The story
The Mid-Pliocene warm period, also called the mid-Piacenzian, lasted from about 3.3 to 3.0 million years ago. Atmospheric carbon dioxide was around 400 parts per million, similar to levels reached by humans in the 2010s, and global temperatures were about 2 to 3 degrees Celsius above pre-industrial. Continents were already close to their modern positions, so it is the most recent time that looks much like our own world but with a similar amount of carbon dioxide.
Evidence includes deep sea sediment cores, fossil plants and ice rafted debris. Forests grew in the Arctic, where northern Canada held beech and larch, and the Greenland and West Antarctic ice sheets were smaller, with sea levels above today's, by an amount that is still debated but probably at least several metres. Researchers use the interval as a test case for climate models, notably through the PRISM project of the US Geological Survey. The warm period ended as carbon dioxide fell and the northern ice sheets began to grow, leading into the Ice Age cycles. Our own relatives, Lucy's species, lived in Africa then.
Why it mattered
- It is a close past analogue for a warmer, high carbon dioxide Earth and is used to test climate models.
- Sea level evidence from this time helps estimate how much ice may be lost under similar warming.
- Its end, with cooling and the growth of northern ice sheets, opened the Ice Age cycles.
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