Pushing the boundaries: Warning of a Planet in Peril

The planet has exceeded six of the nine scientifically established safety limits and has entered the “danger zone”. This is confirmed by a study published in Science Advances at the end of 2023, which suggests that the Earth is outside safe operating space for humanity.

Based on a framework established in 2009, which outlined nine critical thresholds for keeping the Earth’s environment similar to the Holocene period, which began with the end of the last ice age and during which agriculture and modern civilisations evolved, characterised by relatively stable and warm planetary conditions, with this study the researchers warn us of the consequences of having crossed this threshold.

At stake are the limits beyond which major Earth systems such as climate, water and biodiversity can no longer be maintained in the safe and stable conditions that have been observed over the last 10,000 years.

What are the 9 limits of the planet?

The planetary boundaries framework draws on the latest scientific understanding of the functioning of the Earth system to identify a “safe operating space” for humanity, proposing limits beyond which human activities may trigger irreversible changes in the conditions on Earth that enable human life. It identifies nine processes that are critical to maintaining the stability and resilience of the earth system as a whole: biosphere integrity; climate change; land-use change; biogeochemical fluxes; freshwater use changes; new entities; ocean acidification; atmospheric aerosol loading; stratospheric ozone depletion.

Today, human activities such as changes in land use, the alteration of water quantity in rivers and soils, the introduction of synthetic chemicals into the environment and the emission of greenhouse gases into the atmosphere have impacted the interactions of the earth system, causing major disruption of these boundaries.

One of the four limits we have already crossed, climate change, is perhaps the best known of all. The planet has already reached and exceeded the 1.5°C warming limit set by the Paris Agreement. Climate change also causes extreme weather events, such as hurricanes, tsunamis, intense storms, floods and prolonged droughts, which are becoming more frequent and intense.

With regard to the integrity of the biosphere, we have already passed the zone of increasing risk and are now directly in the high-risk zone, which has led to the loss of natural habitats, the extinction of species, affecting biodiversity and ecosystems, causing irreversible environmental changes on a large scale.

Land use change arises from a multitude of human activities. Increased global use of biomass as an alternative to coal, oil and gas, conversion of land use for various purposes and fires are causing rapid changes in forest area, which is leading to its global decline. In addition, deforestation of the Amazon rainforest has increased to such an extent that we have exceeded the planet’s limits in this area.

The fourth frontier that has already been crossed is that of biogeochemical flows. This refers to the introduction of synthetic chemicals into the environment, such as excess phosphorus and nitrogen used in agricultural fertilisers, which have led to widespread eutrophication of freshwater ecosystems, causing a global transgression of this boundary.

Previous studies of planetary boundaries to understand changes in freshwater use only looked at what scientists call “blue water”, which includes surface water and groundwater. In the study published in Science Advances, the researchers also consider “green water”, which does not enter aquifers but is accessible to plants. Unlike previous assessments of planetary boundaries, this new approach indicates a substantial transgression of the limits of freshwater change.

Hundreds of thousands of synthetic chemicals are currently produced and released into the environment. Hundreds of thousands of synthetic chemicals are currently produced and released into the environment. The boundary of “new entities” represents chemical and synthetic substances, e.g. microplastics, endocrine disruptors and organic pollutants, radioactive materials including waste and nuclear weapons, and human modification of evolution, genetically modified organisms and other direct human interventions in evolutionary processes. For many substances, the potentially large and persistent effects of their introduction on earth system processes, especially on the functional integrity of the biosphere, are not well understood and their use is not well regulated. With such a high percentage of untested chemicals being released into the environment, it follows that the safe operating space is currently exceeded.

Of these nine safe limits for the planet, only three have not yet been exceeded: ocean acidification, atmospheric aerosol loading and stratospheric ozone depletion.

Ocean acidification is currently outside the safe operating space. This phenomenon, which worsens as CO2 emissions increase, occurs when, through a set of chemical reactions, seawater absorbs CO2, which lowers the pH of its waters, changing their chemical composition and seriously affecting biodiversity and marine ecosystems.

Quantifying the planetary boundaries of aerosol loading is made difficult by their multiple natural and man-made sources, differences in chemical composition, seasonality, and atmospheric lifetime, and consequently the large spatial and temporal heterogeneity in the distribution and climatic and ecological impacts of aerosols.

The destruction of stratospheric ozone is the only limit of the nine where mankind has successfully acted on the warning signs. Thanks to the Montreal Protocol, an international agreement to protect the stratospheric ozone layer, which regulates the production and consumption of almost 100 man-made chemicals, we have gradually reduced the consumption and production of the various substances that destroy the ozone layer.

Thus, we realise that preserving interactions in the Earth system is fundamental to ensure that human activities do not trigger drastic changes in the state of the Earth, threatening the future of modern civilisations. The great challenge is to find solutions to current problems and alternatives to build an environmentally responsible planet. The future is in our hands.

Supporting documents: