We’ve got some news out of the Paleocene-Eocene Thermal Maximum (PETM) period, and it’s not great for our climate outlook.
It’s not news that temperatures in the tropics have gotten too high at certain points in time for much in the way of life to survive — the 5-million-year-long dead zone of the end-Permian boundary is a good example — but it wasn’t known until recently that one such period was “only” 56 million years ago.
That’s exactly what new research based on newly obtained geological records suggests — that temperatures in parts of the tropics warmed enough at that time that large parts of the tropical biosphere died.
These findings seem to contradict the popular theory that tropical temperatures are limited/regulated to a large degree by an “internal thermostat” — that is to say, that feedback loops and circulation patterns limit the extent of possible warming in the tropics.
As an explanation here about why these findings matter, it should be remembered that the tropics are home to around half of the world’s human population, much of its biodiversity, and much of its remaining good-quality soil (which is eroding at a rapid rate). Despite common fantasies, the “soils” of most of Canada and Siberia are not of the sort that can support intensive agriculture — large-scale agriculture isn’t simply going to move north as temperatures in the tropics continue rising, it’s going to collapse (compounded by extremely high rates of soil erosion and synthetic fertilizer shortages).
The press release notes: “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° Celsius (9° Fahrenheit), from an already steamy baseline temperature, and this study provides the first convincing evidence that the tropics also warmed by about 3° Celsius (5° Fahrenheit) during that time.”
“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,” commented co-author 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.”
What sets this study apart from many earlier ones is the quality of the geological records that were used — which were obtained via a shallow marine sedimentary section deposited in what’s now Nigeria.
“We don’t find 50-million-year-old thermometers at the bottom of the ocean,” Huber noted. “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.”
It’s surprising that they can’t find 50-million-year-old thermometers, but hey, do what you can.
The press release continues: “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. … 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.”
Huber noted that this means that some assumptions about future anthropogenic climate change need to be reshaped.
“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,” he continued.
To put that more bluntly, there’s (if the new work is to be believed) nothing to suggest that anthropogenic climate change won’t lead directly to much of the tropics turning into a dead zone of sorts — one that humans (if they’re still around) likely won’t be able to live in.
The new findings are detailed in a paper published in the journal Science Advances.