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Planetary Boundary

Control Variable(s)

Boundary

The value in brackets indicates the estimated zone of uncertainty

Current Value

Climate change

Atmospheric CO₂ concentration, ppm

Energy imbalance at top-of-atmosphere, (Watts per metre squared, Wm^-2)

350 ppm CO₂ (350-450 ppm)

Energy imbalance: +1.0 W m^-2  (+1.0-1.5 W m^-2)

396.5 ppm CO₂

2.3 W m^-2  (1.1-3.3 W m^-2)

Change in biosphere integrity

Genetic diversity: Extinction rate

Functional: diversity:  Biodiversity Intactness Index (BII)

Genetic: less than 10 extinctions per million species-years (E/MSY), (10-100 E/MSY)  

Functional: Maintain the Biodiversity Intactness Index 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

Stratospheric O₃ concentration, Dobson Units

<5% reduction from pre-industrial level of 290 Dobson Units (5%–10%), assessed by latitude

Only transgressed over Antarctica in Austral spring (~200 DU)

Ocean acidification

Carbonate ion concentration,

average global surface ocean

saturation state with respect to aragonite

(Ωarag )

≥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: (Phosphorus and Nitrogen cycles)

Phosphorus cycle:

Global:  Phosphorus flow from freshwater systems into the ocean

Regional: Phosphorus flow from fertilizers to erodible soils

Nitrogen  cycle:

Global: Industrial and intentional biological fixation of nitrogen.

Phosphorus cycle:

Global: 11 Tg P yr^-1 (11-100 Tg P yr^-1)

Regional: 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 nitrogen to the Earth System, but regional distribution of fertilizer nitrogen is critical for impacts.

~22 Tg P yr^-1

~14 Tg P yr^-1

~150 Tg N yr^-1

Land-system change

Global: area of forested land as % of original forest cover

Biome: area of 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

Global: Maximum amount of consumptive blue water use (km³yr^-1)

Basin: Blue water withdrawal as % of mean monthly river flow

Global: 4000 km³ yr^-1 (4000-6000 km³ 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 km³ yr^-1

Atmospheric aerosol loading

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

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)