The PETM: One true analog for understanding global warming?

by Eric Ellman

Micro-fossils from an era that looks like the Intergovernmental Panel on Climate Change’s worst projections for our future show a robust and complex marine environment, says Andrew Ridgwell, one of the world’s leading Earth system modelers and a visitor to Yale as part of the Flint Lecture Series from February 22-25.

Ridgwell notes that plankton – the essential base of the ocean’s food chain – have previously adapted to the sort of 5-6 degree Centigrade global temperature increase expected to result if the world continues on its “business as usual” emissions curve.  Amidst the usual doomsday scenarios of a world destroyed by climate change, it is comforting to imagine coral reefs and clown fish thriving in a drastically warmed world.

Except that maybe they won’t.  Not until we understand how fast CO2 accumulated during the Paleocene-Eocene Thermal Maximum (PETM), says Ridgwell, should we draw conclusions from it. 

The PETM, which began approximately 55 million years ago (Ma), is arguably the least studied, most important era in Earth’s history. Though it occurred under entirely different circumstances from today – during a hot house climate, and before North and South America were connected by a land bridge – no other warming period functions better as an analog for the experiment mankind currently runs on planet Earth.  If past is prologue, this is the climatic episode we want to understand to better know our future.

How resilient will the planet’s ecosystems prove to be after we’ve pumped similar amounts of CO2 as were responsible for PETM warming into our atmosphere by the middle of the next century? That is the question that concerns Ridgwell, an upbeat Brit who ponders the mechanics of past hothouse climates from sunny headquarters at the University of California Riverside. Did the PETM’s increased levels of CO2 suddenly evolve over a few hundred years (as we produce it today)?  Or did it accumulate over tens of thousands of years, affording time to adapt for marine life? 

Frilly pink and blue “My Little Pony” paraphernalia decorate Ridgwell’s office, adding an artificially buoyant element to ominous realities he considers in his programs.  Using his signature Grid ENabled Integrated Earth System Model (GENIE), and the presence or absence of sand grain-sized fossil Coccolithophorids, the leading calcite producers of the ocean, he uses residue of the past to conjecture on the future of Earth’s oceans.

A recent publication[1] used PETM-aged coccoliths to suggest surprising adaptive abilities. Counter to what had previously been assumed, the calcareous organisms thrived in the past, migrating to cooler high latitude waters, despite lower calcium carbonate saturation levels.  Corals too, he says, have surprising resiliency given enough time: fossil records show them disappearing repeatedly during hothouse climates, only to reappear several million years later.

Ridgwell is gloomier about the present. The current El Niño, he speculates, is probably decimating coral reefs right now. Higher temperatures stimulate corals to eject their dinoflagellate symbionts, he says, speculating that the current trajectory of global warming will probably eliminate reefs in the next 100 years, adding, “we’re gradually dynamiting and destroying them anyway.”  Things further up the food chain will eventually look vastly different, he says, less because of climate change than as a result of human predation, and the eventual evolution of those lucky species “that no one wants to fish out.”

Ridgwell wonders why more people aren’t collectively investigating the PETM, our most useful analog for CO2-driven global warming. Thousands of scientists study the more recent inter-glacials, he says, which were controlled by fluctuating solar radiation. Only a handful of people investigate the far more relevant PETM, when a similar volume of the same greenhouse gas that mankind will likely emit over the next century caused temperatures to soar.

A better understanding of PETM dynamics might clarify nagging challenges to current models regarding soil carbon and methane hydrates, says Ridgwell.  But clearly it’s the PETM as analog that excites him. 

“This is the future.  This is the real deal.  Big release of CO2, temperatures go up.   It’s a sadly familiar story.”


Andy Ridgwell speaks at Yale on Monday, Feb. 22 and Thursday, Feb. 24.  He offers a free one-day workshop on the use of the GENIE Earth systems model on Tuesday, Feb. 23.  For further details consult the YCEI calendar.  To register for the workshop, contact