“Safe” and “just” limits defined for the planet

2023-06-02 06:30:15

A new study published in the journal Nature quantifies “safe” and “fair” limits that should not be exceeded to avoid major impacts on ecosystems and humanity.

The Paris agreement aims to limit global warming to 2°C, or even 1.5°C by the end of the century compared to the pre-industrial era. But according to a study conducted by an international collective of more than forty researchers and published this May 31 in the journal Nature, global warming should be limited to 1°C to respect a safe and fair climate limit. With the world having exceeded this limit with a current warming of 1.2°C, tens of millions of people are already suffering from impacts.

The researchers point out that beyond global warming, science shows that it is necessary to preserve the other biophysical processes and systems that determine the habitability of the planet, regulate the stability and resilience of the Earth system. The collective of researchers has quantified, at the local and global level, for 5 of the 9 planetary limits, “safe” limits which “ ensure stable and resilient conditions on Earth” and “fair” limits that “minimize human exposure to significant harm”. They relate to climate change, the erosion of biodiversity, the freshwater cycle, disruption of the phosphorus and nitrogen cycle, and the increase in aerosols in the atmosphere.

Limits already exceeded

Of the eight limits ensuring the stability and good health of the Earth system, seven exceed the levels designated as “safe” and “fair”. “This means that if a rapid transformation does not occur, it is very likely that irreversible tipping points and widespread impacts on human well-being will be inevitable”explains Johan Rockström, lead author of the article.

Scientists thus estimate that to respect a safe and just limit, it would be necessary to preserve between 50 and 60% of the world’s natural ecosystem, once morest less than 50% currently. It would also be necessary that each square kilometer of ecosystem modified by Man still includes at least 20 to 25% of natural ecosystems. This limit is not respected in two thirds of the terrestrial surface dominated by the Man, note the authors.

Scientists define the safe and fair limit not to be exceeded at the local level for aerosols at 15 μg/m³ of average annual exposure to PM2.5. The study also estimates that no more than 20% of the monthly surface water flow should be modified. This limit would however be exceeded for 34% of these waters. Regarding groundwater, the limit is simple: annual withdrawals below the recharge are required. However, this limit is not respected in 47% of cases.

Finally, in order not to upset the natural cycles of nutrients, the surplus at the global level should not exceed 57 teragrams of nitrogen per year (TgN/year), once morest 119 TgN/year currently. And it would be necessary to add a maximum of between 4.5 and 9 teragrams of phosphorus per year (TgP/year), once morest 10 TgP/year currently.

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