With oceans showing dangerous levels of acidity, seven of nine crucial planetary boundaries – the limits beyond which Earth risks becoming uninhabitable for humans – have now been breached, climate researchers warn. But one of the authors of a new report tells RFI that humanity still has a chance to reverse the trend.

In this underwater photo taken on 5 April 2024, a marine biologist inspects bleached and dead coral around Lizard Island on the Great Barrier Reef, off the north-east coast of Australia.

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The world’s oceans have reached critical levels of acidity, the Potsdam Institute for Climate Impact Research (PIK) said in its annual Planetary Health Check, published this week.

It is the first time ocean acidity has crossed into the danger zone, the report warns, and latest red flag for Earth’s health.

The institute measures the state of the planet’s well-being by nine key thresholds, or “planetary boundaries” – seven of which have now been topped.

In an interview with RFI, climate scientist Levke Caesar, who co-authored the latest assessment, outlines how we might yet help the Earth’s systems recover.

RFI: What is a planetary boundary and what does it tell us?

Levke Caesar: Firstly, we have to understand that our living conditions on Earth – the climate as it is today, ecosystems, predictable seasons, etc – are not guaranteed forever. They are made possible by a climate and Earth system of unusual stability that has prevailed for 12,000 years, known as the Holocene. It is only thanks to this that agriculture, cities and human civilisations have been able to flourish.

Secondly, a healthy planet is characterised by three elements: its stability – predictability, fairly stable and narrow temperature ranges, and the fact that most places on Earth are habitable; its resilience, namely its ability to absorb external shocks – for example, the fact that forests can grow back after a fire, or that the oceans and land absorb around 50 percent of the CO2 we emit; and its habitability, enabled by vital functions such as the water cycle, breathable air, fertile soils and a protective ozone layer.

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Having these three pillars in place means that we are within what we call the “safe operating space” for humanity.

To remain within this space, we need to examine the key systems that regulate these three elements. Scientists have identified nine of them, known as planetary boundaries.

RFI: The United Nations’ Intergovernmental Panel on Climate Change uses the term “tipping points”. Is it the same thing?

LC: The concept of a tipping point is different but related.

Planetary boundaries are warning signs ahead of the cliff edge; tipping points are the edge of the cliff where systems can shift, evolve or fall irreversibly.

As long as we remain below planetary boundaries, within the safe operating space, we will most likely not cross many tipping points, hopefully none. But when we exceed one or more planetary boundaries, we may reach tipping points.

Whether breaching the boundary also means crossing the tipping points depends on the distance between the warning sign and the cliff. That’s a distance that we still need to work on determining scientifically.

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What we do know is that we are in the red zone for several tipping points… They include part of the Greenland and West Antarctic ice sheets, the system of currents in the Atlantic Ocean, coral reefs, permafrost in high latitudes, and the Amazon rainforest, through the combined pressures of climate change and deforestation.

No threshold has been crossed with complete certainty – though evidence is stacking up for West Antarctica and warning signs are multiplying for the others. This is precisely the purpose of planetary boundaries: to avoid finding ourselves on the brink of disaster.

RFI: Is breaching these boundaries reversible?

LC: Yes, the good news is that it’s not too late. We can still reverse this trend and turn back the clock.

This is what happened in the case of the ozone layer, which is one of two of the nine boundaries that are now in the green. The situation was much worse a few years ago, and we have now returned to a safe operating range.

RFI: How?

LC: Through regulation. In the 1970s and 1980s, chlorofluorocarbons and other substances caused a significant depletion of ozone in the stratosphere. Many scientists warned that this was a worrying trend and we need the ozone layer to protect us from ultraviolet rays. They put a lot of pressure on governments.

This led to the Montreal Protocol, which regulated harmful substances. Today, the hole in the ozone layer is closing.

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RFI: Is it appropriate to use the term “limits”? People might think that there’s no point in taking action once the limit has been exceeded.

LC: To be honest, I don’t know. I am a physicist by training, and that’s more of a psychological question.

In any case, the term is correct, because there are indeed limits or thresholds to Earth’s systems, and I would like people to be aware of that. Just as it is important to know that it is not too late and that we can turn back by taking action. We need to focus on that.

When it comes to the climate, for example, returning to a safe operating space requires reducing the concentration of CO2 in the atmosphere to around 350 parts per million (ppm). In 2025, we are at 423 ppm.

For biodiversity loss, we need to return to a species extinction rate of less than ten extinctions per million species per year, compared to more than 100 today.

RFI: The report shows that a seventh boundary, ocean acidification, has been exceeded, with an increase of 30 to 40 percent over the last 150 years. How did you measure this?

LC: We obtained new scientific data this year thanks to new measurement and modelling systems that combine high-resolution satellite readings with local, in situ readings…

We used this data to deduce where we believe the safe threshold lies. We can therefore say that the current acidification of the ocean has exceeded this threshold. The current level of acidity is already harming corals, crustaceans and other organisms that form the basis of marine food networks.

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RFI: What are the main factors?

LC: CO2 emissions and anything that increases the concentration of greenhouse gases, such as changes in land use change, like deforestation.

The ocean and atmosphere typically work to maintain a balance in CO2 pressure. That was the case before we began to significantly alter the Earth’s system. But we are releasing more and more carbon dioxide, which ultimately creates an imbalance.

RFI: Most of the boundaries have been crossed in the last two years. Why have there been so many breaches in such a short time?

LC: We need to differentiate between when the boundary is crossed and when we demonstrate it. In the case of acidification, we now have evidence that the threshold has been crossed, but that probably occurred in 2020…

It is only since last year that we have had the means to produce an annualised assessment. We can also say that, for each of the seven boundaries that have been crossed, there is a trend towards increasing pressure. This is obviously not good at all. It all points towards a need to rapidly correct the trajectory.

We are getting closer and closer to the high-risk zone. For the moment, the planet as a whole is holding up, but the window for returning to the green zone is getting narrower and narrower.

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RFI: Do these nine boundaries work together?

LC: They are all linked. Changing one boundary will have repercussions on one or more others, or even all of them. In both directions, in fact. If we reduce our CO2 emissions, ocean acidification will also decrease. Conversely, if, for example, we use too much fresh water for energy purposes [such as cooling a nuclear power plant], this affects one boundary while favouring another.

The same applies to carbon capture and storage: it may be good for the climate, but it can threaten the integrity of the biosphere by destroying natural spaces.

This is one of the key lessons of this report: these boundaries should not be viewed in isolation, as they are all interconnected to a greater or lesser extent.

RFI: What are your recommendations at this stage?

LC: It is crucial to think about the synergistic processes of planetary boundaries in a holistic way – by not focusing solely on climate change or land use, for example.

RFI: In that case, are UN climate change conferences (Cops) the right place to move forward?

LC: The Cops are not ideal. I am quite frustrated by the lack of results, namely the failure to adopt regulations. But we don’t have any better alternatives, so it’s better than nothing.

All the Cops [on climate, biodiversity and desertification] should be combined into one, focusing on the Earth system and its health.

This article has been adapted from the original version in French by RFI’s Géraud Bosman-Delzons.

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