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799. Safe and Just Earth System Boundaries

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799. Safe and Just Earth System Boundaries

In 2009, Dr. Johan Rockström et al. of the Stockholm Resilience Centre proposed the concept of "planetary boundaries" (the limits within which the Earth system can maintain its own resilience) and "a safe operating space for humanity". This concept defines nine systems, including climate change, biodiversity, land-use change and biogeochemical cycles, which are closely linked to the food system. It warns that if human activities exceed the limits of each system, we may reach a "tipping point" where irreversible changes in the global environment occur rapidly. It has been some time since we entered the geological era of the Anthropocene, when human activities began to have a profound impact on the Earth's systems, but human over-exploitation and abuse of the Earth's finite resources, which is closely linked to the food system, is said to have already exceeded its limits on a global scale. The resilience of the Earth system and the human thicket are already stretched beyond their limits.

Earth system resilience and human well-being are inextricably linked. On the other hand, the extent of unsustainable exploitation and abuse of resources is highly dependent on social and economic factors, and the impact on the Earth system varies from country to country and from social stratum to social stratum. For this reason, some researchers have called for defining the limits of the Earth system in terms of human security and equity, and for assessing these limits not only at the global level but also at the regional level.

Against this background, an international group of researchers led by Dr. Rockström and colleagues published the paper Safe and just Earth system boundaries in Nature on 31st of May.

Based on the concept of planetary boundaries, the paper identified five critical Earth system boundaries: climate, biosphere, water, nutrient cycles, and aerosols; defined "safe" and "just" Earth system boundaries; and attempted to quantify the negative impacts that Earth system changes could have on humanity. In this study, "safe" is defined as a boundary that maintains the stability and resilience of the Earth system, and "just" is defined as a boundary that minimizes negative impacts (harm) on humanity. The safe and just Earth system boundaries proposed in this research will allow us to assess the health of the Earth at spatial scales ranging from local to global, and in the extreme case of biodiversity, at scales as small as one square kilometer.

Based on models, literature reviews, and professional judgment, the researchers conducted an assessment that considered factors such as tipping point risk, degradation of Earth system functions, historical deviations, and impacts on humanity. The assessment found that most systems have already passed safe and just limits.

Here’s an overview for each of the 5 critical Earth system boundaries.

Climate boundary

The global average temperature is already 1.0°C above pre-industrial temperatures, and warming beyond 1.5°C or 2.0°C increases the risk of a tipping point. Because even 1.0°C of warming will increase the number of people exposed to survival-threatening wet-bulb temperatures, and because further warming would be inconsistent with the principle of "leaving no one behind," the fair, safe, and equitable limit is set at 1.0°C. Since it will be difficult to return to this limit in the near future, it is necessary to set a more stringent limit from the perspective of equity. Therefore, adaptation and mitigation measures are needed to mitigate the negative impacts.


Biosphere boundary

In defining the safe and just limits of the biosphere, this study focused on indicators of the conservation of intact natural ecosystems at the global level and the degree of functional integration of all ecosystems, including urban and rural, at the local level. The safe limit is to maintain at least 50-60% of natural ecosystems intact, and the just limit requires the application of particularly stringent criteria. At the local level, 20-25% per square kilometer should be left undisturbed in human-centered agricultural and urban landscapes, but this criterion is currently not met in two-thirds of human-centered areas.


Freshwater boundary

To keep freshwater systems in balance, it is necessary to prevent more than 20% of water runoff in rivers and streams in any given month and to limit groundwater use to less than the rate of recharge. Currently, only 66% of terrestrial areas, home to less than half of the world's population, meet this freshwater runoff standard, and 47% of areas have groundwater use that exceeds the rate of recharge.


Fertilizer and nutrient boundary

When farmers overuse chemical fertilizers, rainwater carries nitrogen and phosphorus fertilizers into rivers and oceans, causing eutrophication that damages ecosystems and degrades drinking water quality. The study assessed agricultural nitrogen and phosphorus surpluses that minimize water pollution and greenhouse gas emissions at global and regional levels. Meanwhile, many poor regions are far from equitable in their lack of access to adequate fertilizers. Globally, nitrogen and phosphorus fertilizers and their use far exceed safe and equitable limits, but while there are regions where fertilizer use needs to be reduced, equity considerations for low-income countries with limited access to nitrogen and phosphorus fertilizers from a food security perspective should also be considered.


Aerosol pollution boundary

This study proposed to assess the safety boundary in terms of the risk that differences in aerosol concentrations between the northern and southern hemispheres could disrupt the weather system, as represented by wind and monsoon patterns. On the other hand, although aerosol concentrations have not yet reached weather-altering levels, from an equity perspective, many parts of the world are exposed to fine particle pollution (known as PM2.5), which is estimated to be responsible for 4.2 million deaths each year and requires immediate reduction.


The study found that most of the quantified safe and just Earth system boundaries have already been crossed, with the exception of aerosol concentration differences between the northern and southern hemispheres. 52% of the world's land area has crossed two or more safe and just Earth system boundaries, representing 86% of the world's population. 5% of land area and 28% of the world's population have crossed more than one boundary. Hot spots across Earth system boundaries are concentrated in densely populated areas, raising serious intergenerational equity concerns.

The safe and just Earth system boundary concept raised in the paper emphasizes the need for governments to put in place regulatory and incentive-based systems and create a policy environment for implementation, suggesting that agreements like the Paris Agreement on climate change will be needed for fresh water, air, ecosystems, and fertilizers.


Rockström, J., Gupta, J., Qin, D. et al. Safe and just Earth system boundaries. Nature (2023). https://doi.org/10.1038/s41586-023-06083-8


Contributors: Solongo TUMUR and IIYAMA Miyuki (Information Program)


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