Sharks may benefit from rising temperatures - but not from rising CO2

Sharks and rays have inhabited Earth’s oceans for about 450 million years. These cartilaginous fish have survived five mass extinctions, adapting to environmental changes that wiped out countless other species.

Despite their resilience, modern threats endanger their survival like never before. Overfishing and habitat destruction have placed more than a third of all species at serious risk. Now, climate change introduces another challenge to sharks and rays.

A team of international researchers led by palaeobiologist Manuel A. Staggl from the University of Vienna sought to understand how past climate fluctuations affected the biodiversity of sharks and rays.

They focused on the Jurassic (200-143 million years ago) and Cretaceous (143-66 million years ago) periods, a time when these species thrived in diverse environments.

By analyzing fossilized teeth and comparing them to climate records, they identified key environmental factors that influenced shark populations.

How climate change shaped shark evolution

The evolution of sharks and rays has always been closely linked to environmental conditions. During the Jurassic and Cretaceous periods, the oceans underwent significant changes.

The expansion of shallow coastal waters created new habitats, while rising global temperatures affected marine ecosystems. These periods saw an explosion in shark and ray diversity, demonstrating their ability to adapt to shifting conditions.

Fossil evidence suggests that the diversity of these species fluctuated with environmental changes. The researchers found that three main factors influenced their survival.

Higher global temperatures and an increase in shallow water areas had a positive effect, allowing sharks and rays to expand into new regions.

However, a third factor – rising carbon dioxide (CO2) levels – had a clearly negative impact. This discovery sheds new light on how future climate change might affect these species.

Rising CO2 and shark diversity

One of the most significant findings of the study was the negative effect of CO2 concentrations on shark and ray biodiversity. Higher CO2 levels in the ocean have long been suspected to harm marine life, but this study provides clear evidence of its impact.

This is the first time that the negative impact of CO2 in the ocean has been so clearly described. While the exact mechanisms remain unclear, laboratory studies on modern sharks and rays suggest direct physiological consequences.

Increased CO2 can alter their sensory abilities, affecting their ability to navigate and hunt. It can also influence skeletal development during embryonic stages, potentially leading to deformities.

The fossil record confirms that past increases in CO2 levels contributed to the extinction of certain shark and ray species. This suggests that current climate change, driven by human activity, could have similar long-term consequences. If CO2 levels continue to rise, the survival of many species may be at risk.

Climate opportunities for sharks

Despite the dangers, climate change may not be entirely detrimental to sharks and rays. In previous geological periods, rising sea levels and increasing temperatures actually benefited their populations.

Expanding warm-water regions and the creation of new shallow coastal environments provided ideal conditions for these predators.

“The resulting habitats in shallow seas that cover large continental areas are real biodiversity hotspots; sharks and rays were able to colonize them very quickly and efficiently thanks to their adaptability,” explained Staggl.

During the Jurassic and Cretaceous periods, temperatures were significantly higher than today, leading to the expansion of the tropics and subtropics. With fewer seasonal changes, marine ecosystems became more stable, supporting a wider range of species.

If similar conditions emerge in the future, some sharks and rays might find new habitats to thrive in. However, the speed of modern climate change presents a unique challenge.

Unlike past warming events that occurred over millions of years, today’s changes are happening in just decades. This rapid shift could make adaptation difficult, even for a group of species that has survived multiple extinctions.

A rapidly changing world

Assuming that sharks and rays will simply adapt to climate change would be overly optimistic.

“The environment is currently changing particularly quickly – unfortunately probably too quickly for the animals and their ecosystems,” said Staggl.

The current rate of warming and ocean acidification may surpass their ability to adjust. Overfishing and habitat destruction further complicate the situation.

Many shark species are already struggling due to declining populations, and climate change may add additional stress. Warmer oceans can disrupt food chains, forcing sharks to migrate in search of prey.

Changes in ocean currents and rising acidity levels may also alter their breeding patterns. If these species cannot adapt quickly enough, many could face extinction.

Protecting sharks from climate change

Urgent action is needed to protect sharks and rays from multiple environmental threats. Conservation efforts must address overfishing, habitat loss, and the growing effects of climate change.

These predators play a crucial role in maintaining marine ecosystems, and their decline could have widespread consequences.

“Because without the top predators, the ecosystems would collapse,” warned Jürgen Kriwet, professor of palaeobiology at the University of Vienna.

Sharks and rays regulate the balance of ocean life, keeping populations of other marine species in check. Without them, ecosystems could become unstable, affecting biodiversity and fisheries that millions of people depend on.

“By protecting sharks and rays, we are investing directly in the health of our oceans and therefore also in the people and industries that benefit from these ecosystems,” said Kriwet.

Preserving these species is not just about maintaining biodiversity – it is about ensuring the stability of marine environments.

The future of sharks and rays depends on immediate and sustained conservation efforts. Their survival will determine the health of the oceans for generations to come.

The study is published in the journal Biology.

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