Humans Have Exceeded Six of the Nine Boundaries Keeping Earth Habitable

Scientists find we are “well outside the safe operating space for humanity” in a new study meant to assess the health of our planet

two trucks on rocky ground in front of lush trees
A quarry in the Cerro Blanco Forest in southern Ecuador, which is facing threats from construction and deforestation. MARCOS PIN / AFP via Getty Images

Human activities have caused the Earth to exceed six of nine boundaries necessary for keeping the planet healthy, pushing the environment “well outside the safe operating space for humanity,” scientists warn in a new study published last week in Science Advances.

According to the paper, Earth’s ability to sustain human society depends on nine primary “planetary boundaries,” or global systems that are key indicators of its health. Of these nine limits, humans have blown past six: climate change, biosphere integrity (which includes biodiversity), freshwater availability, land use, nutrient pollution and novel entities (meaning human-made pollution, such as microplastics and radioactive waste). Only the categories of ocean acidification, air pollution and ozone depletion remain within the constraints.

These boundaries are not “tipping points” that are impossible to return from. Instead, they signal humans’ impact on the environment, scientists say.

“Crossing six of the nine boundaries isn’t a guarantee of disaster,” Katherine Richardson, an Earth systems scientist at the University of Copenhagen in Denmark and lead author of the study, tells Mongabay’s Elizabeth Claire Alberts. “It’s a wake-up call; it’s like your blood pressure. If your blood pressure is [elevated], it doesn’t necessarily mean you’re going to have a heart attack. But it does tell you the risk is too high, and you try and put it down.”

The international group of scientists that conducted the research, which draws from 2,000 studies, calls it the “first scientific health check for the entire planet,” per the Guardian’s Damian Carrington. They say the “most worrying” finding is that all four of the biological boundaries, which cover living things, are already at or near the most dangerous levels.

Of note is the severity of nutrient pollution, namely the overabundance of phosphorus and nitrogen in Earth’s waters. These two elements are widely used as crop fertilizers and cause issues like algal blooms and ocean dead zones when they are released haphazardly into ecosystems.

a diagram showing each of the boundaries, with six over the limits, and biosphere integrity the farthest over the limit
A diagram from the study shows some of the factors involved in evaluating each boundary. Only three of the nine boundaries remain in the green, safe operating zone. Richardson et al. 2023

The study emphasizes that these nine factors are all deeply intertwined. In simulations, undermining one factor, like climate, undermined others, such as biodiversity. However, strengthening one helped the others, in turn. For example, cleaning up land and returning forests to late-20th-century levels would store more planet-warming carbon dioxide in plant matter, helping to address climate change.

“I think it’s a challenge to explain to people how these things interact,” Victor Galaz of the Stockholm Resilience Center, who focuses on climate governance and was not involved in the study, tells Scientific American’s Meghan Bartels. “You pull on one end, and actually you’re affecting something else. And I don’t think people really understand that.”

The planetary boundaries framework sets each threshold based on what would keep the planet’s conditions similar to how they were during the pre-industrial era—the 10,000-year period of human history during the Holocene epoch before society began burning massive amounts of fossil fuels.

Most of the actual metrics used in the study, however, are slightly higher than pre-industrial levels to reflect Earth’s inherent ability to adjust to stressors. For example, carbon dioxide levels prior to the Industrial Revolution measured 280 parts per million, but the number used as the planetary boundary is 350 parts per million. Global average carbon dioxide has clocked in at more than 419 parts per million this week.

“We know for certain that humanity can thrive under the conditions that have been here for 10,000 years,” Richardson tells the Guardian. “We don’t know that we can thrive under major, dramatic alterations, [and] humans’ impacts on the Earth system as a whole are increasing as we speak.”

Scientists first defined the planetary boundaries in 2009—when they reported that three of them had already been surpassed—and updated the framework in 2015.

This year’s latest update is the first to set numerical limits for all the boundaries, as certain metrics had previously been undefined. For example, the paper includes new details on how to evaluate “functional integrity,” or the productivity of plants, which is one of the factors in the biosphere integrity boundary. They posit that to remain at a healthy level, humans should use no more than 10 percent of plant biomass—but currently, we use about 30 percent, for food and fuel. Prior to the Industrial Revolution, that figure was at 2 percent, reports Scientific American.

Unfortunately, this poses a problem for those who hope to address climate change by repurposing large amounts of biomass for fuel. As Richardson tells Michael Le Page of New Scientist, using more plant matter for energy would result in increased deforestation and further loss of biodiversity.

However, this doesn’t mean humanity has lost all feasible solutions to combat Earth’s dire trajectory. In fact, the one improving boundary, the ozone boundary, is recovering thanks to the combined global efforts initiated by the Montreal Protocol in 1987, according to the researchers, who say that this scale of coordinated change is necessary to address the other categories.

“From a policy point of view,” Galaz tells Scientific American, “we don’t need more information to act.”

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