Ancient Evidence That CO2 Does Not Control Climate
Climate scientists continue to be fascinated with the Paleocene–Eocene Thermal Maximum (PETM), which took place about 55 Myr ago. This period of sudden global warming and increasing atmospheric CO2 represents a possible model our present era of warming climate and growing CO2 emissions. Studying the PETM, therefore, may provide insight into climate system sensitivity and feedbacks. Just such a study, reported in Nature Geoscience, found that CO2 forcing alone was insufficient to explain the PETM warming. Scientists speculate that other processes and/or feedbacks, hitherto unknown, must have caused a substantial portion of the warming during the Paleocene–Eocene Thermal Maximum. Simply put, CO2 did not cause the PETM climate change.
In an article entitled “Carbon dioxide forcing alone insufficient to explain Palaeocene–Eocene Thermal Maximum warming,” Richard E. Zeebe, James C. Zachos and Gerald R. Dickens state: “The onset of the PETM was marked by a global increase in surface temperatures by 5–9 °C within a few thousand years. At nearly the same time, a substantial carbon release occurred, as demonstrated by a large drop in the 13C/12C ratio of surficial carbon reservoirs.” Estimates for the carbon release during the onset of the PETM and emissions from anthropogenic sources over the past 50 years seem to be of a similar order of magnitude (~1 Pg C y-1).
Many scientists think that the PETM was triggered by a massive injection of CO2 into Earth's atmosphere. Using deep-sea carbonate dissolution records and stable carbon isotope records across the PETM, in combination with carbon cycle modeling, the authors arrived at an estimate of the amount of carbon released during the PETM. They used the long-term ocean–atmosphere–sediment carbon cycle reservoir model LOSCAR to constrain the total mass of the carbon input.
Interestingly, there were “profound differences” in observed Atlantic and Pacific carbon absorption during the event. This was detected by variation in the calcite compensation depth (CCD) in the different ocean basins. Various results from the study are shown in the figure below.
Zeebe et al. concluded that the maximum initial input is constrained to approx 3,000 Pg C (Pg stands for peta gram, which is the same as a billion metric tons). This estimate of the maximum initial carbon input was also found to be largely independent of its duration, meaning whether it happened in a single giant blast or over 5,000 years made no difference to the simulation response. The pattern of the carbon input scenario required by the model to match observations (Fig. 1a) seems to be consistent with a pulsed input pattern of carbon release from oceanic gas hydrate reservoirs—in other words, an eruption of ocean floor methane.
Regardless of the source or composition, Earth's climate underwent a significant rapid warming. What is more interesting is that the warming that occurred could not have been caused by the amount of CO2 the researchers' study found. Here are the authors' conclusions:
At the accepted equilibrium climate sensitivities of 1.5–4.5 °C warming per doubling of CO2, our calculated 1.7-fold increase in CO2 would at most have caused ~3.5 °C warming during the PETM main phase. This constitutes an enigma because proxy records globally indicate surface warming by 5–9 °C. If the temperature reconstructions are correct, then feedbacks and/or forcings other than atmospheric CO2 caused a major portion of the PETM warming. The origin of this additional warming is unknown at present. Possible causes of the excess warming include increased production and levels of trace greenhouse gases as a consequence of the climatic warming (such as CH4). Regardless, this mismatch poses a challenge for our understanding of past episodes of strong and rapid global warming.
Here is more evidence that CO2 on its own is not a strong enough greenhouse gas to cause the magnitude of temperature change that took place during the PETM. Once again, if CO2 is to be the driver of climate change a number of unidentified but very significant “feedbacks” are needed. Of course there are other explanations: the change was really caused by methane, with CO2 just the remnant of its passing; or perhaps the Earth was suddenly warmed by increased solar activity or a nearby supernova, leaving the increase in CO2 as a natural response to a warming planet. No one knows, but the conclusion is clear—then as now, CO2 was not the main driver of global warming.
The authors' postulated sudden pulse of CO2, equivalent to 1,000 years of human emissions at current rates, could not have caused the PETM. And the atmosphere at the end of the Paleocene started with more than twice the amount of atmospheric CO2 as Earth's current atmosphere. Today, Earth's climate and atmosphere are noticeably different from then, the climate warmer and the Atlantic Ocean much smaller. Even under Paleocene conditions, CO2 falls far short of being able to cause sudden significant global warming.
Earth shortly after the PETM.
“Undoubtedly, the climatic boundary conditions before the PETM were different from today's—including different continental configuration, absence of continental ice and a different base climate, which limits the PETM's suitability as the perfect future analogue,” state the authors, somewhat disappointed at not finding a clear link to CO2 and modern global warming. “Nevertheless, our results imply a fundamental gap in our understanding of the amplitude of global warming associated with large and abrupt climate perturbations. This gap needs to be filled to confidently predict future climate change.”
We most assuredly have a fundamental gap in our understanding—a gap that will continue to exist as long as climate scientists insist on blaming climate change on atmospheric CO2 levels. It is CO2 that is the feedback, not the primary driver. Mankind's direct release of CO2 into the atmosphere cannot come close to causing another PETM.
The importance of CO2 has been constantly overestimated and all evidence of its impotence ignored by mainstream climate scientists. And despite assertions to the contrary, until the real causes for climate change are found we will not be able to predict future climate change.
Be safe, enjoy the interglacial and stay skeptical.