Human activity is breaching most of the boundaries that will keep the planet "stable and resilient", according to a landmark report.
Published in Nature today, the study quantified "planetary boundaries" for aerosol pollution, fresh water, nutrient cycles, biodiversity, land use and climate, beyond which the planet is longer "safe and just" for humans.
The report by the Earth Commission sets "hard limits" for the six categories at local and global scales, according to lead author Johan Rockstrom from the Potsdam Institute for Climate Impact Research.
"Science is showing clearly that we are at risk of destabilising the entire planet and its life support systems," Professor Rockstrom said.
An earlier iteration of the report from 2009 focused on the stability and resilience of the planet (safe boundary). The latest report also considers human safety and justice (just boundary), according to Earth Commission co-chair Joyeeta Gupta from the University of Amsterdam.
"These boundaries can be seen as health indicators for people and the planet."
Let's take a look at each of the planetary boundaries to see how we are faring.
Aerosols
There is evidence that differences in atmospheric aerosol load between the Northern and Southern Hemispheres can trigger "regional-scale tipping points", where rainfall patterns are substantially changed.
Observational studies suggest that increasing reflective aerosols like sulphates and nitrates in the Northern Hemisphere, lowers the rainfall in that hemisphere's tropical monsoon region.
Conversely, rainfall is increased in the Southern Hemisphere's tropics.
The researchers have considered the impact, and potential impact, of aerosols from natural, and human sources, including solar geoengineering — where aerosols may be injected into the stratosphere to influence weather or climate.
According to their assessment, differences in aerosol levels between the hemispheres aren't impacting rainfall at this point and we are still within the safe boundary.
But aerosols such as bushfire smoke and pollution also cause asthma, health complications and premature death.
Given the adverse health risks posed by aerosols, a "just boundary" is defined as exposure to air with an average concentration of 15 micrograms-per-cubic-metre of particulate-matter-2.5, over 12 months.
But the takeaway is that 85 per cent of the world's population is currently exposed to higher concentrations of aerosols than the defined boundary. That's estimated to cause around 4.2 million deaths each year.
Aerosols can be reduced by limiting fossil fuels, deforestation, and overgrazing.
(Image: Reuters/Truth Leem)
Phosphorus
There are at least two big dangers of excessive fertiliser use: eutrophication from using too much, and running out.
Phosphorus is essential to life, and intensive agriculture depletes the soil of phosphorus, which then needs to be replenished with fertiliser.
But the phosphorus we use in inorganic fertiliser is a finite resource mined mostly from phosphate rock.
There are some indications that we've already reached peak supply — after which production will decline.
Declining phosphorus supply may have serious implications for feeding people, said Charles Warren, who researches plant physiology and soil biogeochemistry at the University of Sydney.
"As the availability of phosphorus goes down, of course the price will go up, and that becomes an issue of equity, because it'll be the poorer nations that will suffer the most," said Dr Warren, who was not involved in the research.
"The areas of the globe with the most phosphorus-deficient soils are often those poorer nations. It's the tropics and the subtropics.
"In terms of eutrophication, we are exceeding the planetary boundary for excess phosphorus use as a fertiliser, though not to the same extent as nitrogen."
(Image: ABC Rural/Rob Napier)
Nitrogen
Nitrogen is easier to replenish than phosphorus, however the nitrogen used in fertiliser is made from fossil fuels. In the soil it is partially converted to nitrous oxide — a potent greenhouse gas.
Too much nitrogen fertilisation also causes eutrophication, where runoff into waterways leads to algal blooms and oxygen depletion.
These can cause fisheries and ecosystem collapse, and drinking water contamination.
On local scales, there are some instances where there is room for more nitrogen use, not less.
But on a global scale, we're using far in excess of the safe (and just) limit.
While there is room for some excess use without causing eutrophication, we're breaching that by more than 60 million tonnes annually, worldwide.
(Image: Getty Images/Li Ziheng)
Groundwater
"Unsafe" levels of groundwater extraction happen where the rate of water being harvested is higher than the rate an aquifer is recharged.
This can cause reduced stream and lake levels, wetland loss, biodiversity loss, reduced water quality, land subsidence, and also intergenerational water shortages.
For instance, research from 2021 found that Iran was facing "a state of water bankruptcy". Having doubled its rate of groundwater extraction in a few years, the country has widespread aquifer depletion and salinisation.
The authors of that paper said their findings pointed to "dire, worsening water security risks across the country".
Groundwater is a major source of income and drinking water for many countries.
In the United States, around half the population relies on groundwater for drinking, and in many parts of Australia, groundwater is the primary water source for agriculture.
Aquifers are replenished at different rates depending on location, but some may take tens of thousands of years, and so rates of extraction need to be regionally specific.
Water levels in 47 per cent of groundwater basins globally are declining, according to today's report.
Where depleted aquifers have "already caused significant environmental impacts", the authors recommend the urgent reduction of extraction and protection of catchments.
(Image: ABC Open/haworthdavid)
Surface water
Research shows that altering river flow has significant impacts on biodiversity and ecosystem function, according to report co-author Stuart Bunn from Griffith University's Australian Rivers Institute.
"Our definition of safe is to actually protect freshwater ecosystems not just for their biodiversity but also acknowledging the essential services that freshwater ecosystems provide," Professor Bunn said.
"Once we start altering flow regimes by more than 20 per cent we start to see significant impacts to freshwater systems and those services."
According to today's report, 34 per cent of global land area is exceeding this flow regime threshold. In other words, we're breaching this boundary for some, but not the majority of land surface.
However, more than half the world's population relies on waterways with altered flows beyond the safe boundary.
(Image: ABC Goldfields-Esperance/Andrew Chounding)
Intact ecosystems
Ecosystem services are the invaluable functions of the natural world that make the planet habitable.
Oxygen, biodiversity, cooling, water filtration, storm and flood buffering, medicine, and food are all products of ecosystem services.
It is possible for ecosystems to remain "intact" and provide these services even if they are modified by humans.
The authors have determined that 50 to 60 per cent of the Earth's surface needs to be intact.
Currently we're at about 45 –50 per cent, said study co-author David Obura of marine conservation organisation CORDIO East Africa.
"So we're just below or at that natural ecosystem area boundary."
(Image: ABC Foreign Correspondent/Natalie Whiting)
Modified ecosystems
Given the huge area of the Earth's surface we occupy, modified ecosystems are still necessary for the overall health of the planet.
Having access to nature within modified systems is also considered important for human physical and mental health.
The report recommends that all modified habitat needs to be interspersed with around 25 per cent natural vegetation, CORDIO's Mr Obura said.
"It has to be high-function to provide services to people," he said.
"What we've determined is that about 50 per cent of [current] land area meets this earth system boundary, and half is below that boundary."
(Image: ABC Central West/Hugh Hogan)
1C warming
One degree Celsius of warming is considered the "just boundary".
But we are now at around 1.2C of warming on average above pre-industrial levels.
Beyond 1C we're already experiencing impacts from more extreme weather events, which disproportionately impact poorer regions, according to Professor Rockstrom.
"[This is] scientific evidence of the need for collective action on decarbonising the world economy as fast as we can."
Professor Gupta said a 1C target was significantly more "just" than 1.5C.
"What we are trying to show through our paper is that even at 1C, there is a huge amount of damage taking place where tens of millions of people are exposed, for example, to temperature extremes.
"This will only get worse and worse as we move into 1.5 and 2C."
(Image: Getty Images/visionandimagination.com)
1.5C warming
The boundary for safe warming is set at 1.5C, and is not yet breached.
However, the World Meteorological Organisation now predicts that there is a 66 per cent chance the world will have at least one year above 1.5C average warming within the next five years.
Warming of 1.5C poses significant risks and greatly increases the likelihood of hitting tipping points that lead to irreversible change on a global scale, Professor Rockstrom said.
"Five of the 16 known climate tipping [points] are likely to be transgressed at 1.5C."
These include the collapse of the Greenland ice cap, rapid melting of permafrost, and collapse of a climate-altering current in the north Atlantic.
"That's a safety limit which in itself is not well understood in the world, that 1.5C is a dangerous point."
(Image: Getty Images/Stone)
