Heeding Heated History

Short version: Increasingly strong evidence shows that natural changes in carbon dioxide have been the main control on Earth's climate history and that the climate changes have greatly affected living things.

Friendlier but longer version: During the late 1700s and early 1800s, scientists were building the geologic time scale, drawing “lines” to separate history into blocks of time that could be given names. Fossils showed the species that lived at different times, and the lines were usually drawn when many species became extinct before new species evolved to take over the “jobs” left vacant by the extinctions. Those early geologists didn’t know why the species went extinct, but they knew that something big happened.

Since then, an immense amount of effort has gone into learning what happened. In one case about 65 million years ago, a giant meteorite impact killed the dinosaurs and ended the Mesozoic Era, to start the Cenozoic Era. Changing climate was responsible in other cases, and climate changes may prove to have been the main drivers in most of the big extinctions. Climate change was probably very important in how the meteorite killed the dinosaurs, too; for most of them, it didn’t fall on their heads but instead blocked the sun with dust it kicked up, causing great cooling for a few years, among many changes.

We’ll look briefly at three big changes, and then see what they say when viewed with the rest of climate history. Don’t worry about memorizing names and dates we’ve already given or the ones coming unless you’re really into that; just get the sense of the story.

Video: Continental Glaciation (3:27)

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Estimates of the history of CO₂ over the last 400 million years (MA), with today on the right. The shaded band is output from a model calculating CO₂ from inputs such as volcanic eruptions and outputs such as fossil-fuel formation; the other curves represent estimates of atmospheric CO₂ from different techniques applied to sedimentary deposits. The bars hanging down from the top show the extent of ice at low elevation (very high mountains can have ice even in times with warm temperatures at low elevations where most people live, but low-elevation ice requires cooler temperatures in most places); times with no bar had no ice at low elevation.

Click for a video transcript of "Continental Glaciation".

DR. RICHARD ALLEY: This wonderful plot is from the IPCC and other places. This is a different scale. Today is over here on your right, and the 400 over here is 400 million years-- not 400 years, 400 million years. So this is really deep time.

And what you have plotted on here are two different things. At the top, hanging down in blue is the extent of glaciers at a time. And so there were no ice on the planet, basically at sea level. And then there was a little blip of glaciers, and they went away. And then the glaciers went way down towards the equator-- not all the way by any means-- but they got more than halfway there. And then they melted away into a time with no ice near sea level. And then the glaciers have come back, and we have ice in Antarctica and Greenland today.

So there's the history, and you can think of this as a history is temperature. There was no because it was warm. There was ice because it was cold. There was no ice because it was warm. There was ice because it was cold. OK.

Shown below is the history of CO2. And what you'll notice is when there was no ice, CO2 was high, and this is estimated in various ways. But what you have here is this high CO2 back here in a no ice time. And then when CO2 got low, the ice had grown. And when CO2 went back up to being high, the ice had melted away. And when CO2 got low again, the ice had grown back. And it turns out there's actually a little dip in CO2 right here that goes with this little blip of ice.

And so what we see is a very nice relationship-- high CO2, little or no ice; low CO2, lots of ice. Furthermore, we understand from processes that you can read about in our course and elsewhere, that it is the CO2 causing the changes in ice and not, primarily, the ice causing the changes in the CO2. And this is something you just can't see from this correlation, but we get it from other sources.

Now, try to walk you through a few events in climate history. We're going to start with this one back here, The Great Dying-- a time when volcanic CO2 raises the temperature and seems to have made it so hot near the equator that large creatures couldn't live there. We then will walk you into the Paleocene-Eocene Thermal Maximum-- a time when some formerly living carbon came out of C4 methane or other sources, belched out fairly rapidly and made it warm. And we'll finish up with the Ice Ages. This is a time when features of Earth's orbit have driven temperature changes, but those temperature changes have been amplified and made global by CO2.

Credit: Dutton Institute(link is external). "EARTH 104 Module 5 Continental Glaciation(link is external)." YouTube. November 19, 2014.

Source: From CCSP, 2009: Past Climate Variability and Change in the Arctic and at High Latitudes. A report by the U.S. Climate Change Science Program and Subcommittee on Global Change Research [Alley, R.B., J. Brigham-Grette, G.H. Miller, L. Polyak, and J.W.C. White (coordinating lead authors)]. U.S. Geological Survey, Reston, VA, 257 pp. modified from IPCC (2007)

Activate Your Learning

What information is plotted on the figure above? What does this data tell us about the relationship between CO₂ in the atmosphere and surface temperature over the past 400,000,000 years of Earth history?

Click for answer.