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Climate 101is a Mashable series that answers provoking and Seung Ha (승하) Archivessalient questions about Earth’s warming climate.

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Big snakes, alligators, giant tortoises, and flying lemurs thrived in a balmy Arctic some 50 million years ago. It was a time when the potent greenhouse gas carbon dioxide had spiked in Earth's atmosphere, making the high polar regions downright tropical.

Scientists who investigate past climates, called paleoclimatologists, have collected bounties of evidence that CO2 has long been a dominant lever on Earth's temperature. The evidence exists in chemicals stored in fossils, which indicate how much CO2 once saturated the atmosphere. Now, paleoclimate researchers have published the most comprehensive history to date of Earth's past CO2, starting after the dinosaurs went extinct some 66 million years ago (likely from an asteroid impact). The research, showing the strongest link yet between past CO2 levels and global temperatures, was recently published in the scientific journal Annual Review of Earth and Planetary Sciences.

The 66 million-year geologic story shows an overall trend of gradual, naturally declining CO2 over tens of millions of years, concluding in the geologically recent ice ages. Crucially, this history also reveals the extreme, unnatural, skyrocketing rise in CO2 levels over the last 150 years.

"CO2 has of course changed before, but it's happened in slow and predicable ways," said James Rae, a paleoclimatologist from the University of St Andrews who led the new research. "What's happening now is so much faster than anything in the geologic record. There's nothing in comparison to what’s happening now."

What's happening now is humans have grown extremely proficient at digging up prodigious amounts of some of the most carbon-rich materials on Earth ("fossil fuels") and are burning them. Much of this carbon ends up in the atmosphere. "You couldn't design a better way to put more CO2 into the atmosphere," Rae said.

"You couldn't design a better way to put more CO2 into the atmosphere."

Crucially, the latest research also demonstrates a strong correspondence between CO2 levels and past reconstructions of global temperatures. In short, when CO2 levels were high, so was temperature; when CO2 levels dropped, so did temperatures.

"This paper is a tour de force, an overview of what we know about the history of CO2 over last 60 [million years] that is broad in scope and comprehensive in its level of detail," said Maureen Raymo, a paleoclimatologist and marine geologist at Columbia University. It also paints a picture of modern CO2 trends. "Humans are now a geological scale force of nature" as we exert control over the planet's climate, Raymo, who wasn't involved in the research, added.

The chart below shows the strong link between past CO2 levels and global temperatures.

The clear CO2-temperature link. Credit: Annual Review of Earth and Planetary Sciences via James rae

The evidence

To determine ancient climes from tens of millions of years ago, paleoclimatologists use fossilized evidence, like dead creatures preserved in ocean mud. These are called "proxies," a term that refers to preserved materials that provide clues or evidence of past environments. For a view of CO2 over the past 66 million years, researchers employed chemical evidence stored in tiny, common, and widespread fossilized shells (called foraminifera) and organic compounds (called alkenones) produced by algae.

These critters are sensitive to changes in the ocean's CO2 chemistry (including acidity), which is controlled by how much CO2 is in the atmosphere. (Oceans soak up CO2 from the air, naturally forming carbonic acid, which makes the oceans more acidic — a process that's happening as you read this.) It's a reliable way to sleuth out Earth's past climate.

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"We're doing environmental forensics," explained Rae. "We're looking at the fingerprints environmental change leaves on the geologic record."

"We're doing environmental forensics."

The foraminifera are a good example. They're made mostly of calcium carbonate, similar to oysters and corals. As the shells grow in the ocean, they mix in a handful of other atoms. One of these is the element boron, which changes slightly in different ocean acidities. Evidence of a more acidic ocean, stored in the fossils, equates to higher atmospheric CO2 levels. This allows paleoclimatologists, like Rae, to construct past CO2 trends over millions of years.

Scientists can also estimate what global temperatures were once like, and compare this to CO2 levels. Past temperatures are sleuthed out by analyzing past sea levels (preserved by ancient beaches), where certain fossils are found (it was clearly quite warm when alligators flourished in the Arctic), the chemical makeup of fossilized shells, and beyond.

In this latest comprehensive research, there's a clear, compelling connection between past CO2 levels (measured by marine proxies) and global temperature.

"Rae and coauthors apply state-of-the-art understanding of how the marine CO2 proxies really work, leading to a much refined CO2 record," noted Tom Marchitto, a paleoclimatologist at the University of Colorado Boulder who had no role in the research. "Its correspondence with global temperature over the past 66 million years is very striking, and supports CO2 as the principal driver of global climate change."

Importantly, paleoclimatologists don't rely on any single proxy to reveal environmental conditions in the deep past. They use multiple proxies, like the shells and algae used in the new research.

"These two methods produce very similar results, lending confidence that they are reliable proxies for atmospheric CO2," explained Kathleen Johnson, a paleoclimatologist at the University of California, Irvine, who had no role in the study.

A foraminifera shell as seen through a microscope. Credit: University of St Andrews

But why stop this particular CO2 investigation at 66 million years ago (for now, anyway)? Well, as you know, Earth experienced a dramatic extinction event at the time, significantly altering ecosystems and the life therein. Famously, the dinosaurs disappeared. Researchers simply have a much better understanding of life (like foraminifera shells) since that dramatic, asteroid-triggered environmental change on Earth, explained Rae.

But 65 or 66 million years is more than enough time to establish what's largely controlled the planet's climate. As Mashable previously reported:

"CO2 levels were extremely elevated during the age of the dinosaurs (which ended 65 million years ago), perhaps at some 2,000 to 4,000 ppm. Tremendous CO2 emissions, from incessant and extreme volcanism, heated Earth and allowed dinosaurs to roam a sultry Antarctic. But over millions of years, Earth’s natural processes (specifically the slow, grinding, but potent process of rocks absorbing CO2 from the atmosphere, dubbed 'the rock thermostat') gradually reduced CO2 levels."

Eventually, by the start of the Industrial Revolution, CO2 levels hovered around a stable 280 parts per million, or ppm. Since then, they've shot up to around 416 ppm. That quick jump is unprecedented in the geologic record.

"We have become a geologic force."

"Geologic records like this one present us with some loud warnings," said Marchitto, noting that today's CO2 levels are the highest they've been in some 3 million years. (Sea levels then were some 30 feet higherthan today, but possibly much more). "The history of the human race has never seen anything like this, and it continues to climb," Marchitto said.

What's more, the rate of atmospheric CO2 is rising much fasterthan at any point since the dinosaurs died. How much faster? "We can confidently say that the rate of anthropogenic CO2 rise is around 10 times faster than the fastest natural CO2 change seen over the last 66 million years," said Johnson.

Humanity's having a potent influence.

"We have become a geologic force," said Marchitto.

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