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Shells say tropics once got hot enough to kill

As the world warmed millions of years ago, conditions in the tropics may have made it so hot some organisms couldn’t survive.

Theories dating to the 1980s suggest that as the rest of the earth warms, tropical temperatures would be strictly limited, or regulated by an internal “thermostat.”

These theories are controversial, but important because the tropics and subtropics make up half the earth’s surface area, greater than half of the earth’s biodiversity, and over half its human population.

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.

Will hotter temps cause a mass exodus from the tropics?

The Paleocene-Eocene Thermal Maximum (PETM) period 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. The new study, published in the journal Science Advances, offers the first convincing evidence that the tropics also warmed by about 3 degrees Celsius (5 F) during that time.

“We don’t find 50-million-year-old thermometers at the bottom of the ocean.”

“The records produced in this study indicate that when the tropics warmed that last little bit, a threshold was passed and parts of the tropical biosphere seems to have died,” says Matthew Huber, professor in the earth, atmospheric, and planetary sciences department at Purdue University. “This is the first time that we’ve found really good information, in a very detailed way, where we saw major changes in the tropics directly associated with warming past a key threshold in the past 60 million years.”

The study is unique because of the quality of the geological records utilized. Geological records from the PETM are difficult to find, especially from an area of the tropics, Huber says. The research was based on a shallow marine sedimentary section deposited in Nigeria.

“We don’t find 50-million-year-old thermometers at the bottom of the ocean,” Huber says. “What we do find are shells, and we use the isotopes of carbon and oxygen within the shells, complemented by temperature proxies from organic material, to say something about the carbon cycle and about the temperature in the past.”

Only carnivores could handle Late Triassic tropics

Scientists used two research methods 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.

If the tropics are not able to control temperature and don’t possess an internal thermostat, that should reshape future thinking about climate change, Huber says.

“If you say there’s no tropical thermostat, then half of the world’s biodiversity—over half of the world’s population, the tropical rainforests, the reefs, India, Brazil—these populous and very important countries have nothing to prevent them from warming up substantially above conditions that humans have been used to.”

The trends in temperature increases in the tropics are similar to those found in other parts of the world, but other records have been very sparse and limited until now.

The National Science Foundation funded the work. The NSF-supported National Center for Atmospheric Research developed the model used in the study.  Computing was provided by ITaP’s Research Computing.

Source: Purdue University

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