That carbon belching from our factories causes global warming is well-known, but beyond that, the science becomes controversy. The details of how much a unit of carbon dioxide raises global temperature is hotly debated in climate change literature.

The value most accepted comes from the Intergovernmental Panel on Climate Change, which finds that temperatures will rise by 1.5 to 4.5 degrees Celsius with a doubling of CO2. And the threshold for "catastrophic" climate change that would terminate lifestyles as we know it is 2 C.

But this conclusion, like all derived from scientific endeavor, is uncertain. People who doubt climate change have picked up on the uncertainty to suggest that hidden factors—the "unknown unknowns"—in the climate system may come into play as the Earth grows warmer. And these factors could suddenly alter the way the planet responds to CO2 and make it pretty near impossible to predict how warm or cool the world will be in a hundred years.

But a new study published in Nature challenges this argument by exploring warm periods in Earth's deep history. The study finds that the Earth's climate in the past responded to CO2 in a manner similar to today; hidden factors did not suddenly kick in to alter the relationship between CO2 and temperature.

The "factors" are natural phenomena such as water vapor, clouds, sea ice, dust and vegetation—all of which exert varying pulls on the climate. Water vapor and sea ice, for instance, are potent warming agents, while dust and sea ice are cooling agents.. When scientists use computer models to simulate climate change, they translate all these factors into into code and enter them into Earth's virtual reality.

But using modeling to understand how temperatures respond to CO2 can be challenging. That's because models depend on how well the computer codes reflect the natural world, David Lea, a paleoclimatologist at the University of California, Santa Barbara, who was not affiliated with the study, said in a phone interview.

Paleoclimatologists instead look into Earth's history to a time when the planet was warmer than it is today, about 3 million years ago. It was the Pliocene, an epoch when the earliest human ancestors evolved in Africa. The world was 2 to 3 C warmer than today, and CO2 levels were roughly similar, as well, the Nature study finds. The Arctic was free of sea ice.

Decoding Earth's CO2 history
It is quite difficult to reconstruct temperatures 3 million years ago, of course. For this, scientists rely on proxies—traces of climate's imprint contained in sediment collected from ocean beds or from the shells of creatures that lived back then. Scientists travel the oceans and drill into the deepest seabeds to collect such data.

In the Nature study, the scientists focused on getting data on CO2 levels in the atmosphere back then. Doing so stretched them to the limits of experimentation; the oldest direct measurements of CO2 (captured in tiny bubbles of air trapped within polar ice) stretch back only 800,000 years.

"Beyond that we have to rely on indirect, or proxy, methods to reconstruct CO2," said Miguel Martinez-Boti, a paleoclimatologist at the University of Southampton and the lead author of the study, in an email.

Martinez-Boti and his colleagues dug out sediments from the Caribbean Sea and tropical Atlantic Ocean. From the sediments, they isolated microscopic marine creatures called foraminifera that lived and died during the Pliocene. The shells of these creatures contained boron, and analyzing this element revealed to the scientists the levels of CO2 in the atmosphere.

Using the bugs as proxies, the scientists assembled a comprehensive record of CO2 in the atmosphere from 2.3 million to 3.3 million years ago. The CO2 fluctuated between 350 and 400 parts per million, reaching a maximum of 450 ppm at one point. In comparison, the world presently is hovering at 400 ppm.

The CO2 data set was called fantastic by Ana Christina Ravelo, a paleoclimatologist at the University of California, Santa Cruz, who was not affiliated with the study.

"In my mind, this is probably some of the best data that exists from this time period," she said in a phone interview.

Searching ice ages for clues
Martinez-Boti and his colleagues went a step further and looked at how CO2 levels affected temperature in the Pliocene. And they also looked at whether this relationship remained constant at other points in Earth's history.

The ice ages of the last 800,000 years are well-known for the mammoths and mastadons that roamed North America, but that is not the only attraction for paleoclimatologists. High-quality measurements of CO2 and temperature are available for the epoch.

So Martinez-Boti and his colleagues chose this epoch for study, and they initially found that CO2 raised temperatures by more than in the Pliocene. But when they accounted for the greater ice-sheet coverage during the ice age (ice sheets are cooling agents), they found both epochs were roughly identical.

In both, a doubling of CO2 resulted in raising temperatures by about 3 C.

This was similar to the IPCC's proposal that temperatures will rise by 1.5 to 4.5 C per doubling of CO2.

"This suggests that the IPCC range is adequate to explain the climate response in the near future as we approach Pliocene-like conditions," Martinez-Boti said.

An unprecedented experiment
So how does this finding negate the suggestion that "unknown unknown" climate factors might influence a warmer world, making it nearly impossible to simulate the future using climate models? This suggestion has been made by Judith Curry, a professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology, and others (ClimateWire, Sept. 26, 2014).

The study shows that when temperatures in the Pliocene rose by 3 C, additional unknown climate factors did not suddenly kick in, Martinez-Boti said.

There are caveats to the study's findings. Paleoclimate studies of this sort depend on how well scientists can reconstruct past conditions using proxies. The scientists have put together a great CO2 data set for the Pliocene, but they used temperature data sets from literature that may be imperfect, said Ravelo of UC Santa Cruz.

"We still don't have a great idea, a really accurate idea, of what temperature was globally," she said.

And although the past does provide a remarkable lens into our planet's future, there is a big difference between the pace of climate change today and the changes back then, said Lea of UC Santa Barbara.

"The experiment we are doing right now is pretty much unprecedented in Earth's history," he said. "These changes in the Pliocene and the [ice ages] took place over a million years, and we are changing the planet now on time scales of hundreds of years."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500