Until when can we exceed the planetary limits?

In recent months, several scientific papers have drawn attention to the fact that new planetary boundaries have been crossed (here, or leaves). Many media echoed this worrying information.

But what do these famous planetary limits mean? How to interpret these overruns? And finally, do we really have to worry about it?

The Earth system has been functioning for 11 years under the Holocene regime

To answer these questions, remember that the researchers who study planetary boundaries come from a disciplinary field called Earth system sciences.

These apprehend the planet as an entity that involves complex interactions between the atmosphere, the lithosphere, the hydrosphere and the biosphere (living things). And like any system, the Earth is endowed with adaptive capacities that make it possible to maintain a state of dynamic equilibrium between these elements – we speak of a “regime” to designate this state of relative stability.

But sometimes this balance is broken, to the point that the Earth system begins to function very differently.

For example, the Quaternary era (which began about 2,6 million years ago) is marked by regular changes in the climatic regime. Because of variations in the position of the Earth relative to the Sun, the climate of our planet regularly changes from a glacial regime (which can last up to 100 years) to an interglacial regime (generally shorter).

We have thus lived for more than 10 years in an Earth system regime that geologists call the Holocene.

The Holocene regime proved to be particularly favorable to the development of the human species. The good news is that this regime is expected to last for another 10+ years. The bad news is that we are threatening the balance of this regime. In other words, we are on the verge of crossing a tipping point.

Exceeding a planetary boundary is crossing a tipping point that takes us out of the Holocene

The scientific literature on planetary boundaries is largely based on this tipping point concept. What is it about ?

In a regime such as that of the Holocene, the terrestrial ecosystem is endowed with regulatory capacities that allow it to withstand disturbances – what are called “negative feedbacks”. For example, if CO emissions₂ increase abnormally, the oceans will sequester part of this CO₂ and thus limit climatic disturbances.

Unfortunately, it happens that these shock absorbers end up yielding, like a rubber band on which one would have pulled too much. It is then “positive feedback” that will be put into action.

For example, as it warms, permafrost will release large quantities of methane into the atmosphere, which will increase the greenhouse effect and therefore global warming.

Once they are triggered, these phenomena will amplify and accelerate the upheaval, to the point of making any return to normal impossible. The change of regime then becomes inevitable: the climate will find a new point of equilibrium, characterized by a greenhouse effect and a temperature much greater than those of the Holocene.

Some scientists refer to the scenario of a "hothouse" climate regime, which would have cataclysmic effects on all the variables of the Earth system.

But beware: Crossing a planetary boundary is not the same as crossing a limit!

However, scientists are faced with a major problem: it is extremely difficult to determine with precision the moment when a tipping point occurs.

Aware of the dangers of exceeding such a limit, scientists invite decision-makers to avoid crossing the lower limit of uncertainty. It is this lower limit that they propose to call "planetary boundary" (planetary boundary).

To better understand the difference between limit and border, let's imagine the case of a frozen lake whose ice thickness would gradually thin as one moves away from its shore. Even knowing the thickness of the ice at several points, it is very difficult to determine how far the ice will break under the weight of a person. At most, we can say that beyond five meters, for example, the risk appears. It is this value that is equivalent to a “boundary”.

In terms of climate, models show that below a concentration of 350 ppm of CO₂ in the atmosphere, the Holocene regime is not threatened. Above 500 ppm, on the other hand, the climatic shift is almost certain. The planetary boundary is somewhere between these two poles.

However, today we have crossed the 420 ppm bar : we have therefore crossed the planetary climate frontier.

But have we crossed the tipping point? It remains a mystery. The only thing we know for sure is that we are playing with fire. A bit like a person who has decided to move forward on a frozen lake beyond the safety zone mentioned above...

Beyond the climate, several planetary boundaries have already been crossed

This observation is all the more worrying since the climate is not the only element of the Earth system suffering serious damage.

Biodiversity is dangerously threatened, even though it determines the resilience of the biosphere. The biogeochemical cycles of nitrogen and phosphorus have been profoundly disturbed by intensive agriculture, to the point of causing vast dead zones within the oceans. Deforestation has generated imbalances in the water and climate cycles which today take on a global dimension.

More recently, the impact of chemical pollutants which has been singled out, or even the worrying drop in water content in the soil.

Of nine planetary variables monitored today, five are the subject of a documented border crossing – and even six, if we take into account the latest published study.

This does not mean that the worst is certain. But the multiplication of these alarms must clearly challenge us: we are on the point of provoking an exit from the Holocene, the consequences of which would be cataclysmic.

The transition must not only be climatic, but ecosystemic

What lessons can be drawn from this for human societies?

First, it must be understood that the climate is central to maintaining planetary balances and that it is urgent to stop anthropogenic emissions of greenhouse gases.

Then, we must integrate the plural dimension of the problem. Because despite its importance, the resolution of climate change must not take place to the detriment of other planetary variables. For example, increasing the use of biomass or clouding the atmosphere to limit solar radiation could have catastrophic effects on other fundamental variables of the Earth system.

Finally, it is undoubtedly necessary to favor solutions that attack the root of the problem, by ceasing to imagine that we will be able to go beyond planetary limits thanks to technology alone.

Respecting these limits presupposes just as many economic, social, cultural, political or even geopolitical innovations. In other words, it is undoubtedly a question of going beyond another limit: that of our imagination.

Aurelien Boutaud, Doctor of environmental science and engineering, Mines Saint-Etienne - Institut Mines-Télécom et Natacha Gondran, Teacher-researcher in environmental assessment, Mines Saint-Etienne - Institut Mines-Télécom

This article is republished from The Conversation under Creative Commons license. Read theoriginal article.