Climate change is the largest threat to ocean life

Local populations of marine animals are more vulnerable to global warming-induced extinction than terrestrial animals, showed new research published in Nature. The paper’s conclusions suggest that stopping ocean warming by mitigating carbon emissions is the most important action to protect biodiversity in the ocean. Though climate change also makes land animals more vulnerable to extinction, their biggest threat remains deforestation/habitat loss.

In this post, I’ll explain the research and what it means for conservation.

Pinsky et al. 2019

With the global average temperature increasing bit by bit, climate change appears to creep along. But, this ‘slow creep’ is really extreme temperatures that push up the average and create chaos during hotter heat waves. Higher extreme temperatures are the most common climate change-related cause of death in the animal kingdom (humans included). Understanding the maximum temperature an animal can experience and how close that animal lives to the edge of its temperature range is important data that would help predict species’ vulnerability to rising temperatures. A species at the edge of its temperature range could be living comfortably for most of the year, but an extreme heat wave beyond its maximum temperature would kill it unless the animal can find refuge from the heat. On land, these refugia could be shaded areas under trees or rocks; in the ocean, refugia are colder (deeper) water.  

Researchers, led by Malin Pinsky, measured the maximum temperature of over 400 cold-blooded animals (both terrestrial and aquatic). Humans are warm-blooded, we regulate our internal body temperature by burning calories for warmth and sweating to cool down; our maximum temperature is somewhere around 41° C or ~106° F. However, the internal temperature of cold-blooded animals is determined by their surrounding temperature so heat waves beyond an animal’s maximum temperature would kill them.

Once the maximum temperature of the animals was measured, the scientists mapped how close those animals were, geographically, to the limit of their temperature range. Finally, using climate and extreme temperature modelling, the researchers were able to determine which populations are most vulnerable to dying due to rising extreme temperatures.

Trouble for fish

The results of the study show that the most vulnerable populations of animals on Earth are terrestrial animals without access to refugia. Fortunately, refugia (e.g. shade) are relatively easy to find on land, so the number of animal populations vulnerable in this way is fairly low.

But when refugia are taken into account, the results show that many marine populations are highly vulnerable to local extinction caused by extreme water temperature, much more so than terrestrial animals with access to refugia. Ocean animals are highly vulnerable to climate change because refugia in the ocean means finding cooler water, not an easy task.

There are two ways to find cooler water in the ocean—go deeper or move towards the poles. Going deeper is easy for pelagic fish like tuna, but bottom-dwelling fish who already live in the deepest part of their ecosystem are in trouble. They could migrate towards the poles to find cooler water, but necessary habitat may not exist and if they could survive, they would likely experience increased resource competition from unfamiliar animals that already live there.

The results of the paper also show that the most vulnerable fish populations are those living closest to the equator where the dominant ecosystems are coral reefs. Since coral reefs cannot move with the fish and only exist in shallow water, many fish reliant on the ecosystem could perish due to climate change (to say nothing of the coral itself, also highly vulnerable to warm water).

How should conservation respond?

This paper furthers evidence that extreme water temperature (and climate change in general), is the biggest threat to marine biodiversity. Frankly, no other ocean issue comes close to the potential scale of devastation wrought by excess carbon dioxide in the atmosphere.

With carbon emissions permeating every aspect of global humanity, curbing emissions to save species seems bleak, but optimism can be found in knowing exactly what needs to be done to save marine life: reduce excess carbon in the atmosphere.

Ending overfishing will help species adapt and deal with climate change. Unfortunately, typical ocean conservation measures like marine protected areas do nothing to stop climate change. We need better fishery management to address overfishing, but large-scale carbon mitigation will come from policy and institutional change that environmental NGOs, activists, and politicians need to work towards. You have input on policy and institutional change with your vote. Register to vote in the U.S., here. On an individual level, the best thing you can do to reduce emissions is watch your diet.

Max Mossler

Max Mossler

Max studied environmental perception & policy in grad school. He thinks a lot about how other people think about the planet. He is the managing editor at Sustainable Fisheries UW.

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