Ice and Heat: Fourteen Climate Turning Points in Earth's Past

The path begins when microbial photosynthesis began to fill the air and shallow seas with free oxygen, roughly 2.46 to 2.06 billion years ago. By the end of the interval oxygen had reached as much as 10 percent of today's level, turning a nearly oxygen-free atmosphere into an oxidizing one.

Dated to about 2.4 to 2.1 billion years ago, this episode of at least three ice ages overlaps the rise of oxygen. The evidence is thick glacial diamictite in the Huronian Supergroup, which rivals the thickness of Quaternary glacial deposits.

The Sturtian, from about 717 to 660 million years ago, lasted roughly 57 million years and is the longest and most severe glacial event preserved in the rock record after the Huronian. Whether it was one Snowball Earth or several regional glaciations is still open.

Named in 1950 by Douglas Mawson and Reg Sprigg for the Adelaide suburb of Marino, this later Cryogenian glaciation is dated by a 2025 paper to between 639 and 635 million years ago. Its end is credited to volcanic carbon dioxide and the dissolution of clathrate hydrates.

Around 305 million years ago the vast equatorial coal forests of Europe and North America were altered, and may have broken into isolated refuges that encouraged dwarfism and extinction. Cooler and drier conditions as a brief ice age arrived are the main cause given.

About 183 million years ago, in the Early Jurassic, large igneous province volcanism pumped out carbon dioxide, warmed the planet and acidified the oceans. The resulting nutrient surge and oxygen loss make this possibly the most extreme ocean deoxygenation of the entire Phanerozoic.

Roughly 56 million years ago a carbon release drove global average temperatures up by 5 to 8 degrees Celsius for about 200,000 years. The source of that carbon is still under research, and the event is often called the best past analogue for modern warming.

The hypothesis here is that around 49 million years ago blooms of the freshwater fern Azolla in the Arctic Ocean sank and were buried over about 800,000 years. The resulting drawdown of carbon dioxide is speculated to have helped turn the greenhouse world into the icehouse one.

From about 33.9 to 33.4 million years ago the planet crossed from the Eocene into the Oligocene with a faunal turnover. In Europe it is called the Grande Coupure, when many endemic mammal groups vanished and others arrived from Asia.

From 5.96 to 5.33 million years ago the Mediterranean was cut off from the Atlantic and nearly dried out, leaving a basin 3 to 5 kilometers below normal sea level. The Zanclean flood ended it when the Strait of Gibraltar reopened.

About 8,200 years ago global temperatures dropped rapidly for two to four centuries, marking the start of the Northgrippian stage of the Holocene. Atmospheric methane fell by 80 parts per billion, a 15 percent reduction.

Sulfate aerosols from at least one eruption of uncertain origin darkened the skies over Constantinople in March 536 and cooled European summers by as much as 2.5 degrees Celsius. Crop failures and famine followed, and the cold lasted to about 660 as the Late Antique Little Ice Age.

From about 950 to 1250 CE the North Atlantic region was warm, but proxy records show peak warmth came at different times in different places, so it was not a globally uniform event. That is why some researchers prefer the name Medieval Climatic Anomaly.

The trail ends with a regional cooling centered on the North Atlantic, conventionally the 16th to 19th centuries, though some experts prefer about 1300 to 1850. It was not a true ice age, and the proposed causes include low solar output, volcanic activity and ocean circulation changes.