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RESEARCH ( RESEARCHARTICLE
<br />Climate change
<br />Biosphere Genetic
<br />integrity diversity Novel entities
<br />Functional
<br />diversity
<br />Land - system Stratospheric
<br />change ozone depletion
<br />Atmospheric aerosol
<br />Freshwater loading
<br />use fto
<br />Phosphorus 10117110MINI/
<br />. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
<br />Nitrogen Ocean acidification ® Beyond zone of uncertainty (high risk)
<br />O In zone of uncertainty (increasing risk)
<br />Biogeochemical flows 0 Below boundary (safe)
<br />0 Boundary not yet quantified
<br />Fig. 3. The current status of the control variables for seven of the nine planetary boundaries. The
<br />green zone is the safe operating space (below the boundary), yellow represents the zone of uncertainty
<br />(increasing risk), and red is the high -risk zone. The planetary boundary itself lies at the inner heavy circle.
<br />The control variables have been normalized for the zone of uncertainty (between the two heavy circles);
<br />the center of the figure therefore does not represent values of 0 for the control variables. The control
<br />variable shown for climate change is atmospheric CO2 concentration. Processes for which global -level
<br />boundaries cannot yet be quantified are represented by gray wedges; these are atmospheric aerosol
<br />loading, novel entities, and the functional role of biosphere integrity. Modified from (1).
<br />million years -about 1 per million species -years
<br />(39) -and add a large uncertainty bound, raising
<br />the boundary to 10 per million species - years. The
<br />risk is that, although the Earth system can tol-
<br />erate a higher -than- background level of extinc-
<br />tions for a time, we do not know what levels of, or
<br />types of, biodiversity loss may possibly trigger non-
<br />linear or irreversible changes to the Earth system.
<br />The second control variable aims to capture the
<br />role of the biosphere in Earth - system functioning
<br />and measures loss of biodiversity components at
<br />both global and biome /large ecosystem levels. Al-
<br />though several variables have been developed at
<br />local scales for measuring functional diversity
<br />[e.g., (40)], finding an appropriate control varia-
<br />ble at regional or global levels is challenging. For
<br />the present, we propose an interim control var-
<br />iable, the Biodiversity Intactness Index (BII) (4I).
<br />BII assesses change in population abundance as a
<br />result of human impacts, such as land or resource
<br />use, across a wide range of taxa and functional
<br />groups at a biome or ecosystem level using pre-
<br />industrial era abundance as a reference point. The
<br />index typically ranges from 100% (abundances
<br />across all functional groups at preindustrial levels)
<br />to lower values that reflect the extent and degree
<br />of human modification of populations of plants
<br />and animals. BII values for particular functional
<br />groups can go above 1000/. if human modifications
<br />to ecosystems lead to increases in the abundance
<br />of those species.
<br />Due to a lack of evidence on the relationship
<br />between BII and Earth - system responses, we pro-
<br />pose a preliminary boundary at 90% of the BII
<br />but with a very large uncertainty range (90 to
<br />30 %) that reflects the large gaps in our knowl-
<br />edge about the BII - Earth - system functioning
<br />relationship (42, 43). BII has been so far applied
<br />to southern Africa's terrestrial biomes only (see
<br />fig. S3 for an estimation of aggregated human
<br />pressures on the terrestrial biosphere globally),
<br />where the index (not yet disaggregated to func-
<br />tional groups) was estimated to be 84 %. BII
<br />ranged from 69 to 91% for the seven countries
<br />where it has been applied (41). Observations across
<br />these countries suggest that decreases in BII ad-
<br />equately capture increasing levels of ecosystem
<br />degradation, defined as land uses that do not al-
<br />ter the land -cover type but lead to a persistent
<br />loss in ecosystem productivity (41).
<br />In addition to further work on functional mea-
<br />sures such as BII, in the longer term the concept
<br />of biome integrity -the functioning and persist-
<br />ence of biomes at broad scales (7)-offers a prom-
<br />ising approach and, with further research, could
<br />provide a set of operational control variables (one
<br />per biome) that is appropriate, robust, and scien-
<br />tifically based.
<br />Stratospheric ozone depletion
<br />We retain the original control variable 103 con-
<br />centration in DU (Dobson units)] and boundary
<br />(275 DU). This boundary is only transgressed
<br />over Antarctica in the austral spring, when 03
<br />concentration drops to about 200 DU (44). How-
<br />ever, the minimum 03 concentration has been
<br />steady for about 15 years and is expected to rise
<br />over the coming decades as the ozone hole is
<br />repaired after the phasing out of ozone - depleting
<br />substances. This is an example in which, after a
<br />boundary has been transgressed regionally, hu-
<br />manity has taken effective action to return the
<br />process back to within the boundary.
<br />Ocean acidification
<br />This boundary is intimately linked with one of
<br />the control variables, CO2, for the climate change
<br />PB. The concentration of free H` ions in the sur-
<br />face ocean has increased by about 30% over the
<br />past 200 years due to the increase in atmospheric
<br />CO2 (45). This, in turn, influences carbonate chem-
<br />istry in surface ocean waters. Specifically, it lowers
<br />the saturation state of aragonite (52_,g), a form of
<br />calcium carbonate formed by many marine orga-
<br />nisms. At Q_n < 1, aragonite will dissolve. No
<br />new evidence has emerged to suggest that the
<br />originally proposed boundary ( >t80% of the pre-
<br />industrial average annual global Q_,g) should
<br />be adjusted, although geographical heterogeneity
<br />in S2_a is important in monitoring the state of
<br />the boundary around the world's oceans (fig. S4).
<br />Currently, Q-,g is approximately equal to 84% of
<br />the preindustrial value (46). This boundary would
<br />not be transgressed if the climate - change bound-
<br />ary of 350 ppm CO2 were to be respected.
<br />Biogeochemical flows
<br />The original boundary was formulated for phos-
<br />phorus (P) and nitrogen (N) only, but we now
<br />propose a more generic PB to encompass human
<br />influence on biogeochemical flows in general. Al-
<br />though the carbon cycle is covered in the climate -
<br />change boundary, other elements, such as silicon
<br />(47 48), are also important for Earth- system func-
<br />tioning. Furthermore, there is increasing evidence
<br />that ratios between elements in the environment
<br />may have impacts on biodiversity on land and in
<br />the sea (49 -51). Thus, we may ultimately need to
<br />develop PBs for other elements and their ratios,
<br />although for now we focus on P and N only.
<br />A two -level approach is now proposed for the
<br />P component of the biogeochemical flows bound-
<br />ary (see also the supplementary materials). The
<br />original global -level boundary, based on the pre-
<br />vention of a large -scale ocean anoxic event, is
<br />retained, with the proposed boundary set at a
<br />sustained flow of 11 Tg P year' from freshwater
<br />systems into the ocean. Based on the analysis of
<br />Carpenter and Bennett (3), we now propose an
<br />additional regional -level P boundary, designed
<br />to avert widespread eutrophication of freshwater
<br />systems, at a flow of 6.2 Tg P year' from fer-
<br />tilizers (mined P) to erodible soils.
<br />Given that the addition of P to regional
<br />watersheds is almost entirely from fertilizers, the
<br />regional -level boundary applies primarily to the
<br />world's croplands. The current global rate of ap-
<br />plication of P in fertilizers to croplands is 14.2 Tg
<br />P year' (5-9, 53). Observations point toward a few
<br />agricultural regions of very high P application
<br />rates as the main contributors to the transgres-
<br />sion of this boundary (Fig. 2 and fig. S5A) and
<br />suggest that a redistribution of P from areas
<br />1259855 -6 13 FEBRUARY 2015 • VOL 347 ISSUE 6223 sciencemag.org SCIENCE
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