In the late 1980s, as the world's governments were waking up to the problem of climate change, the mud at the bottom of the ocean near Antarctica revealed a surprise. Earth had lived through rapid global warming before.
About 55 million years ago global temperatures spiked. Then, as now, sea levels rose, the oceans became more acidic, and species disappeared forever.
Little wonder, then, that researchers view this ancient event – known as the "Palaeocene-Eocene Thermal Maximum" or PETM – as a potential goldmine of useful information for understanding modern climate change.
We now know that the PETM was one of the most rapid and dramatic instances of climate change in Earth's history. Its causes are still up for debate, but there seem to be eerie parallels with the causes of modern climate change. What is absolutely clear is that the PETM's effects were far-reaching. It may have altered the course of life on Earth.
The geologists who studied those Antarctic sediments in the 1980s published their findings in 1991. They reported that the shells of tiny planktonic fossils in the muds had betrayed the rapid temperature swings.
The PETM seems to have been caused by greenhouse gases just like modern-day climate change
More precisely, it was the oxygen isotopes locked away in those shells. At around the 55-million-year mark, the amount of "heavy" oxygen-18 in the shells rose relative to "lighter" oxygen-16.
That greater abundance of oxygen-18 is a sure sign that conditions were getting warmer. Water evaporates more readily at higher temperatures, and it's the "light" oxygen-16 that is most easily vapourised. This means that warmer water contains more oxygen-18, and the plankton living in warmer water incorporate more of the stuff into their shells.
Those planktonic shells turned out to be useful for another reason. They hinted at exactly why ocean temperatures rose.
This is because of the carbon they contain.
Today's global warming is not simply a rerun of the PETM
Like oxygen, carbon exists in different isotopic forms. At exactly the same time that the plankton shells became rich in oxygen-18, they also began carrying much more carbon-12 relative to carbon-13. The oceans must suddenly have gained a big supply of carbon-12.
This is something that generally happens after a massive injection of carbon-rich greenhouse gases – carbon dioxide (CO2) or methane – into the atmosphere.
In other words, the PETM seems to have been caused by greenhouse gases just like modern-day climate change.
But today's global warming is not simply a rerun of the PETM. Earth was a very different place 55 million years ago.
One of the biggest concerns today is that the Antarctic ice sheet is shrinking because of climate change. This wasn't a problem during the PETM, because there probably was no Antarctic ice sheet. Even before the onset of the PETM, global temperatures were several degrees warmer than they are now.
All researchers agree that the unusually warm conditions lasted about 170,000 years
Some researchers think the pace of climate change during the PETM distinguishes the event from today too. A controversial study published in 2013 made the case.
Researchers examined another set of muds that formed at the bottom of the ocean 55 million years ago, this time in the north-west Atlantic. They found banding in the muds that they argued was formed by annual cycles.
When they traced the oxygen and carbon isotope blips associated with the PETM, they found that they were contained in just 13 bands. This means, they said, that the PETM temperature surge came in just 13 years.
This does not imply that the PETM came and went in little more than a decade. All researchers agree that the unusually warm conditions, with global temperatures at least 5 °C above average, lasted about 170,000 years.
Modern climate change doesn't have such a dramatic trigger
What it would imply is that global temperatures ramped up to that 5 °C figure in just 13 years. Today, in contrast, global temperatures have risen about 1 °C since the late 19th century.
If PETM climate change really were so rapid, there would be implications for the event that triggered the warming. To create such a rapid rise in global temperature, the atmosphere would have had to be flooded with greenhouse gases almost literally overnight.
Perhaps the release of gases from the melting of a huge carbon-rich comet that flew too close to the Earth would do the trick. Modern climate change doesn't have such a dramatic trigger.
But it's important to stress that many researchers strongly reject that 13-year figure.
There are all sorts of problems with the idea, says Richard Zeebe at the University of Hawaii at Manoa. Most importantly, it is physically impossible for the oceans to heat up that quickly.
Most researchers think the PETM warming really took place over a long period
The Earth's oceans contain a vast amount of water, and heating it up takes time. Even if there was a massive and sudden injection of CO2 into Earth's atmosphere, the oceans simply could not heat up in just 13 years.
"You can heat up the atmosphere relatively quickly, but it takes centuries to millennia to heat up the oceans," says Zeebe.
Other researchers now suggest that the 13 bands in the rock must each represent centuries, not single years. That's if the bands are real at all: some sediment drilling experts say they might simply be an artefact of the drilling process the researchers used to extract the muds.
Most researchers think the PETM warming really took place over a long period, but exactly how long is still up for discussion.
One 2011 estimate suggests that the carbon was released over a period of perhaps 20,000 years.
Such a slow release is very different from today. It might indicate that the greenhouse gases came from the relatively gradual release of gases from volcanic activity.
It looks like the carbon was released into the atmosphere over about 1500 years
Research published in 2014 points to a middle ground. Gabriel Bowen at the University of Utah in Salt Lake City and his colleagues examined the carbon isotopes preserved in soils that formed 55 million years ago in what is now Wyoming.
Whereas the ocean sediments tell us about conditions in the PETM oceans, the soils sample the PETM atmosphere, which responds more rapidly to climate change.
The by-now-familiar surge in carbon-12 popped up again, this time preserved in carbonate nodules that grew in the soil. In this case, it looks like the carbon was released into the atmosphere over about 1500 years: a timescale that looks more similar to today's atmospheric changes.
The ancient soils also indicate the pace of carbon emissions.
The researchers calculated that something approaching 1 billion tonnes of carbon entered the ancient atmosphere each year. That is within an order of magnitude of the current annual release rate of 9.5 billion tonnes.
In light of these findings, the PETM looks like a more reasonable model for today's climate change.
When the oceans warm up a little, vast deposits of methane that are "frozen" in the seabed begin to melt
Bowen and his colleagues made another discovery in Wyoming. They realised that there were actually two distinct pulses of warming 55 million years ago.
A few thousand years before the PETM itself, a vast quantity of carbon-rich greenhouse gases entered the atmosphere from an unidentified source, again at a rate of about 1 billion tonnes per year.
The environment seemed seems to have almost brushed off this "pre-onset event". Atmospheric temperatures rose, but within a couple of thousand years they fell again. Conditions had apparently returned to normal.
The fall in atmospheric temperatures probably came about because the oceans absorbed the heat from the pre-onset event. That might have paved the way for the PETM itself.
When the oceans warm up a little, vast deposits of methane that are "frozen" in the seabed begin to melt. The methane – a potent greenhouse gas – bubbles up, enters the atmosphere and raises global temperatures.
I think that in general the jury is still out
This leads to more ocean warming, triggers more methane release from the seabed, and causes atmospheric temperatures to rise more, and so on. Soon the planet becomes very warm, which is exactly what happened 55 million years ago during the PETM.
Something similar might be happening today. As the modern oceans warm there is good evidence that methane is once again bubbling up from the seabed. The PETM offers us a preview of where that can lead.
However, all of these explanations for the onset of the PETM are still just proposals. There is no scientific consensus on the exact cause of the PETM, beyond the fact that it clearly involved a release of greenhouse gases from somewhere.
"I think that in general the jury is still out," says Bowen.
While the PETM's exact cause is still elusive, its effects are clear
Under the more controversial scenarios, like the idea of a passing comet, the trigger for the event and the pace of climate change have very few parallels with the warming our planet is now experiencing.
Under the more plausible scenarios, like the snowballing release of methane from beneath the sea, the parallels with today are clear.
Regardless, while the PETM's exact cause is still elusive, its effects are clear.
Even back in 1991 when it was first described, it was evident that the PETM was a killer.
Other microbes may have taken advantage of those oxygen-poor conditions
Some of the microfossil species preserved in the Antarctic sediments disappeared as the warming began. The species impacted were those that lived deep in the oceans. They experienced their most severe extinction in tens of millions of years.
Curiously, many microscopic species that lived in the shallower ocean waters actually flourished – an early sign that there were winners and losers as the climate changed.
It was probably a combination of factors that killed the deep-sea species. The warmer temperatures would have been unwelcome, but there may also have been less oxygen available in that warmer water.
However, other microbes may have taken advantage of those oxygen-poor conditions.
Some oceanic sediments from the time contain high quantities of an iron-rich magnetic mineral called magnetite. Some species use magnetite in their bodies: either because of its hardness (it makes good teeth) or its magnetic properties (it can allow some species to orientate themselves with the Earth's magnetic field).
The world's coral reefs faced one of their five greatest crises since they first evolved
Iron can build up in poorly-oxygenated water, so the conditions in the PETM oceans might have led to a radiation of microscopic species using magnetite.
Seawater changed in other ways that were clearly harmful. When the oceans absorb greenhouse gases like carbon dioxide, the process produces a mild acid in the water, lowering the pH: a phenomenon known as ocean acidification. We know it is happening in the world's oceans today, and it happened 55 million years ago too.
Then, as now, ocean acidification was bad news for marine species that build skeletons out of calcium carbonate, because this solid mineral begins to dissolve when the pH drops. Acidification might have been a factor in the deep-sea extinction, and it also affected some shallow living species.
In particular, the world's coral reefs faced one of their five greatest crises since they first evolved 550 million years ago.
There were changes on land too.
In the Arctic, plenty more rain than usual fell during the PETM, probably because stronger ocean evaporation in the tropics delivered more water vapour to higher latitudes.
Seas might have risen by as much as 30m
Geologists have also found evidence, from the styles of rock that formed 55 million years ago, that dry coastal environments were deluged by rising sea levels.
There was little ice to melt, so the sea level rise was probably modest: perhaps in the region of 5m, caused by the expansion of water as it becomes warmer. However, in a worst-case scenario the sea level rise could have been more severe.
For instance, there was magmatic activity in the north Atlantic at roughly this time. That might have warmed up the ocean crust and pushed it upwards, making the oceans shallower than usual and accentuating any sea level rise. Consider factors like this and seas might have risen by as much as 30m.
For life on land, the warm PETM conditions led to dramatic changes.
In Wyoming, plant ranges shifted hundreds of kilometres north as temperatures rose. Conifers apparently disappeared from the area entirely, only returning as temperatures fell after the PETM.
There is strong evidence that about 40% of the mammalian fauna got smaller during the PETM
Some plant species disappeared from the tropics too, but there is evidence that plant diversity actually rose overall here. That may have been a consequence of both the warmer conditions and higher levels of the carbon dioxide plants use to make their food.
The PETM also marks the moment when many of the mammal groups that dominate the world today – including horses, cattle and other hoofed animals – appeared and spread across the northern continents. They probably did so probably in response to the warmer conditions.
But members of these familiar animal groups would have looked odd to our eyes.
"There is strong evidence that about 40% of the mammalian fauna got smaller during the PETM," says Ross Secord at the University of Nebraska in Lincoln. "Nothing appears to have gotten larger."
Some mammals became very small indeed.
In 2012, Secord and his colleagues looked at fossils of Sifrhippus sandrae, a species of early horse that lived in what is now Wyoming.
At the onset of the PETM, when horses first appeared in the fossil record, Sifrhippus was diminutive: it weighed about 5.6kg.
As the temperatures rose, Sifrhippus became even smaller. 130,000 years into the PETM, some adults probably tipped the scales at about 3.9kg: a modest weight for a domestic cat.
At the end of the PETM, as temperatures dropped, Sifrhippus grew again.
Other mammalian herbivores shrank too, and so did some mammalian carnivores.
These size changes might be down to something called Bergmann's rule, says Secord. This says that warm-blooded animals tend to be relatively small in warm regions and larger in cold ones.
When atmospheric CO2 levels rise, the leaves and shoots of plants may become less nutritious
That could be because, in cold regions, it is useful to have a larger body – and a smaller relative surface area – to prevent losing too much body heat.
But Bergmann's rule is usually used to explain why animals in the tropics are smaller than those at higher latitudes, not to explain why animals grew to different sizes as a response to global warming.
Other researchers have suggested other reasons for the PETM changes in mammal size.
In 2013, Philip Gingerich at the University of Michigan in Ann Arbor suggested the mammals might have been responding to changes in vegetation brought on by the PETM.
We have learned a lot about the PETM in the quarter-century since its discovery
When atmospheric CO2 levels rise, the leaves and shoots of plants may become less nutritious and harder for herbivores to digest. If that happened during the PETM, it could have led to slower animal growth, and herbivores might have begun to shrink. Carnivores, forced to target smaller prey, might have followed suit.
Peter Stiling at the University of South Florida in Tampa has investigated this. In 2007 he found that, in a high-CO2 atmosphere, oak leaves did carry less nitrogen. "As a result, herbivores often eat more to compensate," he says.
But there's no direct evidence as yet that herbivores, or the carnivores that eat them, grow more slowly and become smaller adults as a consequence.
We have learned a lot about the PETM in the quarter-century since its discovery, but clearly there are plenty of questions left to answer.
Our particular branch of the primate tree had flourished to such a degree that the world really had become the planet of the apes
One of the most intriguing is whether the warming 55 million years ago was instrumental in the evolution of the first true primates: the group that ultimately gave rise to our species.
Modern primates appeared and spread at the beginning of the PETM, alongside horses and other hoofed animals. Their early fossil record is patchy, but they appear at almost exactly the same time in Asia, Europe, the Americas and Africa.
Within a few tens of millions of years, our particular branch of the primate tree had flourished to such a degree that the world really had become the planet of the apes. About 5 million years later, the first upright apes we recognise as our direct ancestors appeared.
Would primates have become so successful if the PETM had never happened? No one can say for sure.