For example, a temperature of 20-22 degrees Celsius is quite comfortable for a person. We run a little hotter – and less efficiently – because additional energy is needed to remove the heat. Moreover, scientists say that temperature dependence for all species follows an asymmetric bell-like curve: biological processes increase up to a certain temperature, reach a maximum, and then drop sharply when the temperature gets too hot. Many animal species can live quite comfortably at temperatures much lower or higher than humans, but scientists have found a certain degree that overlaps with the possible existential temperature ranges of all animals, plants, and microbes living on land, in air, and in water. Could this be a coincidence?

The most comfortable temperature

Recently, a research team in New Zealand noticed that the number of marine species does not peak at the equator, as is commonly believed. On the contrary, their numbers are decreasing and reaching their peak even in the subtropics. Research has revealed that this decline in the number of living things has become even deeper since the last ice age, approximately 20,000 years ago. And this situation is exacerbated by global warming of the oceans. When the graphs of living species according to annual average temperature were examined, it was revealed that their numbers decreased after the threshold value. 20 degrees Celsius.

Research in Tasmania modeled the growth rates of microbes and multicellular organisms and found that the most stable temperature for their biological processes is also 20 degrees. This model was created based on other studies showing that the most stable temperature for biological molecules is 20 degrees.

Scientists from Canada, Scotland, Germany, Hong Kong and Taiwan drew attention to these “coincidences” and decided to study this issue in more detail to find general patterns of the influence of temperature on life. They say: “Surprisingly, everywhere we looked, we continued to see 20 degrees Celsius as the baseline temperature not only for marine species but also for many biodiversity indicators.”

Examples show that A temperature of more than 20 degrees leads to a decrease in various important indicators:

• Tolerance of marine and freshwater species to low oxygen content.
• Productivity of marine pelagic (living in open water) and benthic (living on the seabed) algae and the response level of fish to feed.
• Global species richness of pelagic fish, plankton, benthic invertebrates and fossil molluscs.
• Genetic diversity.

Globally, the temperature range experienced by reef fish and invertebrates is the narrowest among species with a geographical distribution around 20 degrees. The same effect is also seen in microbes.

Although many species have evolved to live in warmer and colder temperatures, most species live comfortably at 20 degrees. Additionally, extinction rates for fossil species such as sponges, lampreys, molluscs, sea mats, starfish and sea urchins, worms and crustaceans were lower at 20 degrees.

Temperature niches are expanding as species evolve and adapt to life in temperatures above and below 20 degrees. This means that Most species can survive at 20 degrees, even if they live in warmer or colder places.

The mathematical model predicts that at a temperature of 20 degrees the thermal gap should be minimal and biological processes should be most stable and efficient. This should maximize species richness in all areas of life, from bacteria to multicellular plants and animals. The model thus provides a theoretical explanation for this “20 degree effect.” But there is also an unpleasant consequence: animals and plants living in warm regions of the planet are less adaptable than those common in colder regions. Tropical species are less likely to adapt to colder or warmer temperatures than those that have spent their entire lives in colder latitudes.

What happens if the temperature increases?

The fact that life appears to be centered around 20 degrees represents a fundamental constraint that threatens the ability of tropical species to adapt to higher temperatures. We know of many examples in the history of the planet where a temperature increase of just a few degrees led to mass extinctions. Moreover, we see this even today, where the rate of extinction of species increases with the acceleration of warming.

As long as species can shift their ranges to adapt to global warming, the 20-degree effect means an annual increase in local species richness to an average of 20 degrees. Above this limit, species richness will decrease.

This means that many marine species that can adapt to global warming by changing their geographic range are unlikely to go extinct due to climate change.

But terrestrial species may not be able to shift their geographic ranges as easily across different geographies, including those altered by cities, agriculture, and other human infrastructures.

What is the reason?

Scientists hypothesize that the 20-degree “ideality” is related to the molecular properties of water interacting with cells. These features may also be why 42 degrees is the limit for most species.