Climate overshoot likely to cause irreversible biodiversity loss, ecosystem changes

With global climate action lagging behind the scale needed to achieve the temperature targets of the Paris Agreement, many scenarios now feature a multiple decades-long ‘overshoot’ of the global warming limits set by policymakers and then use carbon dioxide removal technology (CDR) to reverse dangerous temperature rise by 2100. However, new research from the University of Cape Town (UCT) and University College London (UCL) shows how an overshoot like this is likely to cause major and often irreversible damage to biodiversity and ecosystems across the world.

The study, published in Philosophical Transactions of the Royal Society B: Biological Sciences, covered more than 30,000 species around the world and found that for more than a quarter of the locations studied, the chances of returning back to pre-overshoot ‘normal’ are either uncertain or non-existent.

The research team from UCT and UCL explored the implications of a temperature overshoot for global biodiversity, which is already affected by climate change as warming causes mass die-offs in forests and coral reefs, alters species distributions and reproductive events, and many other parameters. “A study such as this, whereby the risks to biodiversity of overshooting and then returning back under the 2.0°C temperature target are explicitly modelled, has long been missing from the research on climate change,” the research team said.

They found that, for most regions, exposure to unsafe temperatures will arrive suddenly as further warming means many species will simultaneously be pushed beyond their thermal niche limits. However, the return of these species to conditions comfortably within their thermal niches will be gradual and lag behind the temperature decline. In fact, the effective overshoot for biodiversity risks is projected to be almost twice as long as the actual temperature overshoot of around 60 years.

Tropical regions are most affected by these risks, with over 90% of species for many locations in the Indo-Pacific, Central Indian Ocean, Northern Sub-Saharan Africa, and Northern Australia pushed outside of their thermal niches. And in the Amazon, one of the most species-rich regions of the world, more than half of the species will be exposed to potentially dangerous climate conditions.

Concerningly, for about 19% of the total number of sites studied, including the Amazon, it is uncertain whether the share of exposed species will ever return to pre-overshoot levels. And a further 8% of sites are projected to never return to those levels at all. This means that the overshoot, while reversing the course of global temperature, can cause irreversible impacts to nature  due to species extinction and radical transformations of ecosystems.

Dr Andreas Meyer of the African Climate & Development Initiative (ACDI) at UCT said: “In the Amazon, this could mean replacement of forests with grasslands, and as a consequence, the loss of an important global carbon sink, which would have knock-on effects on multiple ecological and climatic systems as well as our ability to curtail global warming,”

The study underscores how important a temporal perspective can be in discussions of overshoot scenarios, which often dismiss the ‘journey’ of dangerously rising temperatures as long as the ‘final destination’ is within the agreed limits — and thus may underplay rapid and deep emissions reductions called for in the recent IPCC reports. Moreover, the authors note that CDR itself is also likely to have negative impacts on ecosystems: for instance, large-scale forest planting or biofuels production require a lot of land and water and may even have secondary effects on the climate system.

Dr Joanne Bentley of the ACDI said: “It is important to realise that there is no ‘silver bullet’ solution for mitigating climate change impacts. We have to rapidly reduce greenhouse gas emissions. Many carbon dioxide removal technologies and nature-based solutions, such as afforestation, come with potential negative impacts. Our study shows that should we find ourselves overshooting the 2.0°C global warming target, we could pay dearly in terms of loss of biodiversity, compromising the provision of the ecosystem services that we all rely on for our livelihoods. Avoiding a temperature overshoot should be a priority, followed by limiting the duration and magnitude of any overshoot.”