SESD, Rockstrom 2009 - PB

What are planetary boundaries?

limits within which humanity can safely operate while maintaining Earth System stability

How many planetary boundaries are there?

9

How many planetary boundaries can currently be quantified?

7

What are the 9 planetary boundaries?

Climate change; ocean acidification; stratospheric ozone depletion; nitrogen cycle; phosphorus cycle; global freshwater use; land-system change; biodiversity loss; atmospheric aerosol loading; chemical pollution

Which planetary boundaries are hardest to quantify?

Atmospheric aerosol loading and chemical pollution

Which planetary boundaries have already been transgressed?

Climate change, biodiversity loss, nitrogen cycle

What is the Holocene?

Current interglacial period beginning about 10,000 years ago

Why is the Holocene important in PB theory?

Earth System stability during the Holocene provides the baseline for safe human development

How have humans pushed the Earth System outside Holocene stability?

Through industrialisation, fossil fuel use, land-use change, pollution and resource exploitation

What is resilience?

Ability of a system to absorb shocks, adapt and persist while maintaining function

What is meant by erosion of resilience?

Stable systems become vulnerable to abrupt, non-linear changes when thresholds are crossed

What are thresholds in PB theory?

Critical tipping points causing abrupt non-linear transitions in coupled human-environment systems

What is the difference between a threshold and a planetary boundary?

Threshold = tipping point; boundary = precautionary limit set before the threshold

Why are planetary boundaries set before thresholds?

To provide a safety buffer and reduce risk of abrupt Earth System change

What influences the placement of planetary boundaries?

Scientific evidence and normative judgements about acceptable risk

Why is uncertainty important in PB theory?

Scientific uncertainty means boundaries are placed conservatively at the lowest point of uncertainty

What does the PB approach focus on?

Biophysical Earth System processes that regulate planetary stability

Why are PBs important for resilience?

Crossing thresholds could undermine regional and global social-ecological resilience

What are the major biogeochemical cycles included in PBs?

Nitrogen, phosphorus, carbon and water cycles

What is meant by top-down global processes?

Processes operating across multiple regions simultaneously

What is meant by bottom-up global processes?

Global changes emerging from many local interactions

What is the ethical time horizon in PB theory?

A timeframe balancing present-day decisions with long-term sustainability for future generations

What is the climate change planetary boundary?

Limiting global warming to no more than 2°C above pre-industrial levels

What control variables are used for climate change?

Atmospheric CO₂ concentration and radiative forcing

Why is crossing climate thresholds dangerous?

Could disrupt regional climates, thermohaline circulation and cause major sea-level rise

What is thermohaline circulation?

Global ocean circulation driven by temperature and salinity differences

What is eustatic sea-level rise?

Global sea-level rise caused mainly by melting ice and thermal expansion

What geographical shift is associated with climate change?

Poleward shift of subtropical regions by ~4° latitude

What is ocean acidification?

Reduction in ocean pH caused by absorption of atmospheric CO₂

Why are oceans important in the carbon cycle?

They act as a major carbon sink through CO₂ dissolution and biological uptake

How much has ocean pH decreased since pre-industrial times?

About 0.1 pH units

What does a 0.1 pH decrease represent chemically?

About 30% increase in hydrogen ion concentration

What forms of calcium carbonate do marine organisms use?

Aragonite and calcite

How does acidification affect marine organisms?

Reduces calcium carbonate saturation, weakening shells and skeletons

What is aragonite undersaturation?

Seawater becomes corrosive to aragonite shells

Why is ocean acidification especially dangerous?

It is occurring rapidly, giving organisms little time to adapt

Which ecosystems are especially threatened by acidification?

Coral reefs

What is the ocean acidification boundary?

Maintain oceanic aragonite saturation at ≥80% of pre-industrial levels

What is the role of stratospheric ozone?

Filters harmful ultraviolet radiation

What caused the Antarctic ozone hole?

Chlorofluorocarbons (CFCs) and polar stratospheric clouds

How do CFCs destroy ozone?

Sunlight releases chlorine radicals that break down ozone into oxygen

How does global warming affect stratospheric ozone?

Cools the stratosphere and promotes polar stratospheric cloud formation

Why is the ozone boundary difficult to define globally?

No clear threshold exists for extra-polar ozone depletion

What international agreement helped reduce ozone depletion?

Montreal Protocol

How do humans alter the nitrogen cycle?

Industrial ammonia fixation, agriculture, fossil fuel combustion and biomass burning

What is eutrophication?

Nutrient enrichment causing excessive algal growth and oxygen depletion

What threshold shift can eutrophication cause?

Clear oligotrophic waters become turbid eutrophic waters

What is reactive nitrogen?

Biologically available nitrogen compounds produced from atmospheric N₂

Why is nitrous oxide important?

It is a fast-acting greenhouse gas

Why is phosphorus different from nitrogen?

Phosphorus is a finite fossil mineral resource

How does phosphorus enter human systems?

Geological weathering and fertiliser use

What are oceanic anoxic events (OAEs)?

Periods when oceans become depleted in oxygen

How much higher is anthropogenic phosphorus input than natural background levels?

8–9 times higher

What is the sixth mass extinction?

Current period of accelerated biodiversity loss caused by humans

Why is biodiversity important for resilience?

Different species provide different functional responses to environmental change

What is currently greater: extinction or speciation?

Extinction

What percentage of assessed species are threatened with extinction?

About 25%

How has the geography of extinctions changed recently?

More extinctions are occurring on continents rather than islands

Why is biodiversity linked to other PBs?

It supports ecosystem functioning and resilience across Earth systems

What is global freshwater use concerned with?

Human alteration of river flow, runoff and atmospheric moisture cycles

What is green water?

Soil moisture and evapotranspiration flows

What is blue water?

Rivers, lakes and groundwater

How does land degradation affect freshwater systems?

Reduces green water flows and moisture recycling

How are green and blue water connected?

Green water influences blue water availability

Why is freshwater use a planetary concern?

Human demand may already exceed the safe operating space

What drives land-system change?

Agricultural expansion and intensification

What is the land-system boundary?

No more than 15% of ice-free land converted to cropland

How much land is currently under cultivation?

About 12%

Why should cropland be concentrated in productive regions?

To reduce degradation and pressure on marginal land

How does land-system change affect other boundaries?

Influences biodiversity, climate and water cycles

What are aerosols?

Organic or inorganic particles suspended in the atmosphere

What are primary aerosols?

Aerosols directly emitted into the atmosphere, e.g. dust or diesel particles

What are secondary aerosols?

Aerosols formed through atmospheric chemical reactions

How do aerosols affect climate?

Scatter solar radiation and modify cloud reflectivity and persistence

How do aerosols influence the hydrological cycle?

Alter cloud formation and precipitation patterns

What is the Elevated Heat Pump effect?

Aerosols over northern India heat the atmosphere aloft and strengthen monsoon circulation

How do aerosols shift Indian monsoon rainfall?

Warm air aloft and cooler surface temperatures shift rainfall toward the Himalayas

What health impacts do aerosols have?

Fine particulate pollution causes cardiopulmonary disease mortality

What pollutants are included in aerosol loading?

Particulates, tropospheric ozone, sulphur oxides and nitrogen oxides

What is chemical pollution?

Release of harmful chemicals such as heavy metals, radioactive compounds and persistent organics

How can chemical pollutants affect ecosystems?

Through contamination, bioaccumulation and biomagnification

Why is chemical pollution difficult to regulate as a PB?

There are 80,000–100,000 chemicals with interacting effects

What body systems are harmed by chemical pollution?

Immune, endocrine and nervous systems

What neurological conditions are associated with chemical exposure?

ADHD and ASD

Why may chemical pollution require multiple sub-boundaries?

Different chemicals have different thresholds and impacts

How are chemical pollutants transported globally?

Often attached to aerosols

Why are planetary boundaries interdependent?

Crossing one boundary can reduce the safe space for others

How can freshwater and land-use boundaries interact?

Water stress can intensify land-use pressures and vice versa

How does Amazon deforestation affect atmospheric processes?

Alters convection, precipitation and aerosol emissions

How does water availability affect aerosols from vegetation?

Influences both the amount and type of aerosols emitted

Why is the Amazon aerosol-water interaction a feedback loop?

Water affects aerosols, aerosols affect climate and precipitation, which then affects water availability again

How can Amazon feedbacks affect Tibet?

Through atmospheric circulation changes influencing regional temperatures

How does Amazon deforestation weaken convection?

Reduced evapotranspiration lowers atmospheric moisture and latent heat release

Why can warm but dry air weaken convection?

Rising air becomes shallower and less buoyant without moisture

What does the Amazon contribute to the atmosphere?

Large amounts of heat and moisture

What is the ITCZ?

Region where trade winds converge, causing rising air and heavy rainfall

Why is the ITCZ sensitive to Amazon deforestation?

It depends on regions of strongest heating and convection

How can Amazon deforestation shift the ITCZ?

Weakening Amazon convection may shift the ITCZ northward

How can ITCZ shifts affect the jet stream?

Changing tropical heating alters atmospheric circulation patterns

Why do Tibet climate changes matter?

They affect Asian water resources