About 304 million years ago, rapid warming upended a Earth that had settled into a long ice age.
At that time, vast ice sheets covered part of the Southern Hemisphere. The climate was not completely stable, however. Geological records show alternations between colder phases and warmer periods, linked in particular to variations in atmospheric carbon dioxide.
The episode studied stands out for its speed on a geological timescale. Earlier research estimated that the CO₂ concentration rose from about 350 to 700 parts per million. This increase is thought to have occurred over approximately 300,000 years, following a pronounced glaciation phase.
The change also appears in ancient ocean temperatures. Fossilized shells indicate that surface waters rose from about 25 °C to 29 °C. This 4 °C increase accompanied a retreat of continental ice and a rise in sea level.
To understand the significance of this episode, we need to examine the reactions that followed the warming. A rise in temperature can alter the oceans, ice and carbon cycle. These changes can then intensify warming rather than simply accompany it.
The new study published in PNAS examines this ancient episode as a possible crossing of a climate threshold. In this type of situation, a relatively limited disturbance can trigger much more significant changes. The climate system then no longer responds in a strictly proportional way to the initial cause.
The oceans also experienced a substantial loss of oxygen. Estimates from previous research indicate that about 20% of the seafloor may have experienced oxygen-free conditions at the time. This change coincided with a decline in marine biodiversity recorded in rocks from this period.
In other words, this ancient warming was not simply a matter of a few additional degrees. It shows how several mechanisms can act together: rising CO₂, retreating ice, warming waters and declining oxygenation. Together, they can amplify an initial climate disturbance over a relatively short period.
This ancient period does not reproduce the modern world, whose continents, oceans and ecosystems are different. Nevertheless, it offers a real case for studying how an icy planet responds to a rapid increase in carbon. Researchers can now clarify which mechanisms triggered the observed amplification and how far it altered the climate.