Human activities have destabilized biophysical systems and processes that regulate the functioning of life support systems on Earth, pushing the planet beyond a “safe” zone, scientists have warned. According to a study, published on September 13th in the journal “Science Advances“, six of nine planetary boundaries that define a safe operating space for humanity have already been crossed. From global warming to biosphere integrity, from pollutants and plastic to nitrogen and phosphorus cycles, from freshwater to land system change: Unprecedented human disruption has thrown the Earth system out of balance. And the international team of 29 scientists warns that the pressure on all those boundary processes is increasing. “We can think of Earth as a human body, and the planetary boundaries as blood pressure,” explains lead author Katherine Richardson from the University of Copenhagen. “Over 120/80 does not indicate a certain heart attack but it does raise the risk and, therefore, we work to reduce blood pressure.” And the results of our planet’s health check don’t look good. „Science and the world at large are really concerned about the rising signs of dwindling planetary resilience, manifested by the transgression of planetary boundaries, which brings us closer to tipping points, and closes the window to have any chance of holding the 1.5°C planetary climate boundary,” said co-author Johan Rockström, Director of the Potsdam Institute for Climate Impact Research (PIK).

The study is an update to the first and second Planetary Boundaries publications released in 2009 and 2015 in which the novel entities boundary, biosphere integrity and the boundary of atmospheric aerosol loading were not yet quantified. The recent study, for the first time, provides a complete check-up of all nine processes and systems that determine the stability and resilience of the planet. “This update on planetary boundaries clearly depicts a patient that is unwell, as pressure on the planet increases and vital boundaries are being transgressed,” said Rockström. “We don’t know how long we can keep breaching these key boundaries before combined pressures lead to irreversible change and harm.” An illustration that resembles an irregular pie chart shows the current state of the nine planetary boundaries. In the middle, there is a round green area – the safe operating space for humanity. The nine processes are shown as wedges. The boundary of atmospheric aerosol loading, e. g. through desert dust and soot from combustion or wildfires, remains in the green area and the boundary is not transgressed yet, even though regional transgressions do occur, e. g., in South Asia, where regional precipitation patterns are affected in monsoon regions. This could likely lead to significantly lower rainfall, ultimately affecting biosphere integrity. Anthropogenic ocean acidification is still in the green area but it lies at the margin of the safe operating space, and the trend is worsening since CO2 emissions continue to rise. The scientists give green light with regard to stratospheric ozone depletion which is now within the safe operating space. “The boundary for ozone depletion, for example, while not transgressed globally, was headed for increasing regional transgressions. Though it still is exceeded today over Antarctica, it is now slowly recovering – thanks to global initiatives, catalyzed by the Montreal Protocol,” highlights Richardson.

In the pie chart, the length of the wedges in the nine areas symbolizes what the current state of the corresponding process is, in relation to the distance from the planetary boundary (end of the green area) and the Holocene baseline (origin of the diagram). The wedges are coloured from yellow to red according to the risks associated with each. Purple indicates the high-risk zone. Some wedges may be long but may still not be purple or at a later stage because a transgression of the planetary boundary is not yet associated with very high risks for the planet while in other cases already a “small” overshoot (short wedge) results in a big risk. The boundary of “biosphere integrity” includes the largest wedge with purple ends when it comes to the planetary boundary for changes in genetic diversity. The planetary functioning of the biosphere ultimately rests on its genetic diversity but also on its functional role in regulating the state of Earth system. „Of an estimated 8 million plant and animal species, around 1 million are threatened with extinction and over 10% of genetic diversity of plants and animals may have been lost over the past 150 years. Thus, the genetic component of the biosphere integrity boundary is markedly exceeded,” the authors write. For the second component of biosphere integrity, the functional integrity, which has so far not been quantified, a control variable was introduced. The researchers found that this boundary had in fact been crossed since the late 19th century, a time of considerable acceleration in land use globally with strong impacts on species. “Next to climate change, integrity of the biosphere is the second pillar of stability for our planet. And as with climate, we are currently destabilising this pillar by taking out too much biomass, destroying too much habitat, deforesting too much land etc.,” explains co-author Wolfgang Lucht. “Our research shows that mitigating global warming and saving a functional biosphere for the future should go hand in hand.”

The first quantification of the novel entities boundary shows that it is transgressed. Novel entities include synthetic chemicals and substances (e.g., microplastics, endocrine disruptors, and organic pollutants); anthropogenically mobilized radioactive materials, including nuclear waste and nuclear weapons; and human modification of evolution, genetically modified organisms and other direct human interventions in evolutionary processes. The scientists write that the impacts of these novel entities on Earth system as a whole remain largely unstudied. They stress that the planetary boundaries framework is only concerned with the stability and resilience of Earth system, i.e., not human or ecosystem health. For this reason, “it remains a scientific challenge to assess how much loading of novel entities Earth system tolerates before irreversibly shifting into a potentially less habitable state”. The authors write that hundreds of thousands of synthetic chemicals are now produced and released to the environment. For many substances, the potentially large and persistent effects on Earth system processes of their introduction, particularly on functional biosphere integrity, are not well known, and their use is not well regulated. They point to the fact that humanity has repeatedly been surprised by unintended consequences of this release, for example in the case of the release of insecticides such as DDT and the effect of chlorofluorocarbons (CFCs) on the ozone layer. “For this class of novel entities, then, the only truly safe operating space that can ensure maintained Holocene-like conditions is one where these entities are absent unless their potential impacts with respect to Earth system have been thoroughly evaluated,” the scientists conclude.

Biogeochemical flows reflect anthropogenic perturbation of global element cycles and this boundary was also crossed and it has the largest purple wedge indicating a high risk. Currently, the framework considers nitrogen and phosphorus as these two elements constitute fundamental building blocks of life, and their global cycles have been markedly altered through agriculture and industry, the scientists write. Land systems change is currently coloured red: This boundary focuses on the three major forest biomes that globally play the largest role in driving biogeophysical processes, that means tropical, temperate, and boreal. On the basis of 2019 land-cover classification maps derived from satellite observations, the current state of the regional biomes is similar to that in 2015 although, for most regions, the amount of deforestation has increased since the last update of the study in 2015. Land-use conversion and fires are causing rapid change in forest area and deforestation of the Amazon tropical forest has increased such that it has now transgressed the planetary boundary, the study finds. With respect to water, the warning light shines in orange. The freshwater boundary now addresses both green water (held in soil and plants in farms, forests etc.) and blue water (rivers, lakes etc) - both boundaries are transgressed. Rockström described it as „a true breakthrough” that a safe space for humanity on Earth has now been scientifically quantified, „providing a guide for action and the first full picture of our planet’s capacity to buffer stress“. He said that having this knowledge at hand marks an important step for more systematic efforts to protect, recover and rebuild planetary resilience. This is echoed by the last sentence of the study: „Scientific insight into planetary boundaries does not limit, but stimulates, humankind to innovation toward a future in which Earth system stability is fundamentally preserved and safeguarded.” (ab)