Do the tropics have an internal thermostat?

Published 2017-03-23

Photo: NCAR

Science Daily March 6, 2017

New research findings show that as the world warmed millions of years ago, conditions in the tropics may have made it so hot some organisms couldn't survive. Longstanding theories dating to the 1980s suggest that as the rest of Earth warms, the tropical temperatures would be strictly limited, or regulated by an internal 'thermostat.' These theories are controversial, but the debate is of great importance because the tropics and subtropics comprise half of Earth's surface area, greater than half of Earth's biodiversity, as well as over half Earth's human population. But new geological and climate-based research indicates the tropics may have reached a temperature 56 million years ago that was, indeed, too hot for living organisms to survive in parts of the tropics.

The Paleocene-Eocene Thermal Maximum (PETM) period occurred 56 million years ago and is considered the warmest period during the past 100 million years. Global temperatures rapidly warmed by about 5 degrees Celsius (9 F), from an already steamy baseline temperature, and this study provides the first convincing evidence that the tropics also warmed by about 3 degrees Celsius (5 F) during that time. These results are unique because geological records from the PETM are typically hard to find, especially in tropical regions. To overcome this limitation, researchers can analyze the carbon and oxygen isotopic composition of shells, which tell a story about the carbon cycle and temperatures from the past. Two research methods were used to judge the temperature during the PETM, one utilizing isotopes in shells, while the other examined organic residues in deep-sea sentiments. The biotic records left behind from living organisms indicate they were dying at the same time the conditions were warming.

This research has important implications in the context of climate change. If temperature buffering in the tropical regions did not occur in the past, the future of these regions is uncertain when exposed to rapidly increasing temperatures.

Journal Reference:
1. Joost Frieling, Holger Gebhardt, Matthew Huber, Olabisi A. Adekeye, Samuel O. Akande, Gert-Jan Reichart, Jack J. Middelburg, Stefan Schouten, Appy Sluijs. Extreme warmth and heat-stressed plankton in the tropics during the Paleocene-Eocene Thermal Maximum. Science Advances, 2017; 3 (3): e1600891 DOI: 10.1126/sciadv.1600891