Mammals' time on Earth is half over, scientists predict

It's been about 250 million years since reptile-like animals evolved into mammals. Now a team of scientists is predicting that mammals may have only another 250 million years left.

The researchers built a virtual simulation of our future world, similar to the models that have projected human-caused global warming over the next century. Using data on the movement of the continents across the planet, as well as fluctuations in the chemical makeup of the atmosphere, the new study projected much further into the future.

Alexander Farnsworth, a paleoclimate scientist at the University of Bristol who led the team, said the planet might become too hot for any mammals — ourselves included — to survive on land. The researchers found that the climate will turn deadly thanks to three factors: a brighter sun, a change in the geography of the continents and increases in carbon dioxide.

"It's a triple whammy that becomes unsurvivable," Farnsworth said. He and his colleagues published their study Monday in the journal Nature Geoscience.

Scientists have been trying for decades to foretell the fate of life on Earth. Astronomers expect that our sun will grow steadily brighter and, in about 7.6 billion years, may engulf the Earth.

But life probably will not make it that long. As the sun hurls more energy at the planet, Earth's atmosphere will heat up, causing more water to evaporate from the oceans and continents. Water vapor is a potent greenhouse gas, and so it will trap even more heat. It may get hot enough in 2 billion years to boil away the oceans.

In 2020, Farnsworth turned his attention to the future of Earth as a way to distract himself from the pandemic. He came across a study predicting how the continents will move around the planet far in the future.

In his primary research, Farnsworth builds models of ancient Earth to reconstruct the climates of the past. But he thought it would be interesting to use his models to see what life will be like on Pangea Ultima. The climate he ended up with took him by surprise.

"This world was very toasty," he said.

Farnsworth enlisted Christopher Scotese, a retired geophysicist from the University of Texas who had crafted the Pangea Ultima model, and other experts to run more detailed simulations of that far-off future, tracking the atmosphere moving over the oceans, the supercontinent and its mountains.

"They did quite a lot, which I'm quite impressed by," said Hannah Davis, an earth systems scientist at GFZ German Research Centre for Geosciences, who was not involved in the research.

Under a range of possible geological and atmospheric conditions, the researchers found, Pangea Ultima will be much hotter than today's continents. One reason for the drastic change is the sun. Every 110 million years, the energy released by the sun increases by 1%.

Pangea Ultima will also influence the climate thanks to its topography, which will include vast stretches of flat land far from the ocean. On today's Earth, rainwater and carbon dioxide react with minerals on the sides of mountains and hills, which then get carried out to sea to fall to the sea floor. The result is that carbon dioxide is steadily pulled out of the atmosphere. But when Earth becomes home to Pangea Ultima, that conveyor belt will slow down.

If Pangea Ultima behaves like previous supercontinents, it will become studded with volcanoes that belch carbon dioxide, the model found. Thanks to the turbulent movements of molten rock deep in the Earth, the volcanoes may release vast surges of carbon dioxide for thousands of years — blasts of greenhouse gases that will make temperatures rocket up.

Currently, humans are heating the planet by releasing more than 40 billion tons of carbon from fossils fuels each year. If global warming continues unabated, biologists fear it will lead to the extinction of a number of species, while people will be unable to survive the heat and humidity in large swaths of the planet.

On Pangea Ultima, Farnsworth and his colleagues concluded, things will probably get far worse for mammals like us. The researchers found that almost all of Pangea Ultima could easily become too hot for any mammal to survive. They might disappear in a mass extinction.

Farnsworth granted that a few mammals might eke out an existence in refuges on the fringes of Pangea Ultima. "Some areas in the northern and southern peripheries could be survivable," he said.

Wolfgang Kiessling, a climate scientist at University of Erlangen-Nuremberg in Germany, who was not involved in the study, said that the model did not take account of a factor that might mean a lot for the survival of mammals: the gradual decline in the heat escaping from the Earth's interior. That decline might lead to fewer volcanic eruptions, and less carbon dioxide put in the atmosphere.

"Mammals may survive somewhat longer than modeled," he said — maybe 200 million years, give or take.

Eric Wolf, a planetary climate scientist at the University of Colorado, who was not involved in the new study, said the research could one day help us spot life on other planets. As scientists begin using powerful space telescopes to peer at planets in other solar systems, they may be able to measure their continental arrangements to infer what kinds of life might survive there.

"We're trying to prepare ourselves for the many worlds we are going to see," Wolf said.