Scott Wing spent more than a decade in the badlands of Wyoming's Bighorn Basin, most of it thirsty, sunburned, and down on his hands and knees, digging endlessly through the dirt.
But he had never found anything like the fossil he now held in his hand: an exquisitely preserved leaf embossed on beige rock. Wing let out a jubilant laugh as he uncovered a second fossil and then a third. Each leaf was different from the others. Each was entirely new to him.
And then he started to cry.
This was exactly what he'd been searching for. When these strange fossils formed 56 million years ago, the planet was warming faster and more dramatically than at any point in its history - except the present.
Recounting the moment recently in his office at the Smithsonian's National Museum of Natural History, Wing recalled the uneasy reaction of the field assistant with whom he'd been hiking. The young man looked understandably nervous that his supervisor was shedding tears over a handful of rocks.
"I said, 'You just have to realise, I've been looking for this . . . since you were a kid. I'm unreasonably happy right now, but I'm not crazy,'" Wing chuckled. "So, that was the first really good set of plant fossils from the PETM. It was definitely a moment that I won't forget."
The PETM is the Paleocene Eocene Thermal Maximum - an ungainly name for the time that's considered one of Earth's best analogues to this era of modern, human-caused global warming. In a matter of a few thousand years, huge amounts of carbon were injected into the atmosphere, causing global temperatures to rise between 5C and 8C.
The rapid climate change disrupted weather, transformed landscapes, acidified oceans and triggered extinctions. It took more than 150,000 years for the world to recover.
If history is allowed to repeat itself, the consequences for modern life could be similarly long-lasting - which is why Wing is so determined to understand this ancient climate catastrophe.
"To me, it doesn't lead me to be fearful," Wing said. "It leads me to feel responsible. It leads me to feel that we need to be more informed."
The first major evidence for the PETM was uncovered in the early 1990s by scientists looking at the transition from the Paleocene, the epoch after the extinction of the dinosaurs, to the Eocene, when modern mammal orders first emerged.
There was something strange about the thin band of sediment that represented the boundary between these two epochs: its ratio of carbon isotopes - different forms of the same element - was skewed. Further research revealed that between 4 trillion and 7 trillion tons of carbon - the rough equivalent of the planet's entire current reserve of fossil fuels - had flooded the atmosphere in this period. It came from the decomposed remains of ancient algae and plants, so it contained a larger amount of carbon 12 - the isotope that is preferred for photosynthesis.
This "spike" in carbon 12 served as a marker of the PETM and allowed researchers to start tracking the effects of this sudden climate shift in rocks and fossils around the world.
Chalk deposits at the bottom of the ocean began to dissolve as carbon dioxide made seawater more acidic. Fossils of tiny, deep sea-dwelling creatures showed evidence of an oxygen shortage - a sign that the water was getting warmer. Everywhere in the ocean, creatures adapted to the changed environment - or they died out.
On land, mammals got smaller. Ancient ancestors of horses, tiny to begin with, shrank 30 per cent to the size of house cats. Abigail Carroll, a paleoclimatologist at the University of New Hampshire, said this was probably an adaptation to the warmer weather: smaller bodies are easier to keep cool.
Weather also got wilder. Geologists have uncovered huge rocks that were carried long distances by intense floods - which happens when dry spells are followed by extreme rains.
And then there are the plants in Wing's collection at the National Museum of Natural History. Before the PETM, fossils suggest, Wyoming looked more like Florida - a lush, subtropical forest shaded by stately sycamores, silvery birches and waving palm trees.
But as the world warmed, the Bighorn Basin transformed. The fossils Wing finds from this period belong to plants that typically grow in hot, arid places even farther south - spindly bean plants and relatives of poinsettia and sumac. These plants must have migrated north when the weather changed, following their preferred environment to ever higher latitudes.
A swarm of ravenous herbivores apparently followed. Many of Wing's fossils are perforated with bite marks left by insects more numerous and diverse than the ones that preceded them.
The source of all this mayhem remains uncertain. Some have suggested the flood of carbon that set off the PETM came from volcanic eruptions or even a comet impact. But the most popular theory suggests that reservoirs of solid methane buried in seafloor sediments were released when the ocean's temperature and chemistry changed.
Methane is a potent greenhouse gas, short-lived but harder-hitting than carbon dioxide. Once it set global warming in motion, the rising temperatures may have triggered the release of even more methane and unlocked additional carbon sources - wildfires, shifting ocean currents, soil microbes that breathe out greenhouse gases - in a chain reaction that changed the planet.
To scientists today, many of the phenomena observed during the PETM will feel familiar - so familiar "it's almost eerie", Wing said.
Humans burning fossil fuels have produced the same kind of carbon isotope spike researchers find in 55-million-year-old rocks. The ocean has become about 30 per cent more acidic and it's losing oxygen - changes that are already triggering die-offs.
The world has witnessed dramatic weather extremes: deadly heat waves, severe storms, devastating droughts. In response to these shifts, plants and animals are showing up in new places at unusual times. There's even evidence that some species, such as birds called red knots, are getting smaller as a result of the warmer climate.
Still, the past is an imperfect predictor of what might happen as the modern world continues to warm. For one thing, Earth on the eve of the PETM was already much hotter than it is today. With the poles unfrozen and the sea levels high, ancient creatures didn't have to worry about the effects of melting ice, as we do today.
And the pace at which we are changing the climate outstrips anything in the geologic record. The carbon surge that set off the PETM unfolded over the course of as long as 5000 years. At our current rate, humans will produce a comparable surge in a matter of a few centuries.
"In all the major ways it's more perilous now than it would have been then," Wing said.
But for scientists trying to predict our peril, the PETM is an invaluable reference. Jeff Kiehl, a senior scientist at the National Centre for Atmospheric Research, uses research by Wing and others to test models of the interplay between carbon and climate.
"We don't have data for the future but we do have data from the past," Kiehl said. "This is where Scott's work has played a critical role."
Data from the PETM and other times of global warming can be used to answer the questions that haunt modern climate scientists: How much will the Earth warm if atmospheric carbon doubles? What will happen to the world's water as a result? How long will it take for things to return to normal?
This week, Wing and his colleagues at the Smithsonian have gathered 17 experts for a symposium on ancient climate. Over two days, they will try to reconstruct a timeline of Earth's temperature and atmospheric carbon levels since complex life began roughly a half-billion years ago.
"Science has finally gotten us to a point where we have some idea of what the consequences are of the things that we do," Wing said. "Now the question is, can we use that knowledge in something that starts to approach a wise way?"