Earth’s ancient climate reveals a history of extreme transformations—from a greenhouse world of sweltering temperatures and high sea levels to a “snowball earth” in which glaciers reached the equator. In The Story of Earth’s Climate in 25 Discoveries, Donald R. Prothero explores the astonishing connections between climate and life through the ages, telling the remarkable stories of the scientists who made crucial discoveries. The following facts offer insight into our climate’s past and present.

1. About 700–900 million years ago, the earth froze over, creating a “snowball earth.”

Discoveries of glacial sediments in the ancient tropics of 700–900 million years ago prove that the earth had sea-level ice close to the equator, so the planet was almost completely frozen over at this time—and again about 1.7 billion years ago. The earth might have remained a completely frozen planet with just bacteria living on it (much like Mars is now) were not for volcanic eruptions beneath the ice sheets, which released tons of greenhouse gases. This geological activity eventually warmed the earth up again and melted all the ice.

2. About 300 million years ago, the earth’s atmosphere contained almost 35 percent oxygen (compared to 21 percent today), and gigantic insects roamed the tropical swamps.

During the Carboniferous Period, enormous swamps full of primitive plants like ferns, horsetails, and club mosses covered the tropics. There were no termites yet, so these plants did not decay; instead, they fell into these swamps and were buried, eventually turning into coal. These plants pulled lots of carbon dioxide from the atmosphere and released enormous amounts of oxygen. Oxygen levels were so high that there were dragonflies the size of eagles and gigantic millipedes almost 9 feet long! Sadly, much of this coal has been burned during the Industrial Revolution, releasing the carbon dioxide that had once been trapped in the earth’s crust back into the atmosphere and contributing to modern global warming.

3. During the latter half of the Age of Dinosaurs (100–70 million years ago), greenhouse climates were so warm that there was no polar ice, and the seas rose to drown the Great Plains of North America.

Today, if you travel through western Kansas, eastern Colorado and Wyoming, and the Dakotas, you will find huge deposits of chalky limestones and shales, full of the fossils of marine reptiles, fish, and more that once swam in a tropical seaway that connected the Arctic Ocean to the Gulf of Mexico.

4. The Ice Ages of the past 2.5 million years have come and gone at least 25 times, covering Canada and much of the northern United States in more than a mile-thick sheet of ice, and turning the areas in the central United States into a frozen tundra.

If you travel across the northern half of North America, you can find all sorts of evidence of the Ice Ages, from the Great Lakes (sculpted by major glaciers flowing from Canada) to the Finger Lakes in New York, to dry lake beds in Nevada and Utah that were once filled with water when the climate was cooler and wetter. In many of these glacial deposits, you can find the remains of mammoths, mastodons, bison, horses, camels, and saber-toothed cats that once roamed ancient North America.

5. Climate is now changing more rapidly than at any time in the geological past, and it is not due to natural causes, but due to the human burning of fossil fuels that release greenhouse gases.

We can rule out the possibility that our current global climate change is due to “natural variations in climate.” Scientists have sampled ice cores from Antarctica dating back almost 800,000 years, at least seven complete glacial-interglacial cycles. At no time in this interval was carbon dioxide more than 280 parts per million, even in the warmest interglacial. That is the natural range of climate variability. Today, it is over 415 parts per million and rising rapidly from only 315 parts per million in 1960. The year 2023 shattered all previous climate records as the warmest year ever recorded, and 2024 is already warmer than 2023 at the same time last year.