Global warming increases the risk of Ectotherm heat shutdowns

Global warming could have dire consequences for ectotherms (cold-blooded animals) on land and in water around the world. According to recent studies, the rate of thermal injuries among ectotherms doubles with each degree of temperature rise.

It may seem obvious that ectothermic animals have been seriously affected by global warming. It is well known that their body temperature and consequently their biochemical processes depend on the temperature of the environment and sunlight.

However, the fact that thermal damage doubles for every degree that the ambient temperature exceeds the animal’s tolerance limit surprised even the researchers who conducted the new study.

There are five researchers Aarhus University zoophysiologists who recently published their results in a well-known scientific journal Nature, where the study is featured on the cover. The researchers based their findings on data from previous studies on ectothermic animals.

There is a well-known relationship between the geographical distribution of ectotherms and their ability to tolerate environmental temperature conditions. They can only survive in temperatures that allow them to develop and reproduce, and in harsh winter and summer temperatures that are neither too cold nor too hot for long periods of time.

Animals are injured when the temperature exceeds the threshold they can tolerate. These damages accumulate over time and ultimately determine whether a species can survive the prevailing temperature conditions.

“And the higher the temperature exceeds the tolerance level of the species, the faster they will accumulate injuries,” explains one of the study’s co-authors, postdoc Lisa Bjeregard Jorgensen.

The researchers analyzed the temperature sensitivity to heat stress of 112 ectothermic species. analysis shows that the rate at which heat damage accumulates more than doubles when the temperature rises by just 1°C

And because this is exponential growth, a 2°C increase in temperature would more than quadruple the rate of thermal damage accumulation, while a 3°C increase would more than eightfold the rate of damage.

The researchers then compared their data on temperature sensitivity with models of expected increases in maximum temperatures due to global warming. These data indicate that the rate of thermal damage for ectotherms globally may increase by an average of 700 percent, and in many terrestrial settings by more than 2,000 percent.

For water ectotherms, the corresponding figures are 180 percent and 500 percent.

Regional analysis suggests huge impacts, especially in the northern temperate zone, which covers most of Europe and North America, and the ocean around the Arctic.

Although the researchers do not know the underlying physiological and biochemical reactions that lead to heat stress and death, their study shows that these processes are extremely sensitive to temperature in all groups of ectotherms. This may indicate that similar processes determine the degree of thermal damage.

“Nor can we predict how many species and individuals are at risk of succumbing to rising temperatures, because the threshold for heat stress varies significantly from one species to another. In addition, many terrestrial ectothermic animals can regulate their temperature by finding shade, thereby reducing the risk of thermal injury. It’s not so easy for aquatic animals,” says Professor Johannes Overgaard, who co-authored the study.

And he adds: “The point is that this very high sensitivity to heat damage means we risk underestimating the effects of future heat waves. Our results show that future warming will have serious consequences – even if not all species will be affected to the same extent.”

Reference: Lisa Bjerregaard Jørgensen, Michael Oersted, Hans Malte, Tobias Van and Johannes Overgaard “Extreme Escalation of Heat Denial Rates in Ectotherms with Global Warming”, 26 Oct 2022 Nature.
DOI: 10.1038/s41586-022-05334-4

The study was funded by the Danish Council for Independent Research.