The tropical ocean became as hot as a jacuzzi during an abrupt warming period, 56 million years ago, NESSC-research published this week in Science Advances shows. The study also suggests that water temperatures rose so quickly and became so hot that a die-off of organisms occurred in the region. The findings imply that tropical temperatures can significantly rise with future climate change and can reach much higher temperatures than previously suspected.

Earth scientist dr. Joost Frieling at Utrecht University, lead author of the publication, tells: “We already knew that surface seawater in mid-latitude areas reached temperatures exceeding today’s tropical seas during the geological past, during times when atmospheric CO₂ levels were much higher than today. Until now, very limited reconstructions of tropical regions were available. The effect of rising CO₂ concentrations on tropical temperatures, in past and future climates, was hence a topic of active debate and broad uncertainty.”

Hot tub
Frieling and his colleagues reconstructed the history of the surface water temperature using sediments deposited in a shallow part of the tropical Atlantic Ocean that existed about 56 million years ago in present-day Nigeria. During this episode, the Paleocene-Eocene Thermal Maximum (PETM), temperatures outside the tropics abruptly rose by at least five degrees Celsius. Climate scientists have considered the PETM a close analogue to the current climate change. The PETM-episode was similarly associated with a rapid rise in atmospheric CO₂ levels and may thus reveal the long-term consequences of a warming climate.

By closely studying the chemical composition of fossils preserved in the sediments, the scientists determined that the surface temperature of the tropical ocean rose three degrees during the PETM, to a maximum exceeding 36 degrees Celsius. Frieling: “Stepping in water with such temperatures would feel like stepping into a hot tub. If the water temperature was any hotter, we’d likely find it too hot and pull out.”

Heat-stress
Furthermore, the findings show that the high water temperatures had a dramatic effect on dinoflagellates, a group of single-celled organisms that form a crucial part of the ocean’s ecosystem. Their numbers and diversity dramatically decreased at the time when the Atlantic hot-tub developed, an indication of a massive die-off. “It was surprising to see that even the hardy dinoflagellates were diminished, particularly because they appear to have benefitted greatly from warmer water temperatures in areas further from the equator,” Frieling tells.

“Land temperatures likely reached values above forty degrees Celsius on average and extremes far beyond that.”

 
The high water temperatures must also have taken their toll on larger organisms like plants and fish, Frieling thinks. “If tiny single-celled organisms suffered significantly from heat-stress, it can be expected that larger organisms were also negatively impacted. Land temperatures likely reached values above forty degrees Celsius on average and extremes far beyond that. At such temperatures, land plants no longer are capable of functioning and would die off.”

Tropical thermostat
Importantly, the findings show that tropical oceans do not have a maximum upper temperature. This means that a rise in CO₂ levels can result in much higher ocean temperatures at the tropics than was previously thought. “Scientists have previously proposed that a climate mechanism, similar to a thermostat, affects tropical regions and effectively keeps the temperature stable even when the rest of the planet gets warmer,” explains Frieling.

“Our results show that there is no such thing as a tropical thermostat. It represents geological support for future climate model scenarios that project temperatures of tropical oceans to rise significantly with future global warming.”

Article
‘Extreme warmth and heat-stressed plankton in the tropics during the Paleocene – Eocene Thermal Maximum’
Science Advances 2017.
J. Frieling, H. Gebhardt, M. Huber, O. A. Adekeye, S. O. Akande, G.-J. Reichart, J. J. Middelburg, S. Schouten, and A. Sluijs