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.

I consider this more cause

I consider this more cause for alarm wrt our CO2 emissions, not less.

Climate models underestimating the warming is not a good thing. A 70% increase in CO2 and a 5–9C increase in temperature is ominous.

Geological Society in London

came across this article today which happens to be about this very topic:

A conference organised by the Geological Society in London this week will bring together scientists from around the world to look at how the world coped with climate change in the past.

yet they of course conclude:

A statement read: “The geological evidence from the 55 million year event and from earlier warming episodes suggests that such an addition [a massive increase in greenhouse gases caused by the activities of mankind] is likely to raise average global temperatures by at least 5 to 6C, and possibly more, and that recovery of the Earth’s climate in the absence of mitigation measures could take 100,000 years or more. Numerical models of the climate system support such an interpretation. In the light of the evidence presented here it is reasonable to conclude that emitting further large amounts of carbon dioxide into the atmosphere over time is likely to be unwise, uncomfortable though that fact may be.”

i added the emphasis to highlight, yet again, its based solely on computer models.

while i understand the importance of models and their usefulness, you name me ONE aircraft manufacture that has ever gone from a pure computer simulation of their product to retail product without massive extensive real world testing. yet we are to take these even more complex models at face value??

Temps first, CO2 second?

There was a new study published in Nature on Oct, 21, 2010 which used high resolution temperature and CO2 estimates right at the PETM boundary – 55.8Mya.

The study found that there was a spike in temperatures (55.81 Mya) which occurred before the CO2 increased (55.76 Mya). There might have been two pulses of warming before the CO2 spiked. Temps increased between 4C to 5C in the spike (and CO2 increased from 900 ppm to 1400 ppm in the spike noted from other sources versus this study).

So again we have Temperatures leading CO2 (in a major warming event that as you noted has particular fascination for the pro-AGW set).

On the Earth maps above, it should be noted that the North Atlantic was just opening up at the time. The deep ocean access to the Arctic may have just opened in the time period and there was great volcanic activity in the opening rift between Europe and Greenland. Sea level was also much higher and much of the continents were flooded. These kind of changes should be considered as influencing the event. This may be a better illustration of continental positions.