Scientists used this fossil, found in Germany, and other data to study the movement processes of prehistoric lizard Orobates pabsti, which lived 300 million years ago. Associated Press file photo

Jens Meyer

Scientists have pieced together enough clues to Earth’s past climate to graph the average temperature from 485 million years ago to the present — back to a time long before dinosaurs and even trees, when the land was either barren or hosted mostly moss, millipedes and primitive insects. This new work shows temperatures spent hundreds of millions of years bouncing up and down.

Earlier attempts to chart our planet’s climate suggested vastly hotter average temperatures — 60 degrees above today’s — that gradually cooled. The new analysis, published in Science, instead shows a series of extreme oscillations but no overall trend.

Because complex plants and animals survived all those fluctuations, extreme temperature swings might be a normal part of Earth’s history. But that doesn’t let us off the hook for climate change. Our climate crisis is unfolding too quickly for many species to adapt. There were similarly abrupt temperature upswings millions of years ago — the result was mass death.

The new research covers the Phanerozoic Eon, which marked the period when living things left fossils of shells and bones, wood and leaves. The fossils show a time of dramatic changes. Around 375 million years ago, fish dragged themselves out of the water on proto-limbs to become the first land vertebrates. Continents became forested 350 million years ago, and dinosaurs emerged 240 million years ago, dying out suddenly 65 million years back.

Looking over this long course of time helps scientists understand how Earth’s climate works and which factors make a planet habitable. “Very large swings in climate are the norm for this planet, and they have been driven very clearly by changes in CO2 concentration,” said Benjamin Mills, a University of Leeds biogeochemist. But the swings have never been big enough to destroy life entirely.

Knowing how Earth’s temperature has behaved also helps scientists test climate models that predict the future. “The only way we can test if these models can actually properly represent what happens when climate gets hot is to compare them to these examples from the paleo record,” said Mills.

The new analysis shows Earth repeatedly jumped between a 60 degree state, known as a greenhouse, and the cooler state, around 30 degrees, known as the ice house. “Right now, we’re in an ice-house Earth,” said paleoclimatologist Jessica Tierney, a co-author of the paper. Our average temperature is around 30 degrees and, for now, we have thick ice on Greenland and Antarctica.

Tierney and her colleagues created the new graph using a combination of clues. The most important comes from the ratio of ordinary oxygen to its heavier isotope, called oxygen 18. The amount of oxygen 18 incorporated into minerals and fossils depends on temperature. With enough temperature samples from different latitudes, it’s possible to estimate a global average.

Scientists have known about past hot spells from the fossil record, which showed crocodiles once roamed the Arctic and palm trees thrived in Antarctica. That record also shows periods where the diversity of life suddenly collapsed.

The worst mass extinction event happened 250 million years ago, when 70% of land species and 90% of marine species were snuffed out. The culprit was a series of volcanic eruptions that caused carbon dioxide levels to spike. A paper published earlier this month in Science re-examines that deadly hot spell and attributes some of the dying to a tipping point in ocean currents, like a giant El Nino event that accelerated the rate of heating.

It wasn’t the extent of the heat that wiped out species. It happens when the Earth’s temperature changes too rapidly for organisms to evolve and adapt. That’s starting to happen now.

One small reassurance is that average temperature, even in the worst of times, never gets above 86 or below 50 degrees. The scientific explanation involves feedback loops. A warming Earth is ice-free and wetter, exposing rocks. Over hundreds of thousands of years, those rocks weather, a process that sequesters carbon dioxide and cools the planet. As it gets colder, ice covers those rocks, the rock-weathering stops and atmospheric carbon starts to build up.

That cycle keeps our planet alive and will continue to do so for millions of years. But it won’t do anything to save humanity from the trouble created by our carbon emissions. We’re already seeing other species pushed to the brink by human activity — from polar bears to coral reefs to wild salmon. The next half a million years might be in the hands of Mother Earth, but the coming decades are in ours.

F.D. Flam is a columnist covering science and host of the “Follow the Science” podcast.