Ancient forests, seaways, and climate: Lessons from the miocene for today's world

Widespread forests once played a significant role in warming the climate during the Middle Miocene Climate Optimum (MMCO), while open Panama and Tethys Seaways served as "shortcuts" for the Atlantic Meridional Overturning Circulation (AMOC). These findings from a recent study published in Palaeogeography, Palaeoclimatology, Palaeoecology led by scientists at the Institute of Atmospheric Physics and the Institute of Tibetan Plateau Research, both part of the Chinese Academy of Sciences, offer valuable insights into the complex relationship between forests, ocean circulation, and climate. They have meticulously pieced together this captivating puzzle, unveiling the profound connections among these factors.

The MMCO, which occurred approximately 16.9-14.7 million years ago, was a period characterized by extensive forest coverage on land, with the exception of Antarctica. During this time, our ancestors and orangutans emerged and thrived, swinging from tree to tree. The researchers sought to understand the climate dynamics during the MMCO by simulating ocean circulation using a coupled model called the Flexible Global Ocean-Atmosphere-Land System Model Grid-Point Version 3 (FGOALS-g3).

Although some proxy data suggested that CO₂ concentrations during the MMCO were as high as those projected for the end of the 21st century, the CO₂ level used in the MMCO simulation was similar to todays'. The results indicated that global average temperature in MMCO was over 3℃ higher than present, with forests playing a more significant role in climate regulation than previously thought. Land temperatures were particularly high in the Sahara and high northern latitudes, which are now covered by deserts and low-growing vegetation but were forested during the MMCO.

In addition to extensive forest coverage, the MMCO featured open Panama and Tethys Seaways, which differs from today's land-sea distribution. The simulation revealed that high-salinity throughflow from the Tethys Seaway combined with fresh throughflow from the Panama Seaway in the western North Atlantic. This finding suggests that the Tethys Seaway compensated for the Panama Seaway's contribution to the AMOC.

"By delving into the depths of our planet's past, we can better comprehend the potential impacts of climate change on our fragile world and devise strategies to navigate the uncertain waters of the future." said Prof. LIU Hailong, the corresponding author of the study.