even a temporary exceedance of 2°C would cause permanent damage to terrestrial species

The history of climate change is one of people slowly coming to terms with the truth. No one, except a small minority, still doubts whether it is real and man-made. Now most are grappling with the reality of trying to slow catastrophic warming, and the difference between solutions and false hopes. The concept of climate transgression is the next thing we need to get to grips with.

If urgent action is not taken, emissions are expected to cause the planet to continue to warm rapidly in the coming decades, pushing global average temperatures beyond the target of the Paris Agreement of limiting warming to 1.5°C to 2°C. °C had to limit, exceeded. This will lead to a period of higher temperatures in the middle of this century. Then, the idea goes, new, but as yet unproven technologies and techniques to remove greenhouse gases from the atmosphere, will eventually bring the temperature back to safer levels.

Until now, scientists weren’t sure what temporarily exceeding (and then boomeranging back down) the Paris Agreement’s temperature target would mean for nature. So, for the first time, we studied the implications of allowing Earth’s temperatures to exceed these precautionary limits and then fall below them again on marine and terrestrial life. In other words, we looked at how damaging the journey of exceeding the 2°C temperature target would be, not just the destination itself.

The results suggest that a temporary overshoot would cause waves of irreversible extinction and permanent damage to tens of thousands of species. Here’s what the world can expect if humanity fails to drastically reduce its emissions this decade, relying instead on future technologies to remove the emissions later.

Harm arrives quickly and leaves slowly

Our study modeled the impact of global temperatures above 2°C for about 60 years between 2040 and 2100 on more than 30,000 species living on land and in the sea. We looked at how many of them would be exposed to temperatures that could hinder their reproduction and survival, and how much time they would be exposed to this risk.

A line graph that
In this scenario where the world exceeds the 2°C target, emissions will not peak until 2040.
Meijer et al. (2022), Author provided

For nature, damage would arrive quickly and disappear slowly, even if the temperature drops again. Just a few years with global temperatures above 2°C can transform the world’s most important ecosystems. Take, for example, the Amazon basin. Some species would remain exposed to hazardous conditions long after global average temperatures stabilized — some remained exposed until 2300. This is because some species, especially those in the tropics, live closer to the limit of heat they can tolerate and so are sensitive to relatively small changes in temperature. And while global average temperatures may eventually return to safer levels, local temperature changes may lag.

The effects of this exposure can be irreversible and include turning the tropical forest into savanna. The world would lose a critical global carbon sink, leaving more planet-warming gases in the atmosphere.

The Coral Triangle in the western Pacific Ocean is one of the most species-rich marine ecosystems and is home to many reef-forming corals, sea turtles, reef fish, and mangrove forests. Our modeling showed that in some communities, all or most species would be simultaneously exposed to hazardous conditions for at least several decades and as long as two centuries. In addition to disrupting a food source for millions of people, disappearing corals and mangroves would remove a natural barrier that protects coastal towns and villages from rising seas and worsening storms.

A variety of corals growing in shallow water with a tropical island in the background.
Tropical corals exist at the limit of their thermal tolerance and are particularly vulnerable to climate change.
Ethan Daniels/Shutterstock

No way home

The consequences of exceeding 2°C for species survival have been neglected by policy makers. Our analysis shows that it cannot be assumed that life will simply recover if the temperature drops below 2°C again. We found that 3,953 species will expose their entire population to temperatures outside the range in which they evolved for more than 60 consecutive years. The Philippine porcupine will be exposed for 99 years, and the Mawa clawed frog a whopping 157 years. Surviving this exposure time is a major challenge for any species.

Relying on carbon dioxide removal and so-called negative emissions technologies to reduce the greenhouse gases in the atmosphere over several decades is too risky to consider. Some of this technology, such as carbon capture and storage, has not yet been shown to work on the scale needed. Other techniques have negative effects on nature, such as bioenergy, where trees or crops are grown and then burned to generate electricity. Rolling out huge plantations while temperatures exceed the internationally agreed-upon “safe” limit would cause species to falter from a warmer climate and shrinking natural habitat.

Delaying drastic emission reductions will mean the world will exceed 2°C, the best scenario. This overrun would incur an astronomical cost to life on Earth that negative emissions technologies will not reverse. The attempt to halt the rise in temperature is not an abstract attempt to bend curves on a graph: it is a fight for a habitable planet.

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