What are the nine planetary boundaries - and have we crossed them?

As humanity continues to expand and develop, we are increasingly pressing up against the limits of what the Earth’s ecosystems can handle. Scientists have identified a set of nine planetary boundaries that define the safe operating space for humanity. These boundaries represent thresholds that, if crossed, could lead to large-scale environmental changes and threaten the stability of the planet’s systems. By respecting these boundaries, we can avoid catastrophic environmental degradation and maintain a stable Earth system. However, some of these boundaries are already in danger of being breached.

Climate Change

Climate change is arguably the most pressing planetary boundary. It is driven primarily by the emission of greenhouse gases such as carbon dioxide (CO₂) and methane (CH₄) due to human activities, including the burning of fossil fuels, deforestation, and industrial processes. The threshold is staying below a 1.5°C to 2°C rise in global temperatures above pre-industrial levels. However, we are rapidly approaching the 1.5°C threshold, with global temperatures already about 1.2°C above pre-industrial levels. Without substantial reductions in greenhouse gas emissions, we risk crossing this boundary, which could lead to more severe climate events, sea-level rise, and disruptions to agriculture and ecosystems.

Biosphere Integrity (Biodiversity Loss)

Biodiversity loss refers to the ongoing extinction of species at rates much higher than natural levels, driven by habitat destruction, pollution, overexploitation, and climate change. The planet’s biodiversity underpins ecosystem services that are crucial for human survival, including food production, water filtration, and climate regulation. The boundary threshold is defined as extinction rates not exceeding 10 species per million species per year. Currently, extinction rates are 100 to 1,000 times higher than natural background rates, meaning this boundary has already been breached. The loss of biodiversity destabilizes ecosystems, leading to the collapse of essential services such as pollination, pest control, and soil fertility.

Land-System Change

The transformation of natural landscapes, such as forests and wetlands, into agricultural or urban areas represents land-system change. This change disrupts ecosystems and reduces biodiversity while contributing to climate change. No more than 15% of Earth’s land surface should be converted for human use. Yet, around 30-40% of global land is now used for agriculture, urbanization, and infrastructure, meaning this boundary has been significantly breached. The continued expansion of agricultural land is driving deforestation and degradation of natural habitats, particularly in tropical regions.

Biogeochemical Flows (Nitrogen and Phosphorus Cycles)

Human activities, especially the use of fertilizers in agriculture, have massively disrupted the natural cycles of nitrogen and phosphorus, two key nutrients that regulate the health of ecosystems. Excessive use leads to nutrient pollution, eutrophication, and dead zones in aquatic systems. For nitrogen, less than 62 million tons per year should be added to the environment, and for phosphorus, less than 6.2 million tons per year. Unfortunately, both boundaries have been exceeded. Nitrogen and phosphorus flows are far above sustainable limits due to overuse in industrial agriculture, contributing to algal blooms, water contamination, and the collapse of aquatic ecosystems, particularly in coastal areas.

Freshwater Use

Freshwater is essential for all life on Earth, but human activities such as agriculture, industry, and urbanization have dramatically altered the natural water cycle. Over-extraction of freshwater for irrigation and other uses leads to drying rivers, depleted aquifers, and stressed ecosystems. The boundary for global freshwater use should not exceed 4,000 km³ per year. We are nearing this threshold, with global freshwater use at around 2,600-3,000 km³ per year. Some regions, like the Middle East and parts of India, have already exceeded sustainable freshwater limits, leading to severe water stress and degradation of aquatic ecosystems.

Ocean Acidification

The oceans absorb roughly 25% of the CO₂ emitted by human activities, which leads to a chemical reaction that makes seawater more acidic. This acidification threatens marine ecosystems, particularly coral reefs, shellfish, and other species that rely on calcium carbonate for their shells and skeletons. The concentration of carbonate ions in the ocean should not fall below a certain level to maintain the health of marine ecosystems. We are approaching this boundary. Ocean acidity has increased by 30% since the industrial revolution, and continued CO₂ emissions could make the ocean hostile to marine life in the coming decades, leading to widespread ecological collapse.

Stratospheric Ozone Depletion

The ozone layer protects life on Earth from harmful ultraviolet (UV) radiation. Chemicals known as chlorofluorocarbons (CFCs), used in refrigerants and aerosols, caused significant ozone depletion in the 20th century, leading to the formation of the ozone hole over Antarctica. The concentration of ozone in the stratosphere should not fall below 220 Dobson Units. Due to international efforts like the Montreal Protocol, which phased out CFCs, the ozone layer is recovering. This boundary is now within safe limits, though it will take several more decades for full recovery.

Atmospheric Aerosol Loading

Aerosols are tiny particles in the atmosphere, often resulting from industrial pollution, vehicle emissions, and biomass burning. These particles can have complex effects on climate and human health, causing respiratory problems and influencing weather patterns. No specific global threshold has been set due to regional variability, but local impacts are significant. Aerosol pollution remains a major concern, particularly in regions with high industrial activity and vehicle emissions. While not globally breached, aerosol pollution has serious localized impacts on air quality and health, especially in densely populated areas like South Asia and China.

Novel Entities (Chemical Pollution)

Novel entities refer to the introduction of novel chemicals into the environment, including plastics, synthetic compounds, and other pollutants that ecosystems have not evolved to handle. These substances can be toxic to wildlife and humans and disrupt natural processes. While no specific threshold has been defined yet, the rapid increase in novel chemical production is a significant concern. The production of plastics, pharmaceuticals, and other synthetic chemicals has skyrocketed, with some of these substances accumulating in ecosystems in harmful concentrations. While this boundary is not fully quantified, it is increasingly seen as a significant risk.

Out of the nine planetary boundaries, four—climate change, biodiversity loss, land-system change, and biogeochemical flows—have already been breached, pushing us into dangerous territory. Others, such as freshwater use and ocean acidification, are nearing their limits. International cooperation, policy changes, and shifts toward sustainable practices are essential if we are to stay within these boundaries and avoid catastrophic environmental consequences.