Picture: Tropical Cottages. Credit: Julius Silver (Pexels).
The mid-Pliocene Warm Period (about 3 million years ago) was a time when temperatures were 2–3ºC warmer than the pre-industrial climate. At the time, CO2 concentrations were similar to present-day values (~400ppm) but polar ice sheets were reduced by more than half the mass compared to today. Given the similarities to future projections, the mid-Pliocene is commonly used as an analogue to understand end-of-the-century climate change.
Much effort has been put into exploring changes in the Northern Hemisphere as there is a larger number of proxy-data from the mid-Pliocene in that hemisphere compared to its southern counterpart. Due to the scarcity of proxies in the south, little is known about the Southern Hemisphere, making global circulation models crucial tools to investigate climate processes during that period.
CLEX researchers and colleagues used model simulations from the Pliocene Model Intercomparison Projects phases 1 and 2 to investigate the Southern Hemisphere’s hydroclimate. The study showed that the November-to-March precipitation (when rainy season peaks over most of the Southern Hemisphere land mass) was significantly reduced both in the Southern Hemisphere tropics and subtropics due to a weakening of the subtropical convergence zones. Mid-Pliocene sea surface temperatures were as much as 9 °C higher than pre-industrial era in the northern hemisphere compared to 4 °C in the southern hemisphere.
This asymmetric warming between the two hemispheres shifted the Intertropical Convergence Zone northward, leading to a reorganisation of the large-scale atmospheric circulation. This intensified the Hadley cells in the Southern Hemisphere and strengthened the subtropical highs.
The South Pacific Convergence Zone, the South Atlantic Convergence Zone, and to a lesser extent, the South Indian Convergence Zone, weakened and shifted south, resulting in drier-than-normal Southern Hemisphere tropics and subtropics.
Understanding the mid-Pliocene climate adds a constraint to future warmer scenarios associated with differing rates of warming between hemispheres. If a similar climate change scenario is projected, South America, South Africa, and South Pacific Islands are expected to receive less summer rain in the future. The consequences for Australia are not as clear as the South Pacific Convergence Zone weakens and moves southwestward in the subtropical part, while the Pacific warm pool and the Coral Sea warms in the mid-Pliocene relative to pre-industrial times, thus suggesting an intensification of the monsoon season instead.
- Paper: Pontes, G.M., Wainer, I., Taschetto, A.S. et al. Drier tropical and subtropical Southern Hemisphere in the mid-Pliocene Warm Period. Sci Rep 10, 13458 (2020). https://doi.org/10.1038/s41598-020-68884-5