Earth system boundaries
It was believed that the earth could always contain us, and that nature's strong resilience would always provide the right conditions for our survival. But now we know that this is not the case. Many of humanity's actions are putting future civilizations and all life on Earth at great risk.
A new study published in Nature has seen 51 researchers from the natural and social sciences from around the world assess the health of the planet by using models and literature reviews.
They propose a safe and just Earth system boundary for climate, biosphere (natural ecosystems, working landscapes), freshwater (groundwater, surface water), fertilizers (phosphorus, nitrogen), and air pollutants on a global and local scale. This is the first boundary assessment to quantify the harm of changes in the Earth system to humans.
Safety and justice
What is the border of security and justice? According to the study's interpretation, secure boundaries ensure the stability and resilience of the planet. A stable and resilient planet is dominated by balanced feedbacks that have the functions and abilities to support humans and all other living things.
Just borders minimize the risk of significant harm to humans. They define significant harm as the widespread, severe, and irreversible negative effects of changes in the Earth system on nations, communities, and individuals. They used three criteria to assess whether people could be protected from significant harm within safe Earth system boundaries: the first was interspecies justice and Earth system stability; The second is intergenerational justice between past and present, present and future generations; The third is intergenerational justice among nations, communities and individuals.
The results were astounding. The researchers found that of all the domains they analyzed, all but air pollution pushed the boundaries of safety and justice. This means that irreversible tipping points and widespread impacts on human well-being will be inevitable unless timely measures are taken.
This chart shows how we have crossed almost all safe and just Earth system boundaries on a global scale. (Photo: Johan Rockström et al)
What was found?
For each Earth system domain, the researchers first quantified the boundaries of safety. They argue that adherence to these safety boundaries will meet the criteria of "interspecies justice and stability of the Earth system" and will protect future generations from significant harm caused by changes in the Earth system (intergenerational justice), but it may not avoid causing significant harm to present generations, especially vulnerable groups.
As a result, they suggest that some boundaries should be set more strictly to protect current human and ecosystem purposes. Specifically, the researchers found:
1. Climate boundaries
To avoid the high risk of triggering dangerous climate tipping points, the Paris Agreement sets global warming of 1.5°C as a safe boundary. Although this boundary has not yet been reached, many people in the world have already been hit hard by climate-related disasters under the current 1.2°C warming scenario.
When warming reaches 1.5°C, hundreds of millions of people may be exposed to an average annual temperature of more than 29°C, which will exceed the human climate niche with fatal consequences. Therefore, the researchers believe that if tens of millions or more of people are to be avoided from major harm, the boundaries of justice should be set at 1°C or below.
2. Biosphere boundaries
A healthy biosphere ensures a safe and just planet by storing carbon, maintaining the global water cycle and soil quality, and protecting pollinators and many other ecosystems.
Based on modelling results for climate, water and species conservation, the researchers propose that globally, 50 to 60 percent of Earth's land needs to have essentially intact natural ecosystems. However, this security boundary has not been reached. Some recent studies suggest that the current ratio is between 45 and 50 per cent, including large tracts of land with relatively small populations, such as parts of Australia and the Amazon rainforest. And even these regions are already under pressure because of climate change and other human activities.
Locally, the researchers believe that each square kilometer of land (farms, towns, cities, or other human-dominated landscapes) needs to contain at least 20 to 25 percent of the seminatural habitat that can accommodate a largely intact natural ecosystem. This boundary has also not been reached. Currently, only one-third of the human-dominated land area meets this threshold.
3. Freshwater boundaries: Maintain the water table and keep rivers from drying up
Too much fresh water can be a problem, it can cause disasters such as floods; But too little is also a problem, and the lack of fresh water can lead to drought, which in turn will hit food production hard.
To keep the freshwater system in balance, the researchers proposed two safe Earth system boundaries, one for changes in the flow of surface water and the other for the decline of groundwater.
They recommend avoiding pumping or adding more than 20 percent of the water from a river or stream in any given month without knowing local surface water flow information. At present, 66 per cent of the world's land area fits this boundary in terms of average annual flows, although less than half of the world's population lives in these areas, indicating a significant impact on human settlements.
Groundwater is also overused. In their view, the safety boundary for annual groundwater landing in all aquifers depended on the average annual recharge, and that groundwater was considered safe if the drop was less than the recharge. Under this standard, 47% of the world's land is currently affected by over-exploitation of groundwater.
4. Boundaries of fertilizers and nutrients
When farmers overfertilize fields, rainwater washes runoff of nitrogen (N) and phosphorus (P) into rivers and oceans. These nutrients can cause eutrophication, damage ecosystems and deteriorate drinking water quality. In this study, the researchers set safe boundaries for agricultural nitrogen and phosphorus excess.
Worldwide, the use of nitrogen and phosphorus has doubled as safe and just borders. However, the researchers point out that in many poor countries, not enough fertilizer is applied in agricultural areas. Therefore, while many countries need to reduce fertilizer use, from an equity perspective, fertilizer use can be increased in some regions.
5. Aerosol contamination boundaries
For aerosols, the researchers propose a safety boundary defined by differences in aerosol optical depth between the southern and northern hemispheres. They found that if the difference in aerosol optical depth between the northern and southern hemispheres reaches 0.15, it can disturb wind patterns and monsoons. In other words, air pollution can actually disrupt weather systems.
At present, the difference in aerosol optical depth in the northern and southern hemispheres is 0.05, which has not yet reached the level that affects the weather. However, much of the world is exposed to dangerous levels of fine particulate pollution (PM 2.5), which kills an estimated 4.2 million people each year. We must reduce these pollutants to safer levels, which are less than 15 micrograms per cubic meter of air.
Take action
Setting boundaries and goals is critical, but restoring the planet to a safe and just planet will require multifaceted action, perhaps especially collective action by governments and businesses within the secure boundaries of the Earth system. In order to stop human civilization from upsetting the balance of the Earth system, we must put an end to many of the planet-destroying behaviors to ensure that the future Earth remains a planet with clean fresh water, clean air, and life abound.
Reference source:
https://theconversation.com/its-not-just-climate-weve-already-breached-most-of-the-earths-limits-a-safer-fairer-future-means-treading-lightly-206678
https://www.nature.com/articles/s41586-023-06083-8
https://earthcommission.org/news/publications/just-world-safe-planet/
Cover image & first image: futureearth.org