By the time it ended, 75 percent of the world's marine organisms were extinct. But very slowly the planet began to warm, gradually eroding the mega-continent of ice around the South Pole, and many of the tiny creatures and plants that had died out were replaced by new organisms. Life began anew on planet Earth.
It happened, not because of a cataclysmic impact by a foreign body, like the one believed to have wiped out the dinosaurs. It happened simply because the planet's climate began to change, apparently fairly quickly, but not nearly as fast as the changes we are seeing today.
It's not particularly important that all of this happened around 440 million years ago. What's important is that the consequences were rapid, at least on a geological time scale, and they were nearly total.
For the first time scientists are able to measure how intense it all was.
"The big breakthrough here is the ability to tell how much of this climate change was felt by temperature change in the tropical oceans, and how big the ice sheets got," Woodward Fischer, assistant professor of geology at the California Institute of Technology, said in a telephone interview.
"We can really tell you how intense this glaciation was, and the thing that's so remarkable about it is there's a relatively large temperature change associated with it" of about 10 degrees Fahrenheit.
That change in average global temperature peaked near the end of what is known as the Late Ordovician period, and it coincided precisely with the second largest mass extinction in the history of the planet. The moral is pretty clear: Climate change can wreak havoc around the Earth, especially if the change happens relatively quickly.
For many years scientists have tried to figure out just what the global climate was like, but obviously 440 million years is a long time ago, and while fossils from that era have been found around the Earth the actual temperature seemed impossible to pin down. To do that, they needed to know how much water was locked up in long-gone ice sheets.
A team of nine scientists from five different institutions convened at Caltech to tackle the problem. Using a new "paleothermometer" developed at Caltech, they analyzed the isotopic chemistry of fossilized marine animal shells collected from Canada and the Midwestern United States. The new technique measures "clumpiness" of heavy isotopes in fossils. The lower the temperature, the clumpier the isotopes. That allowed them to determine the climate, and they thus could figure out the thickness of the ice.
It turns out that the ice cap at the South Pole would have dwarfed what is now Antarctica. It spanned across land masses that later drifted to the equatorial region and became Africa and South America.
"The sea level was probably as high as it's been in the last 500 million years," said Seth Finnegan, a postdoctoral researcher at Caltech and first author of a study published in the journal Science. "Most of the continents were extensively flooded with very shallow inland seas. We don't really have anything comparable to it in the world today. It was a rich habitat for marine life."
But as the ice sheet at the South Pole grew, it sucked water out of those seas, drying the habitat and killing its inhabitants. The world became a very cold, and largely lifeless, place.
Scientists want to learn how all that happened because it may help us understand "the potential climate change we may be headed into," Finnegan said. It's important "to understand how the global biota seems to respond to different changes in the climate system through time."
Yet the world, and its atmosphere, and even the sun were quite different during the Late Ordovician period. Even opposites, in some ways. Scientists have estimated that the earth's atmosphere at that time contained up to 16 times the amount of carbon dioxide that it does today.
Today, the concern is that the amount of carbon dioxide is growing, causing a greenhouse effect that will warm the planet for the next few centuries. So why was the earth a big snowball way back when the atmosphere was much richer with carbon dioxide? The answer shows just how remarkable our planet is.
"As you go back in time, our sun was actually dimmer, and it's getting brighter with time," Fischer said. "The earth has kind of a built in thermostat that sets the carbon dioxide level at what it needs to provide the climate conditions" suitable for life. "That means liquid water on the surface of the earth."
So as the sun brightened over time, carbon dioxide was absorbed by rocks and plants ranging from algae to, eventually, forests, keeping the climate from getting too hot. But that cuts both ways. Some scientists have speculated that a sudden drop in carbon dioxide could have caused the earth to turn bitterly cold in the so-called "snowball earth" periods of glaciation that led to ice ages of varying intensities.
So too little carbon dioxide and the earth turns cold, too much and it turns hot, sustaining life most of the time, but occasionally destroying habitats and all those who depend on them.
That's essentially what this seemingly arcane research is all about.
"When you look back in time you realize that extinction of species has been highly non-random," Finnegan said. "There have been times when many, many have gone all at once, and it's hard not to wonder why that is."
"We are looking at a mass extinction, and there's always the question of how flexible, how plastic, is the biology," Fischer added. "Is there time to adapt?
"We are looking at a time when the climate change was rapid, at least on the geologic time scale we can measure, but it still may not have been anything near the pace that we see in climate change today. None the less, the biology felt this climate change" 440 million years ago.
Far from the polar ice sheet, in a relatively warm area near the equator, the temperature plunged several degrees. That may have taken several hundred thousand years, but it was enough to wipe out most marine organisms.
We're not heading for another snowball earth. But wherever we're going, it isn't going to take several hundred thousand years. And if a faster pace means greater impact, the consequences of the current warming trend could be severe.