
the operational challenge is to capture the im-
portance of subglobal change for the functioning
1259855-2 13 FEBRUARY 2015 •VOL 347 ISSUE 6223 sciencemag.org SCIENCE
Fig. 1. The conceptual framework for the planetary boundary approach, showing the safe operating space, the zone of uncertainty, the position of
the threshold (where one is likely to exist), and the area of high risk. Modified from (1).
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of the Earth system. To do this, we propose the
development of a two-level set of control var-
iables and boundaries. The subglobal-level units
of analysis for these six boundaries are not
identical; they vary according to the role that the
processes play in the Earth system: (i) changes
in biosphere integrity occur at the level of land-
based biomes, large freshwater ecosystems, or
major marine ecosystems as the largest sub-
global unit; (ii) the role of direct, human-driven
land-system change in biophysical climate regu-
lation is primarily related to changes in forest
biomes; (iii) freshwater flows and use occur at
the largest subglobal level in the major river
basins around the world; and (iv) changes in
biogeochemical flows, exemplified by phospho-
rus and nitrogen cycling, aggregate from rela-
tively localized but very severe perturbations
in intensive agricultural zones to affect global
flows of nutrients. We recognize these as crit-
ica l regi ons for Earth-system functioning. Where
appropriate, the updates of the individual bound-
aries (see below) (33) now contain both the glob-
ally aggregated boundary value of the control
variable and its regional distribution function.
Figure 2 shows the distributions and current
status of the control variables for three of the
boundaries where subglobal dynamics are crit-
ical: biogeochemical cycles, land-system change,
and freshwater use.
We emphasize that our subglobal-level focus is
based on the necessity to consider this level to
understand the functioning of the Earth system
as a whole. The PB framework is therefore meant
tocomplement,notreplaceorsupersede,efforts
to address local and regional environmental issues.
Updates of the individual boundaries
Brief updates of all nine of the PBs are given in
this section, and more detailed descriptions of
theupdatesforthreeofthePBsthathaveunder-
gone more extensive revision can be found in (33).
The geographical distribution issues discussed
above are particularly important for five of the
PBs, and their control variables and boundaries
have been revised accordingly (Table 1). Figure 3
shows the current status of the seven bounda-
ries that can be quantified at the global level.
Climate change
We retain the control variables and boundaries
originally proposed—i.e., an atmospheric CO
2
con-
centration of 350 parts per million (ppm) and an
increase in top-of-atmosphere radiative forcing of
+1.0 W m
–2
relative to preindustrial levels (1).
The radiative forcing control variable is the more
inclusive and fundamental, although CO
2
is im-
portant because of its long lifetime in the atmo-
sphere and the very large human emissions.
Human-driven changes to radiative forcing in-
clude all anthropogenic factors: CO
2
, other green-
house gases, aerosols, and other factors that
affect the energy balance (18). Radiative forcing
is generally the more stringent of the two bound-
aries, although the relationship between it and
CO
2
can vary through time with changes in the
relative importance of the individual radiative
forcing factors.
Evidence has accumulated to suggest that the
zoneofuncertaintyfortheCO
2
control variable
should be narrowed from 350 to 550 ppm to 350
to 450 ppm CO
2
(17,18),whileretainingthecur-
rent zone of uncertainty for radiative forcing of
+1.0 to 1.5 W m
–2
relative to preindustrial levels.
Current values of the control variables are 399 ppm
CO
2
(annual average concentration for 2014) (34)
and +2.3 W m
–2
(1.1 to 3.3 W m
–2
)in2011relative
to 1750 (18). Observed changes in climate at cur-
rent levels of the control variables confirm the
original choice of the boundary values and the
narrowing of the zone of uncertainty for CO
2
.For
example, there has already been an increase in
the intensity, frequency, and duration of heat
waves globally (35); the number of heavy rainfall
SCIENCE sciencemag.org 13 FEBRUARY 2015 •VOL 347 ISSUE 6223 1259855-3
Fig. 2. The subglobal distributions and current status of the control variables for (A) biogeochemical flows of P; (B) biogeochemical flows of N; (C)land-
system change; and (D) freshwater use. In each panel, green areas are within the boundary (safe), yellow areas are within the zone of uncertainty (increasing
risk), and red areas are beyond the zone of uncertainty (high risk). Gray areas in (A) and (B) are areas where P and N fertilizers are not applied; in (C), they are
areas not covered by major forest biomes; and in (D), they are areas where river flow is very low so that environmental flows are not allocated. See Table 1for
values of the boundaries and their zones of uncertainty and (33) for more details on methods and results.
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1259855-4 13 FEBRUARY 2015 •VOL 347 ISSUE 6223 sciencemag.org SCIENCE
Table 1. The updated control variables and their current values, along with the proposed boundaries and zones of uncertainty, for all nine planetary
boundaries. In the first column, the name for the Earth-system process used in the original PB publication (R2009, reference 1) is given for comparison.
Earth-system
process
Control
variable(s)
Planetary boundary
(zone of uncertainty)
Current value of
control variable
Climate
change
(R2009:
same)
Atmospheric CO
2
concentration, ppm
Energy imbalance
at top-of-
atmosphere, W m
–2
350 ppm CO
2
(350–450 ppm)
+1.0 W m
–2
(+1.0–1.5 W m
–2
)
398.5 ppm CO
2
2.3 W m
–2
(1.1–3.3 W m
–2
)
Change in
biosphere
integrity
(R2009:
Rate of
biodiversity
loss)
Genetic diversity:
Extinction rate
Functional diversity:
Biodiversity
Intactness Index (BII)
Note: These are
interim control
variables until more
appropriate ones are
developed
< 10 E/MSY (10–100 E/MSY)
but with an aspirational goal of
ca. 1 E/MSY (the background
rate of extinction loss). E/MSY =
extinctions per million species-years
Maintain BII at 90% (90–30%)
or above, assessed
geographically by biomes/large
regional areas (e.g. southern
Africa), major marine
ecosystems (e.g., coral reefs) or
by large functional groups
100–1000 E/MSY
84%, applied to
southern Africa
only
Stratospheric
ozone
depletion
(R2009: same)
Stratospheric O
3
concentration, DU
<5% reduction from pre-
industrial level of 290 DU
(5%–10%), assessed by
latitude
Only transgressed
over Antarctica in
Austral spring
(~200 DU)
Ocean
acidification
(R2009:
same)
Carbonate ion
concentration,
average global
surface ocean
saturation state with
respect to aragonite
(W
arag
)
≥80% of the pre-industrial
aragonite saturation state of
mean surface ocean, including
natural diel and seasonal
variability (≥80%–≥70%)
~84% of the
pre-industrial
aragonite
saturation state
Biogeochemical
flows: (P and
N cycles)
(R2009:
Biogeochemical
flows: (interference
with P and N
cycles))
P Global: P flow
from freshwater
systems into the
ocean
P Regional: P flow
from fertilizers to
erodible soils
N Global: Industrial
and intentional
biological fixation
of N
11 Tg P yr
–1
(11–100 Tg P yr
–1
)
6.2 Tg yr
–1
mined and applied to
erodible (agricultural) soils
(6.2-11.2 Tg yr
–1
). Boundary is a
global average but regional
distribution is critical for
impacts.
62 Tg N yr
–1
(62–82 Tg N yr
–1
).
Boundary acts as a global
‘valve’limiting introduction of
new reactive N to Earth System,
but regional distribution of
fertilizer N is critical for
impacts.
~22 Tg P yr
–1
~14 Tg P yr
–1
~150 Tg N yr
–1
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events in many regions of the world is increasing
(17); changes in atmospheric circulation patterns
have increased drought in some regions of the
world (17); and the rate of combined mass loss
from the Greenland and Antarctic ice sheets is
increasing (36).
Changes in biosphere integrity
We propose a two-component approach, address-
ingtwokeyrolesofthebiosphereintheEarth
system. The first captures the role of genetically
unique material as the “information bank”that
ultimately determines the potential for life to
continue to coevolve with the abiotic component
of the Earth system in the most resilient way
possible. Genetic diversity provides the long-term
capacity of the biosphere to persist under and
adapt to abrupt and gradual abiotic change. The
second captures the role of the biosphere in
Earth-system functioning through the value, range,
distribution, and relative abundance of the func-
tional traits of the organisms present in an eco-
system or biota (7).
For the first role, the concept of phylogenetic
species variability (PSV) (7,33,37)wouldbean
appropriate control variable. However, because
global data are not yet available for PSV, we re-
tain the global extinction rate as an interim con-
trol variable, although it is measured inaccurately
and with a time lag. There may be a considerable
risk in using extinction rate as a control variable,
because phylogenetic (and functional) diversity
maybemoresensitivetohumanpressuresthan
species-level diversity (38). In principle, the bound-
aryshouldbesetatarateoflossofPSVnogreater
than the rate of evolution of new PSV during the
Holocene. Because that is unknown, we must fall
back on the (imperfectly) known extinction rate
of well-studied organisms over the past several
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Earth-system
process
Control
variable(s)
Planetary boundary
(zone of uncertainty)
Current value of
control variable
Land-system
change
(R2009:
same)
Global: Area of
forested land as %
of original forest
cover
Biome:Areaof
forested land as %
of potential forest
Global: 75% (75–54%) Values
are a weighted average of the
three individual biome
boundaries and their uncertainty
zones
Biome:
Tropical: 85% (85–60%)
Temperate: 50% (50–30%)
Boreal: 85% (85–60%)
62%
Freshwater
use
(R2009:
Global
freshwater
use)
Global: Maximum
amount of
consumptive blue
water use (km
3
yr
–1
)
Basin: Blue water
withdrawal as % of
mean monthly river
flow
Global: 4000 km
3
yr
–1
(4000–6000 km
3
yr
–1
)
Basin: Maximum monthly
withdrawal as a percentage
of mean monthly river flow.
For low-flow months: 25%
(25–55%); for intermediate-
flow months: 30% (30–60%);
for high-flow months: 55%
(55–85%)
~2600 km
3
yr
–1
Atmospheric
aerosol
loading
(R2009:
same)
Global: Aerosol
Optical Depth
(AOD), but much
regional variation
Regional: AOD as
a seasonal average
over a region. South
Asian Monsoon
used as a case study
Regional: (South Asian
Monsoon as a case study):
anthropogenic total (absorbing
and scattering) AOD over
Indian subcontinent of 0.25
(0.25–0.50); absorbing
(warming) AOD less than 10%
of total AOD
0.30 AOD, over
South Asian
region
Introduction
of novel entities
(R2009: Chemical
pollution)
No control variable
currently defined
No boundary currently
identified, but see boundary
for stratospheric ozone for an
example of a boundary
related to a novel entity (CFCs)
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million years—about 1 per million species-years
(39)—and add a large uncertainty bound, raising
the boundary to 10 per million species-years. The
risk is that, although the Earth system can tol-
erate a higher-than-background level of extinc-
tions for a time, we do not know what levels of, or
types of, biodiversity loss may possibly trigger non-
linear or irreversible changes to the Earth system.
The second control variable aims to capture the
role of the biosphere in Earth-system functioning
and measures loss of biodiversity components at
both global and biome/large ecosystem levels. Al-
though several variables have been developed at
local scales for measuring functional diversity
[e.g., (40)], finding an appropriate control varia-
ble at regional or global levels is challenging. For
the present, we propose an interim control var-
iable, the Biodiversity Intactness Index (BII) (41).
BII assesses change in population abundance as a
result of human impacts, such as land or resource
use, across a wide range of taxa and functional
groups at a biome or ecosystem level using pre-
industrial era abundance as a reference point. The
index typically ranges from 100% (abundances
across all functional groups at preindustrial levels)
to lower values that reflect the extent and degree
of human modification of populations of plants
and animals. BII values for particular functional
groups can go above 100% if human modifications
to ecosystems lead to increases in the abundance
of those species.
Duetoalackofevidenceontherelationship
between BII and Earth-system responses, we pro-
pose a preliminary boundary at 90% of the BII
but with a very large uncertainty range (90 to
30%) that reflects the large gaps in our knowl-
edge about the BII–Earth-system functioning
relationship (42,43). BII has been so far applied
to southern Africa’s terrestrial biomes only (see
fig. S3 for an estimation of aggregated human
pressures on the terrestrial biosphere globally),
where the index (not yet disaggregated to func-
tional groups) was estimated to be 84%. BII
ranged from 69 to 91% for the seven countries
where it has been applied (41). Observations across
these countries suggest that decreases in BII ad-
equately capture increasing levels of ecosystem
degradation, defined as land uses that do not al-
ter the land-cover type but lead to a persistent
loss in ecosystem productivity (41).
In addition to further work on functional mea-
sures such as BII, in the longer term the concept
of biome integrity—the functioning and persist-
ence of biomes at broad scales (7)—offers a prom-
ising approach and, with further research, could
provide a set of operational control variables (one
per biome) that is appropriate, robust, and scien-
tifically based.
Stratospheric ozone depletion
We retain the original control variable [O
3
con-
centration in DU (Dobson units)] and boundary
(275 DU). This boundary is only transgressed
over Antarctica in the austral spring, when O
3
concentration drops to about 200 DU (44). How-
ever, the minimum O
3
concentration has been
steady for about 15 years and is expected to rise
over the coming decades as the ozone hole is
repaired after the phasing out of ozone-depleting
substances. This is an example in which, after a
boundary has been transgressed regionally, hu-
manityhastakeneffectiveactiontoreturnthe
process back to within the boundary.
Ocean acidification
This boundary is intimately linked with one of
the control variables, CO
2
,fortheclimatechange
PB. The concentration of free H
+
ions in the sur-
face ocean has increased by about 30% over the
past 200 years due to the increase in atmospheric
CO
2
(45). This, in turn, influences carbonate chem-
istry in surface ocean waters. Specifically, it lowers
the saturation state of aragonite (W
arag
), a form of
calcium carbonate formed by many marine orga-
nisms. At W
arag
< 1, aragonite will dissolve. No
new evidence has emerged to suggest that the
originally proposed boundary (≥80% of the pre-
industrial average annual global W
arag
)should
be adjusted, although geographical heterogeneity
in W
arag
is important in monitoring the state of
theboundaryaroundtheworld’soceans(fig.S4).
Currently, W
arag
is approximately equal to 84% of
thepreindustrialvalue(46). This boundary would
not be transgressed if the climate-change bound-
ary of 350 ppm CO
2
were to be respected.
Biogeochemical flows
The original boundary was formulated for phos-
phorus (P) and nitrogen (N) only, but we now
propose a more generic PB to encompass human
influence on biogeochemical flows in general. Al-
though the carbon cycle is covered in the climate-
change boundary, other elements, such as silicon
(47,48), are also important for Earth-system func-
tioning. Furthermore, thereisincreasingevidence
that ratios between elements in the environment
may have impacts on biodiversity on land and in
the sea (49–51). Thus, we may ultimately need to
develop PBs for other elements and their ratios,
although for now we focus on P and N only.
A two-level approach is now proposed for the
P component of the biogeochemical flows bound-
ary(seealsothesupplementarymaterials).The
original global-level boundary, based on the pre-
vention of a large-scale ocean anoxic event, is
retained, with the proposed boundary set at a
sustained flow of 11 Tg P year
–1
from freshwater
systems into the ocean. Based on the analysis of
Carpenter and Bennett (3), we now propose an
additional regional-level P boundary, designed
to avert widespread eutrophication of freshwater
systems, at a flow of 6.2 Tg P year
–1
from fer-
tilizers (mined P) to erodible soils.
Given that the addition of P to regional
watersheds is almost entirely from fertilizers, the
regional-level boundary applies primarily to the
world’s croplands. The current global rate of ap-
plication of P in fertilizers to croplands is 14.2 Tg
P year
–1
(52,53). Observations point toward a few
agricultural regions of very high P application
rates as the main contributors to the transgres-
sion of this boundary (Fig. 2 and fig. S5A) and
suggest that a redistribution of P from areas
1259855-6 13 FEBRUARY 2015 •VOL 347 ISSUE 6223 sciencemag.org SCIENCE
Fig. 3.The current status of the control variables for seven of the nine planetary boundaries.The
green zone is the safe operating space (below the boundary), yellow represents the zone of uncertainty
(increasing risk), and red is the high-risk zone.The planetary boundary itself lies at the inner heavy circle.
The control variables have been normalized for the zone of uncertainty (between the two heavy circles);
the center of the figure therefore does not represent values of 0 for the control variables. The control
variable shown for climate change is atmospheric CO
2
concentration. Processes for which global-level
boundaries cannot yet be quantified are represented by gray wedges; these are atmospheric aerosol
loading, novel entities, and the functional role of biosphere integrity. Modified from (1).
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where it is currently in excess to areas where the
soil is naturally P-poor may simultaneously boost
global crop production and reduce the transgres-
sion of the regional-level P boundary (3,52,54).
The N boundary has been taken from the com-
prehensive analysis of de Vries et al.(5), which
proposed a PB for eutrophication of aquatic eco-
systems of 62 Tg N year
–1
from industrial and
intentional biological N fixation, using the most
stringent water quality criterion. As for the P
boundary, a few agricultural regions of very high
N application rates are the main contributors to
the transgression of this boundary (Fig. 2 and
fig. S5B). This suggests that a redistribution of N
could simultaneously boost global crop produc-
tion and reduce the transgression of the regional-
level boundary.
Because the major anthropogenic perturba-
tion of both the N and P cycles arises from fertil-
izer application, we can analyze the links between
the independently determined N and P bounda-
ries in an integrated way based on the N:P ratio
inthegrowingplanttissueofagriculturalcrops.
Applying this ratio, which is on average 11.8 (55),
to the P boundary (6.2 Tg P year
–1
) gives an N
boundary of 73 Tg N year
–1
.Conversely,applying
the ratio to the N boundary (62 Tg N year
–1
) gives
a P boundary of 5.3 Tg P year
–1
.Thesmalldif-
ferences between the boundaries derived using
the N:P ratio and those calculated independent-
ly, which are likely nonsignificant differences
given the precision of the data available for the
calculations, show the internal consistency in
our approach to the biogeochemical boundaries.
More detail on the development of the P and N
boundaries is given in (33),wherewealsoem-
phasize that the proposed P and N boundaries
may be larger for an optimal allocation of N (and
P) over the globe.
Land-system change
The updated biosphere integrity boundary pro-
vides a considerable constraint on the amount
and pattern of land-system change in all ter-
restrial biomes: forests, woodlands, savannas,
grasslands, shrublands, tundra, and so on. The
land-system change boundary is now focused
more tightly on a specific constraint: the biogeo-
physical processes in land systems that directly
regulate climate—exchange of energy, water, and
momentum between the land surface and the
atmosphere. The control variable has been changed
from the amount of cropland to the amount of
forest cover remaining, as the three major forest
biomes—tropical, temperate and boreal—play a
stronger role in land surface–climate coupling
than other biomes (56,57). In particular, we fo-
cus on those land-system changes that can in-
fluence the climate in regions beyond the region
where the land-system change occurred.
Of the forest biomes, tropical forests have sub-
stantial feedbacks to climate through changes in
evapotranspiration when they are converted to
nonforested systems, and changes in the distribu-
tion of boreal forests affect the albedo of the land
surface and hence regional energy exchange. Both
have strong regional and global teleconnections.
The biome-level boundary for these two types of
forest have been set at 85% (Table 1 and the
supplementary materials), and the boundary for
temperate forests has been proposed at 50% of
potential forest cover, because changes to tem-
perate forests are estimated to have weaker in-
fluences on the climate system at the global level
than changes to the other two major forest
biomes (56). These boundaries would almost
surely be met if the proposed biosphere integ-
rity boundary of 90% BII were respected.
Estimates of the current status of the land-
system change boundary are given in Figs. 2 and
3andfig.S6andin(58).
Freshwater use
The revised freshwater use boundary has retained
consumptive use of blue water [from rivers, lakes,
reservoirs, and renewable groundwater stores
(59)] as the global-level control variable and
4000 km
3
/year as the value of the boundary.
This PB may be somewhat higher or lower de-
pending on rivers’ecological flow requirements
(6). Therefore, we report here a new assessment
to complement the PB with a basin-scale bound-
ary for the maximum rate of blue water with-
drawal along rivers, based on the amount of water
required in the river system to avoid regime shifts
in the functioning of flow-dependent ecosystems.
We base our control variable on the concept of
environmental water flows (EWF), which defines
the level of river flows for different hydrological
characteristics of river basins adequate to main-
tain a fair-to-good ecosystem state (60–62).
The variable monthly flow (VMF) method
(33,63) was used to calculate the basin-scale
boundary for water. This method takes account
of intra-annual variability by classifying flow re-
gimes into high-, intermediate-, and low-flow
months and allocating EWF as a percentage of
the mean monthly flow (MMF). Based on this
analysis, the zones of uncertainty for the river-
basin scale water boundary were set at 25 to 55%
of MMF for the low-flow regime, 40 to 70% for
the intermediate-flow regime, and 55 to 85% for
thehigh-flowregime(tableS2).Theboundaries
were set at the lower end of the uncertainty
ranges that encompass average monthly EWF.
Our new estimates of the current status of the
water use boundary—computed based on grid
cell–specific estimates of agricultural, industrial,
and domestic water withdrawals—are shown in
Figs. 2 and 3, with details in figs. S7 and S8.
Atmospheric aerosol loading
Aerosols have well-known, serious human health
effects, leading to about 7.2 million deaths per
year (64). They also affect the functioning of the
Earth system in many ways (65) (fig. S9). Here,
we focus on the effect of aerosols on regional
ocean-atmosphere circulation as the rationale
for a separate aerosols boundary. We adopt aero-
sol optical depth (AOD) (33)asthecontrolvar-
iable and use the south Asian monsoon as a case
study, based on the potential of widespread aero-
sol loading over the Indian subcontinent to switch
the monsoon system to a drier state.
The background AOD over south Asia is ~0.15
and can be as high as 0.4 during volcanic events
(66). Emissions of black carbon and organic car-
bon from cooking and heating with biofuels and
from diesel transportation, and emission of sul-
fates and nitrates from fossil fuel combustion,
can increase seasonal mean AODs to as high as
0.4 (larger during volcanic periods), leading to
decreases of 10 to 15% of incident solar radiation
at the surface (fig. S9). A substantial decrease in
monsoon activity is likely around an AOD of 0.50,
an increase of 0.35 above the background (67).
Taking a precautionary approach toward uncer-
tainties surrounding the position of the tipping
point, we propose a boundary at an AOD of 0.25
(an increase due to human activities of 0.1), with
a zone of uncertainty of 0.25 to 0.50. The annual
mean AOD is currently about 0.3 (66), within the
zone of uncertainty.
Introduction of novel entities
We define novel entities as new substances, new
forms of existing substances, and modified life
forms that have the potential for unwanted geo-
physical and/or biological effects. Anthropogenic
introduction of novel entities to the environment
is of concern at the global level when these en-
tities exhibit (i) persistence, (ii) mobility across
scales with consequent widespread distributions,
and (iii) potential impacts on vital Earth-system
processes or subsystems. These potentially in-
clude chemicals and other new types of engi-
neered materials or organisms [e.g., (68–71)] not
previously known to the Earth system, as well as
naturally occurring elements (for example, heavy
metals) mobilized by anthropogenic activities.
The risks associated with the introduction of
novel entities into the Earth system are exempli-
fied by the release of CFCs (chlorofluorocarbons),
which are very useful synthetic chemicals that
were thought to be harmless but had unexpected,
dramatic impacts on the stratospheric ozone layer.
In effect, humanity is repeatedly running such
global-scale experiments but not yet applying the
insights from previous experience to new appli-
cations (72,73).
Today there are more than 100,000 substances
in global commerce (74). If nanomaterials and
plastic polymers that degrade to microplastics
areincluded,thelistisevenlonger.Thereisalso
a“chemical intensification”due to the rapidly
increasing global production of chemicals, the
expanding worldwide distribution as chemical
products or in consumer goods, and the exten-
sive global trade in chemical wastes (75).
In recent years, there has been a growing de-
bate about the global-scale effects of chemical
pollution, leading to calls for the definition of
criteria to identify the kinds of chemical sub-
stances that are likely to be globally problematic
(76,77). Persson et al.(73) proposed that there are
three conditions that need to be fulfilled for a
chemical to pose a threat to the Earth system: (i)
the chemical has an unknown disruptive effect
on a vital Earth-system process; (ii) the disruptive
effect is not discovered until it is a problem at the
global scale; and (iii) the effect is not readily
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reversible. The challenge to the research commu-
nity is to develop the knowledge base that allows
the screening of chemicals, before they are re-
leased into the environment, for properties that
maypredisposethemtowardbecomingglobal
problems.
As a first step toward meeting this challenge,
the three conditions outlined above have been
used as the basis for identifying scenarios of
chemical pollution that fulfill the conditions and
as a next step for pinpointing chemical profiles
that fit the scenarios (28). This proposal consti-
tutes a first attempt at adding the Earth-system
perspective when assessing hazard and risk of
chemicals and offers a vision for a systematic ap-
proach to a complex management situation with
many unknowns.
Despite this progress in developing an Earth-
system–oriented approach, there is not yet an
aggregate, global-level analysis of chemical pol-
lution on which to base a control variable or a
boundary value. It may also serve little purpose
to define boundary values and control varia-
bles for a planetary boundary of this complexity.
Nevertheless, there is a potential threat from
novel entities to disrupt the functioning of the
Earth-system and society needs to learn how to
mitigate these unknown risks and manage chem-
icals under uncertainty (28,73).
Some precautionary and preventive actions
can be considered. These may include a stronger
focus on green chemistry (78), finding synergies
with risk-reducing interventions in other fields
such as occupational health (79), paying more
attention to learning from earlier mistakes (80,
81), and investing in science to better under-
stand and monitor vital Earth-system processes
in order to be able to detect disruptive effects
from novel entities as early as possible.
Hierarchy of boundaries
An analysis of the many interactions among the
boundaries (table S3 and fig. S10) suggests that
two of them—climate change and biosphere
integrity—are highly integrated, emergent system-
level phenomena that are connected to all of the
other PBs. They operate at the level of the whole
Earth system (7) and have coevolved for nearly
4 billion years (82). They are regulated by the
other boundaries and, on the other hand, pro-
vide the planetary-level overarching systems with-
in which the other boundary processes operate.
Furthermore, large changes in the climate or in
biosphere integrity would likely, on their own,
push the Earth system out of the Holocene state.
In fact, transitions between time periods in Earth
history have often been delineated by substantial
shifts in climate, the biosphere, or both (82,83).
These observations suggest a two-level hierar-
chy of boundaries, in which climate change and
biosphere integrity should be recognized as core
planetary boundaries through which the other
boundaries operate. The crossing of one or more
of the other boundaries may seriously affect hu-
man well-being and may predispose the trans-
gression of a core boundary(ies) but does not by
itself lead to a new state of the Earth system. This
hierarchical approach to classifying the bounda-
ries becomes clearer by examining in more detail
therolesofclimateandbiosphereintegrityinthe
functioning of the Earth system.
The climate system is a manifestation of the
amount, distribution, and net balance of energy
at Earth’s surface. The total amount of energy
sets the overall conditions for life. In Earth’scur-
rent climate, a range of global surface temper-
atures and atmospheric pressures allows the three
phases of water to be present simultaneously,
with ice and water vapor playing critical roles in
the physical feedbacks of the climate system. The
distribution of energy by latitude, over the land
and sea surfaces, and within the ocean plays a
major role in the circulation of the two great
fluids, the ocean and the atmosphere. These sys-
temic physical characteristics are key spatial de-
terminants of the distribution of the biota and
the structure and functioning of ecosystems and
are controllers of biogeochemical flows.
Biosphere integrity is also crucial to Earth-
system functioning, where the biosphere is de-
fined as the totality of all ecosystems (terrestrial,
freshwater, and marine) on Earth and their biota
(32). These ecosystems and biota play a critical
role in determining the state of the Earth system,
regulating its material and energy flows and its
responses to abrupt and gradual change (7). Di-
versity in the biosphere provides resilience to
terrestrial and marine ecosystems (83,84). The
biosphere not only interacts with the other plan-
etary boundaries but also increases the capacity
of the Earth system to persist in a given state under
changes in these other boundaries. The ultimate
basis for the many roles that the biosphere plays
in Earth-system dynamics is the genetic code of
the biota, the basic information bank that de-
fines the biosphere’sfunctionalroleanditsca-
pacity to innovate and persist into the future.
Planetary boundaries in a
societal context
A proposed approach for sustainable develop-
ment goals (SDGs) (85)arguesthatthestable
functioning of the Earth system is a prereq-
uisit e for thriving societies around the world. This
approach implies that the PB framework, or
something like it, will need to be implemented
alongside the achievement of targets aimed at
more immediate human needs, such as provi-
sion of clean, affordable, and accessible energy
and the adequate supply of food. World devel-
opment within the biophysical limits of a stable
Earth system has always been a necessity [e.g.,
(86,87)]. However, only recently, for a number
of reasons, has it become possible to identify,
evaluate, and quantify risks of abrupt planetary-
and biome-level shifts due to overshoot of key
Earth-system parameters: (i) the emergence of
global-change thinking and Earth-system think-
ing (88); (ii) the rise of “the Planetary”as a rel-
evant level of complex system understanding
(89–92); and (iii) observable effects of the rapid
increase in human pressures on the planet (16).
ThePBapproachisembeddedinthisemerg-
ing social context, but it does not suggest how to
maneuver within the safe operating space in the
quest for global sustainability. For example, the
PB framework does not as yet account for the re-
gional distribution of the impact or its histor-
ical patterns. Nor does the PB framework take
into account the deeper issues of equity and cau-
sation. The current levels of the boundary pro-
cesses, and the transgressions of boundaries that
have already occurred, are unevenly caused by
different human societies and different social
groups. The wealth benefits that these trans-
gressions have brought are also unevenly distrib-
uted socially and geographically. It is easy to
foresee that uneven distribution of causation and
benefits will continue, and these differentials
must surely be addressed for a Holocene-like
Earth-system state to be successfully legitimated
and maintained. However, the PB framework as
currently construed provides no guidance as to
how this may be achieved [although some po-
tential synergies have been noted (54)], and it
cannot readily be used to make choices between
pathways for piecemeal maneuvering within
the safe operatingspace or more radical shifts of
global governance (93).
The nature of the PB framework implies that
two important cautions should be observed when
application of the framework to policy or man-
agement is proposed: boundary interactions and
scale.
Boundary interactions
The planetary boundaries framework arises from
the scientific evidence that Earth is a single,
complex, integrated system—that is, the bound-
aries operate as an interdependent set [e.g.,
(94)] (table S1 and fig. S10). Although a system-
atic, quantitative analysis of interactions among
all of the processes for which boundaries are
proposed remains beyond the scope of current
modeling and observational capacity, the Earth
system clearly operates in well-defined states in
which these processes and their interactions
can create stabilizing or destabilizing feedbacks
(16,90,95).Thishasprofoundimplicationsfor
global sustainability, because it emphasizes the
need to address multiple interacting environ-
mental processes simultaneously (e.g., stabilizing
the climate system requires sustainable forest
management and stable ocean ecosystems).
Scale
The PB framework is not designed to be “down-
scaled”or “disaggregated”to smaller levels, such
as nations or local communities. That said, the
PB framework recognizes the importance of
changes at the level of subsystems in the Earth
system (e.g., biomes or large river basins) on the
functioning of the Earth system as a whole. Als o,
there are strong arguments for an integrated ap-
proach coupling boundary definitions at region-
al and global levels with development goals to
enable the application of “PB thinking”at lev-
els (nations, basins, and regions) where policy
action most commonly occurs [e.g., (85,96)].
This update of the PB framework is one step on
a longer-term evolution of scientific knowledge to
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inform and support global sustainability goals
and pathways. This evolution is needed more
than ever before; there are severe implementa-
tion gaps in many global environmental policies
relating to the PB issues, where problematic
trends are not being halted or reversed despite
international consensus about the urgency of the
problems. The prospect of tighter resource con-
straints and rising environmental hazards is also
unavoidably turning the focus onto global social
equity and the planetary stewardship of Earth’s
life-support system. There is a need for a truly
global evidence base, with much greater integra-
tion among issues, in order to respond to these
global challenges. New research initiatives [e.g.,
Future Earth (www.futureearth.org)] provide evi-
dence that science can respond to this need by
applying Earth-system research to advance a new
generation of integrated global analyses and to
explore options for transformations toward sus-
tainability. This is a clear sign that, as the risks
of the Anthropocene to human well-being be-
come clearer, research is maturing to a point
where a systemic step-change is possible—and
necessary—in exploring and defining a safe and
just planetary operating space for the further
development of human societies.
Methods summary
Our approach to building the planetary bound-
aries framework is described above. We have
implemented the framework through an ex-
pert assessment and synthesis of the scientific
knowledge of intrinsic biophysical processes that
regulate the stability of the Earth system. Our
precautionary approach is based on the main-
tenance of a Holocene-like state of the Earth
system and on an assessment of the level of
human-driven change that would risk destabi-
lizing this state. For the climate change PB, there
is already much literature on which to base
such an assessment. For others, such as strato-
spheric ozone, ocean acidification, extinction
rates,andPandNcycles,wehaveusedestimates
of preindustrial values of the control variable
as a Holocene baseline. Where large, undesira-
ble thresholds exist and have been studied (e.g.,
polar ice sheets, Amazon rainforest, aragonite
dissolution, atmospheric aerosols, and the south
Asian monsoon), quantitative boundaries can be
readilyproposed.Forothers,wherethefocusis
on erosion of Earth-system resilience, the bound-
aries are more difficult (but not impossible) to
quantify, as reflected in larger uncertainty zones.
We used large-scale assessments of the impacts
of human activities on Earth-system functioning
[e.g., IntergovernmentalPanelonClimateChange
(17,18), the International Geosphere-Biosphere
Programme synthesis (16), and chemicals (75,80)]
as sources of community-level understanding
on which to propose PBs. Our update has also
relied on post-2009 assessments of individual
boundaries by the relevant expert research com-
munities; examples include phosphorus (3), ni-
trogen (5), biosphere integrity (7), freshwater use
(5,63), and novel entities [with a focus on chem-
icals (28,73)]. Finally, some new analyses have
been undertaken specifically for this paper: (i) a
freshwater-use PB based on the EWF approach
(33,63); (ii) the linkage of the phosphorus and
nitrogen boundaries via the N:P ratio in grow-
ing crop tissue (33); and (iii) the use of major
forest biomes as the basis for the land-system
change PB (33).
REFERENCES AND NOTES
1. J. Rockström et al., Planetary boundaries: Exploring the safe
operating space for humanity. Ecol. Soc. 14, 32 (2009).
http://www.ecologyandsociety.org/vol14/iss2/art32/
2. J. Rockström et al., A safe operating space for humanity.
Nature 461,472–475 (2009). doi: 10.1038/461472a;
pmid: 19779433
3. S. R. Carpenter, E. M. Bennett, Reconsideration of the
planetary boundary for phosphorus. Environ. Res. Lett. 6,
014009 (2011). doi: 10.1088/1748-9326/6/1/014009
4. S. W. Running, Ecology. A measurable planetary boundary
for the biosphere. Science 337, 1458–1459 (2012).
doi: 10.1126/science.1227620; pmid: 22997311
5. W. de Vries, J. Kros, C. Kroeze, S. P. Seitzinger, Assessing
planetary and regional nitrogen boundaries related to food
security and adverse environmental impacts. Curr. Opinion
Environ. Sust. 5, 392–402 (2013). doi: 10.1016/
j.cosust.2013.07.004
6. D. Gerten et al., Towards a revised planetary boundary for
consumptive freshwater use: Role of environmental flow
requirements. Curr. Opinion Environ. Sust. 5, 551–558 (2013).
doi: 10.1016/j.cosust.2013.11.001
7. G. M. Mace et al., Approaches to defining a planetary boundary
for biodiversity. Glob. Environ. Change 28, 289–297 (2014).
doi: 10.1016/j.gloenvcha.2014.07.009
8. V. Galaz, Global Environmental Governance, Technology and
Politics: The Anthropocene Gap. (Edward Elgar, Cheltenham,
UK, 2014).
9. UN GSP (UN High-Level Panel on Global Sustainability),
Resilient People, Resilient Planet: A Future Worth Choosing
(Report for the 2012 Rio+20 Earth Summit, United Nations,
New York, 2012).
10. WBCSD (World Business Council on Sustainable Development),
Action 2020 Overview (WBCSD, Geneva, Switzerland,
http://action2020.org, accessed 18 June 2014).
11. R. Costanza, L. Graumlich, W. Steffen, Eds., Integrated History
and Future of People on Earth (MIT Press, Cambridge,
MA USA, 2006).
12. S. Sörlin, P. Warde, in Nature’s End: History and the
Environment, S. Sörlin, P. Warde, Eds. (Palgrave MacMillan,
London, 2009), pp. 1–19.
13. R. C. Bishop, Endangered species and uncertainty: The
economics of a safe minimum standard. Am. J. Agric. Econ. 61,
10–18 (1978). doi: 10.2307/1240156
14. T. M. Crowards, Safe minimum standards: Costs and
opportunities. Ecol. Econ. 25, 303–314 (1998). doi: 10.1016/
S0921-8009(97)00041-4
15. W. Steffen, J. Crutzen, J. R. McNeill, The Anthropocene: Are
humans now overwhelming the great forces of Nature?
Ambio 36,614–621 (2007). doi: 10.1579/0044-7447(2007)36
[614:TAAHNO]2.0.CO;2; pmid: 18240674
16. W. Steffen et al., Global Change and the Earth System: A Planet
Under Pressure (The IGBP Book Series, Springer-Verlag,
Berlin, Heidelberg, New York, 2004).
17. IPCC (Intergovernmental Panel on Climate Change), Managing
the risks of extre me events and dis asters to advance
climate change adaptation. A special report of Working Groups
I and II of the IPCC. C.B. Field et al., Eds. (Cambridge
University Press, Cambridge, UK (2012). doi: 10.1017/
CBO9781139177245
18. IPCC (Intergovernmental Panel on Climate Change), Climate
Change 2013: The Physical Science Basis. Summary for
Policymakers., L. Alexander et al., Eds. (IPCC Secretariat,
Geneva, Switzerland, 2013). doi: 10.1017/CBO9781107415324
19. P. J. Crutzen, Geology of mankind. Nature 415, 23 (2002).
doi: 10.1038/415023a; pmid: 11780095
20. K. Richardson, W. Steffen, D. Liverman, Climate Change: Global
Risks, Challenges and Decisions (Cambridge Univ. Press,
Cambridge, UK, 2011).
21. T. M. Lenton et al., Tipping elements in the Earth’s climate
system. Proc. Natl. Acad. Sci. U.S.A. 105, 1786–1793 (2008).
doi: 10.1073/pnas.0705414105; pmid: 18258748
22. M. Scheffer et al., Ear ly-warning sign als for critica l
transitions. Nature 461,53–59 (2009). doi: 10.103 8/
nature08227;pmid:197271 93
23. S. R. Carpenter, W. A. Brock, Rising variance: A leading
indicator of ecological transition. Ecol. Lett. 9, 311–318 (2006).
doi: 10.1111/j.1461-0248.2005.00877.x; pmid: 16958897
24. J. Bakke et al., Rapid oceanic and atmospheric changes during
the Younger Dryas cold period. Nat. Geosci. 2, 202–205
(2009). doi: 10.1038/ngeo439
25. M. Scheffer et al., Anticipating critical transitions. Science
338, 344–348 (2012). doi: 10.1126/science.1225244;
pmid: 23087241
26. R. Wang et al., Flickering gives early warning signals of a
critical transition to a eutrophic lake state. Nature 492,
419–422 (2012). doi: 10.1038/nature11655; pmid: 23160492
27. R. Biggs, S. R. Carpenter, W. A. Brock, Turning back from
the brink: Detecting an impending regime shift in time to avert
it. Proc. Natl. Acad. Sci. U.S.A. 106, 826–831 (2009).
doi: 10.1073/pnas.0811729106; pmid: 19124774
28. M. MacLeod et al., Identifying chemicals that are planetary
boundary threats. Environ. Sci. Technol. 48, 11057–11063
(2014). doi: 10.1021/es501893m; pmid: 25181298
29. C. S. Holling, Resilience and stability of ecological systems.
Annu. Rev. Ecol. Syst. 4,1–23 (1973). doi: 10.1146/annurev.es.
04.110173.000245
30. C. Folke et al., Resilience thinking: Integrating resilience,
adaptability and transformability. Ecol. Soc. 15, 20 (2010).
www.ecologyandsociety.org/vol15/iss4/art20
31. T. P. Hughes, S. Carpenter, J. Rockström, M. Scheffer,
B. Walker, Multiscale regime shifts and planetary boundaries.
Trends Ecol. Evol. 28, 389–395 (2013). doi: 10.1016/
j.tree.2013.05.019; pmid: 23769417
32. T. M. Lenton, H. T. P. Williams, On the origin of planetary-scale
tipping points. Trends Ecol. Evol. 28, 380–382 (2013).
doi: 10.1016/j.tree.2013.06.001; pmid: 23777818
33. Supplementary text, figures, and tables are available on
Science Online.
34. NOAA (National Oceanic and Atmospheric Administration),
NOAA-ESRL Annual CO
2
Data, accessed at: http://co2now.
org/Current-CO2/CO2-Now/annual-co2.html (2014).
35. S. E. Perkins, L. V. Alexander, J. Nairn, Increasing frequency,
intensity and duration of observed global heat waves and warm
spells. Geophys. Res. Lett. 39, L20714 (2012). doi: 10.1029/
2012GL053361
36. A. Shepherd et al., A reconciled estimate of ice-sheet mass
balance. Science 338, 1183–1189 (2012). doi: 10.1126/
science.1228102; pmid: 23197528
37. M. R. Helmus, T. J. Bland, C. K. Williams, A. R. Ives,
Phylogenetic measures of biodiversity. Am. Nat. 169, E68–E83
(2007). doi: 10.1086/511334; pmid: 17230400
38. S. D’agata et al., Human-mediated loss of phylogenetic
and functional diversity in coral reef fishes. Curr. Biol.
24,555–560 (2014). doi: 10.1016/j.cub.2014.01.049;
pmid: 24560574
39. A. D. Barnosky et al., Has the Earth’s sixth mass extinction
already arrived? Nature 471,51–57 (2011). doi: 10.1038/
nature09678; pmid: 21368823
40. N. W. Mason, F. de Bello, D. Mouillot, S. Pavoine, S. Dray, A
guide for using functional diversity indices to reveal changes
in assembly processes along ecological gradients. J. Veg.
Sci. 24,794–806 (2013). doi: 10.1111/jvs.12013
41. R. J. Scholes, R. Biggs, A biodiversity intactness index. Nature
434,45–49 (2005). doi: 10.1038/nature03289;
pmid: 15744293
42. B. Cardinale, Ecology. Impacts of biodiversity loss. Science
336, 552–553 (2012). doi: 10.1126/science.1222102;
pmid: 22556243
43. D. U. Hoope r et al., A global synthesis reveals biodiversity
loss as a major driver of ecosystem change. Nature 486,
105–108 (2012). doi: 10.1038/nature11118;pmid: 22678289
44. BAS (British Antarctic Survey), “Antarctic ozone”www.
antarctica.ac.uk/met/jds/ozone/index.html#data, J. Shanklin,
British Antarctic Survey (2013).
45. Royal Society, Ocean Acidification Due to Increasing
Atmospheric Carbon Dioxide. Policy Document 12/05 (The
Royal Society, London, 2005).
46. J. M. Guinotte, V. J. Fabry, Ocean acidification and its potential
effects on marine ecosystems. Ann. N. Y. Acad. Sci. 1134,
320–342 (2008). doi: 10.1196/annals.1439.013;pmid: 18566099
47. D. J. Conley, Terrestrial ecosystems and the global
biogeochemical silica cycle. Global Biogeochem. Cycles 16,
681–688 (2002). doi: 10.1029/2002GB001894
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