Earth’s climates in half a billion years – 09/19/2024 – Science

The most ambitious reconstruction yet of Earth’s temperature variations over the past half-billion years shows an alternation between “hot” and “cold” states of the planet in the long term, although the cooling trend has prevailed since the extinction of the dinosaurs.

The analysis further indicates that the most important factor behind these back-and-forths is the amount of CO₂ (carbon dioxide or carbon dioxide) in the atmosphere, the same factor that is behind the current human-caused climate crisis. If this data is correct, the influence of carbon dioxide would have surpassed even that of solar radiation, which has gradually become more intense as the Sun evolves.

The work covers 485 million years of the planet’s history — from the Ordovician, when animals and plants were just beginning to colonize dry land, to the Holocene, the geological era in which we are still, according to the official classification.

The reconstruction was carried out by Emily Judd’s team at the Smithsonian National Museum of Natural History (USA) and has just been published in the specialized journal Science. According to Judd’s team, the average temperatures on Earth during this period varied from a minimum of 11 degrees Celsius (120 thousand years ago, at the end of the Ice Age) to a staggering maximum of 36 degrees Celsius (in the Turonian, about 90 million years ago, when the Age of Dinosaurs was in its final phase).

According to the study’s authors, around 41% of the total period studied corresponded to warmer states, or “greenhouses”, with an average temperature equal to or greater than 25 degrees Celsius. The colder states of the biosphere, or “refrigerators”, with temperatures below 22 degrees Celsius, would correspond to 31% of the last half billion years. The remaining time, or 27%, is classified by them as a transition state.

The current average temperature of 15 degrees Celsius is typical of “refrigerators” and has followed this trend for 34 million years. The transitions between one state and another are gradual and extend over phases ranging from 3 million to more than 10 million years.

To arrive at relatively accurate estimates over such a vast span of time, Judd and his colleagues combined computer simulations of Earth’s climate with temperature indications from fossils.

The most important of these is the preservation of isotopes (variants) of the chemical element oxygen in rocks. Among these isotopes, there is the most common form of the atom, with eight protons and eight neutrons in its nucleus, and a “heavier” form, which has ten neutrons.

To build their shells and other parts of their bodies, marine organisms incorporate oxygen, and this is where the study comes in. It turns out that the ratio of atoms of the light and heavy forms of oxygen in marine shells varies with temperature (when it’s warmer, there’s more of the “thin” oxygen), and this acts as a thermometer of the past.

There is yet another detail that researchers needed to take into account. There are other clues indicating that the isotopic ratio of oxygen in past waters varied slightly over time, independently of temperature. Therefore, it was necessary to go hunting for rocks with components that recorded this, which was achieved through the analysis of minerals containing iron oxides.

There are still a number of methodological uncertainties that could significantly affect the numbers proposed by the study in Science, warns Benjamin Mills, from the University of Leeds (United Kingdom), in a comment written at the request of the editors of the specialized journal.

“Their model predicts temperatures that are generally higher during ‘hothouse’ periods than those seen in long-term models of Earth’s carbon cycle,” he writes.

This also has repercussions for understanding the past of living beings. During the warmest periods of the Age of Dinosaurs, for example, the tropical regions of the Earth, according to the new temperature reconstruction, could have hot months exceeding 45 degrees Celsius for long periods.

Current species of animals and plants are simply not equipped to survive these conditions, even when adapted to life in the tropics. But the possibility cannot be ruled out that there were refuges with milder microclimates (local climates) or ancient species that were adapted to such conditions, the researchers write.