During the ice-free Eocene period 56 to 34 million years ago, Earth was 10 ?C warmer than today. "Tropical" animals like crocodiles roamed the polar regions, and Antarctica ? now a frigid desert ? was once warm and covered with lush vegetation. Identification of these plants suggests that Antarctica had a monsoon, something that only occurs in the tropics these days. It could mean that in a warmer world monsoons spread across the planet.
Fr?d?ric Jacques of the Xishuangbanna Tropical Botanical Garden in Mengla, China and colleagues excavated fossil plants from an Antarctic island. As certain plants only grow in particular climates, their fossils provide evidence that such a climate prevailed in the past.
Jacques' fossils came from plants that experience seasonal rainfall. He estimates that 60 per cent of Antarctica's annual rainfall fell in summer during the Eocene, about 6.4 millimetres per day.
That suggests there was a monsoon. In monsoon climates like south Asia today, heavy rains fall in summer but the winter is dry. Eocene Antarctica looks the same.
Finding a monsoon so close to the poles is unexpected, says Gill Martin of the UK Met Office in Exeter, UK. "It tends to be thought of as a tropical thing."
Monsoon rains are driven by seasonal changes in wind direction. Moist sea air blows onto the land during summer, while in winter the winds travel in reverse. For this to happen, the land must warm up much more than the ocean during summer. Today, this only happens in the tropics, but Jacques says that monsoons may proliferate in hotter climates. "It can create new monsoon circulations."
Climatologists aren't sure how monsoons will be affected by climate change. Their best guess is that monsoon winds will diminish, but because warmer air holds more moisture the monsoon rains will get heavier, causing more floods (Nature Climate Change, doi.org/h9p). "In that sense the monsoon is stronger, but the circulation is weaker," Martin says. Air pollution complicates the story by reducing monsoon rains.
Our climate future
Could the monsoons also spread? It's unlikely we'll reach Eocene temperatures unless we burn fossil fuels well into the 2200s. Even then, Antarctica is likely to stay frozen, so an Antarctic monsoon probably isn't on the cards. But Jacques says monsoons could spring up on other continents.
Martin is unconvinced. She says major weather systems like the jet streams operate close to the poles, and could interfere with any new monsoons.
Models by Matthew Huber of Purdue University in Indiana suggest that Eocene monsoons were located in the same places as monsoons today (Journal of Asian Earth Sciences, doi.org/bg4c3j). However, climate models struggle with simulating such hot climates. "Data to really answer the question are few and far between," Huber says.
Journal reference: Gondwana Research, doi.org/h9n.
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