A new study published in the journal Proceedings of the National Academy of Sciences has investigated a period of rapid global warming from 300 to 260 million years ago, when the Earth’s climate went from a glacial icehouse to a hot, ice-free greenhouse. According to the scientists, within about 300,000 years, atmospheric carbon dioxide levels doubled, oceans became anoxic, and biodiversity dropped significantly both on land and in the seas.
Although several other hyperthermal or rapid warming events are known in our planet’s history, this appeared to be one of the fastest ones. Moreover, it was the first such event identified in an icehouse Earth, when the planet had a variety of ice caps and glaciers, comparable to today’s situation. Experts argue that an icehouse climate may be more sensitive to changes in atmospheric carbon dioxide levels than warmer conditions, when the levels of this gas are already higher.
In this study, the scientists homed in on a transition 304 million years ago, known as the Kasimovian-Gzhelian boundary, or KGB. By using multiple proxies, such as carbon isotopes and trace elements from rocks and plant fossils, they estimated that about 9,000 gigatons of carbon were released in the Earth’s atmosphere just before the K-G boundary.
This period of massive carbon release also led to marine anoxia, or a drop in the levels of dissolved oxygen in the oceans. Since melting ice caps release fresh water onto the ocean surface, they create a barrier to deep water circulation and cut off the supply of oxygen, leading to the extinction of many marine species. By measuring uranium isotopes in carbonate rocks from present-day China, the scientists managed to get a proxy for the amount of oxygen in the ocean when the rocks were laid down. They estimate that 23 percent of the seafloor worldwide became anoxic dead zones, leading to high losses of biodiversity.
According to study co-author Isabel Montañez, a paleoclimatologist at the University of California, Davis, the effects of carbon release on ocean anoxia were significantly greater than those observed in other studies of rapid warming during “greenhouse” conditions, in which the baseline level of atmospheric carbon dioxide was already higher. “If you raised CO2 by the same amount in a greenhouse world, there isn’t much effect, but icehouses seem to be much more sensitive to change and marine anoxia,” Professor Montañez said.
The researchers concluded that this massive carbon release may have been triggered by volcanic eruptions that tore through carboniferous coal beds and started a large number of wildfires. The rapid warming caused by these events may have melted permafrost, leading to the release of even more organic carbon in the atmosphere.