How Gaia Changed Along the Ages. CO2 as the Major Driver of Temperature During the Past 65 Million Years
You wanted proof that CO2 determines Earth’s surface temperature? Here is your proof. Another fascinating result of climate science which appeared in a recent paper on “Science.” It summarizes what we know about temperatures and CO2 concentrations during the past 66 million years, with temperatures shown as color-coded. If you want to see both sets of data in graphical form, there you go; from the same paper.
You also see other ecosystem elements affected by temperatures here. Ice caps and sea levels go in step with temperatures, and the biosphere also does. These are not revolutionary results, but they confirm the picture that had been emerging over more than a century of painstaking work.
The Cenozoic started after the great extinction of the previous era, the Mesozoic, when the dinosaurs (at least the non-avian kinds) were wiped out. In the beginning, temperatures rose to levels nearly 15 degrees C higher than today. Then, the atmosphere drifted to lower and lower temperatures to arrive at the epoch we call the “Pleistocene,” where a cycle of ice ages and interglacials started, initially with a period of some 41,000 years, then of 100,000 years. During the whole Cenozoic, the CO2 concentration always changed in step with the temperatures. High CO2 concentrations mean a hot Earth; the reverse holds as well. Today, everything is changing again, with the CO2 concentration and temperature rapidly rising.
The grand Cenozoic cycle gives us some certainties and some unsolved problems. Physics told us long ago that CO2 should affect planetary temperatures. Evidently, it does. It does that mainly indirectly, triggering the release of water vapor, which is a more powerful and more concentrated greenhouse gas. We can also rule out an important role of changes in solar irradiation: there is no evidence that the sun changed its output enough and in the right direction during the Cenozoic.
And here we are with a fundamental question: “Why did all that happen?” What caused the Cenozoic system to vary the atmospheric CO2 concentration and go through this cycle of temperatures, first warming and then cooling?
The atmosphere is not a static reservoir of CO2. Its concentration is the result of the dynamic interaction between input and output. You may think of the atmosphere as a bathtub with a tap and a sink. The level of the water is determined by the balance between the inflow and the outflow. It is the same for CO2.
Of course, during the Cenozoic, there were no humans burning fossil fuels, and the main source of CO2 was outgassing from the depths of Earth. Even today, volcanoes continuously emit CO2 and methane (CH4) that are then transformed into CO2 by bacteria. On the other end of the atmospheric bathtub, CO2 is removed mainly by a chemical reaction called “silicate erosion” that transforms CO2 into carbonates. It is also removed by the sedimentation and burial of organic matter.
Image from https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JB016460
So, we could interpret the decline in CO2 concentrations during the Cenozoic to either less intense volcanic outgassing or faster silicon erosion/sedimentation. Or maybe both because the two are related and form a grand cycle that spans millions of years. Carbon is continuously pushed into the depths of Earth at the subduction zones of continents, and re-emitted out of volcanoes.
At present, there is some indication of reduced volcanism during the Cenozoic but, on the whole, it seems that the decline in CO2 may be mostly related to an evolutionary factor: the increased activity of the biosphere that led to a faster rate of sedimentation of organic carbon. For instance, a major factor in this increase may have been the evolution of baleen whales. Whatever the case, it seems that an imbalance exists between the tap and the sink of the atmospheric bathtub. If not perturbed by other factors, the climate tends to cool down over tens of millions of years.
This trend is sporadically interrupted and reversed by the volcanic phenomena called “Large Igneous Provinces” (LIPs) that push back into the atmosphere huge amounts of sedimented carbon, with the associated warming. It was one of these LIPs that caused the extinction of the dinosaurs and ushered in the Cenozoic era. But there haven’t been important LIPs during the Cenozoic, so the cooling trend has been maintained.
And now? Humans have played the role of a large igneous province by mobilizing again a large amount of carbon buried into sedimentary rocks (they call it “fossil fuels”). By pushing this carbon into the atmosphere, Earth is going backward along the Cenozoic temperature curve: it is warming again. At present, we have already reached the CO2 concentrations of about 15 million years ago. Earth was considerably warmer at that time, but on the whole not enormously different from what it is today. The problem is that humans are continuing to emit carbon dioxide and that’s going to push the system toward higher and higher temperatures.
How much higher? Hard to say. The effect of CO2 on temperature is clear, but its intensity varies depending on several factors: albedo, water vapor phase transitions, other greenhouse gases, and more. On the whole, it may still be possible to keep the temperature increase within acceptable limits if humans cease emissions rapidly enough and if they manage to keep the biosphere running at the levels of activity that cool the planet. Forests and whales can do a lot to limit warming.
And if we fail? Well, Gaia will go through another big warming pulse, like the one at the beginning of the Cenozoic. It will cause a giant extinction, but the biosphere should survive and rebound. Then, the same factors that led to the slow cooling of the Cenozoic will cause a gradual cooling of the new era that will last tens of millions of years. Maybe will be interrupted again by a new large igneous province. Or, who knows, by some smart monkey-like creatures who will indulge again in the suicidal habit of burning fossil fuels, as humans did. The only sure thing is that this new era will not be called the “Anthropocene” because there won’t be humans (anthropoi) in it.
Gaia is an old lady by now, but she is still strong and vital, and she may live several hundred million years more before the Sun becomes too hot for life on Earth to survive. And, as always, she knows best.
Thanks Ugo. I have seen a well supported hypothesis that India slamming into Asia, thrusting up the Himalayas, which make rain fall on them, and give up CO2 into the calcium-containing rock, which washes down to the floor of the Indian ocean, and sits, may be what is cooling our planet off in the last few million years.
Contextually, looking at geological history, it is hard to see our current liberation of terrestrial carbon as catastrophic for life on Earth. It's going to have been a fairly short pulse, as far as Gaia will be concerned, maybe a few more human lifetimes...