Extreme global warming in the ancient past

November 10, 2010, National Oceanography Centre, Southampton
The image shows the the scientific drilling ship JOIDES Resolution docked in Hobart, Tasmania. Credit: John Beck, IODP

Variations in atmosphere carbon dioxide around 40 million years ago were tightly coupled to changes in global temperature, according to new findings published in the journal Science. The study was led by scientists at Utrecht University, working with colleagues at the NIOZ Royal Netherlands Institute for Sea Research and the University of Southampton.

"Understanding the relationship between the Earth's climate and in the geological past can provide insight into the extent of future global warming expected to result from emission caused by the activities of humans," said Dr Steven Bohaty of the University of Southampton's School of Ocean and Earth Science (SOES) based at the National Oceanography Centre in Southampton.

It has been known for some time that the long-term warmth of the Eocene (~56 to 34 million years ago) was associated with relatively high atmospheric carbon dioxide levels. However, scientists were previously unable to demonstrate tight-coupling between variations in atmospheric carbon dioxide and shorter-term changes in global climate.

To fill this gap in knowledge, the authors of the new study focused on one of the hottest episodes of Earth's – the Middle Eocene Climatic Optimum (MECO), which occurred around 40 million years ago.

Algae use photosynthesis to harvest the energy of the sun, converting carbon dioxide and water into the organic molecules required for growth. Different isotopes of carbon are incorporated into these molecules depending on the environmental conditions under which algae grow. Ancient climate can therefore be reconstructed by analysing the carbon isotope ratios of molecules preserved in fossilised algae.

The researchers took this approach to reconstruct variations in carbon dioxide levels across the MECO warming event, using fossilised algae preserved in sediment cores extracted from the seafloor near Tasmania, Australia, by the Ocean Drilling Program. They refined their estimates of carbon dioxide levels using information on the past marine ecosystem derived from studying changes in the abundance of different groups of fossil plankton.

Their analyses indicate that MECO carbon dioxide levels must have at least doubled over a period of around 400,000 years. In conjunction with these findings, analyses using two independent molecular proxies for sea surface temperature show that the climate warmed by between 4 and 6 degrees Celsius over the same period.

"We found a close correspondence between carbon dioxide levels and sea surface temperature over the whole period, suggesting that increased amounts of carbon dioxide in the atmosphere played a major role in global warming during the MECO," said Bohaty.

The researchers consider it likely that elevated atmospheric carbon dioxide levels during the MECO resulted in increased global temperatures, rather than vice versa, arguing that the increase in carbon dioxide played the lead role.

"The change in carbon dioxide 40 million years ago was too large to have been the result of temperature change and associated feedbacks," said co-lead author Peter Bijl of Utrecht University. "Such a large change in carbon dioxide certainly provides a plausible explanation for the changes in Earth's temperature."

The researchers point out that the large increase in atmospheric carbon dioxide indicated by their analysis would have required a natural carbon source capable of injecting vast amounts of carbon into the atmosphere.

The rapid increase in atmospheric carbon dioxide levels around 40 million years ago approximately coincides with the rise of the Himalayas and may be related to the disappearance of an ocean between India and Asia as a result of plate tectonics – the large scale movements of the Earth's rocky shell (lithosphere). But, as explained by Professor Paul Pearson of Cardiff University in a perspective article accompanying the Science paper, the hunt is now on to discover the exact cause.

Explore further: Global warming likely to be amplified by slow changes to Earth systems

More information:
-- Publication: Bijl, P. K., Houben, A. J. P., Schouten, S., Bohaty, S. M., Sluijs, A., Reichart, G-J., Sinninghe Damsté, J. S. & Brinkhuis, H. Transient middle Eocene atmospheric CO2 and temperature variations. Science 330, 819 - 8215 (2010). DOI: 10.1126/science.1193654

-- Science Perspective: Pearson, P. N. Increased atmospheric CO2 during the middle Eocene. Science 330, 763-764 (2010). DOI: 10.1126/science.1197894

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2.8 / 5 (9) Nov 10, 2010
It is generally acknowledged that one cannot infer causality from correlation. To correlate ('correspond') CO2 with ocean temperatures does not, in any way, allow a scientist to take the next step and say that CO2 'causes' warming. In fact, most climate scientists would agree that it is a lagging indicator, not a leading one. The most likely scenario for all of global warming is the following: magma creeps up close to the surface; then volcanoes start erupting, then earthquakes increase in frequency and intensity; the proximity of extremely hot magma at increasingly shorter distance from the crustal plate causes ocean currents to warm up; this extra ocean warmth then results in CO2 trapped as condensate to be released as gas from the oceans; ocean surface temperatures increase; land temperatures increase. What causes the magma to creep upwards? Gravitational effects. What kinds of gravitational effects? Nearness to sun; but more likely alignments of planets in the solar system.
5 / 5 (5) Nov 10, 2010
Lino235, the article says the warming took place over 400,000 years. Planetary alignment and proximity to the sun probably don't last that long.

They also said that this time period coincides with the formation of the Himalayas, which is where they suspect the volcanic activity comes from.
1 / 5 (2) Nov 11, 2010
They also said that this time period coincides with the formation of the Himalayas, which is where they suspect the volcanic activity comes from.

Fluid CO2 from the upper mantle is observed in CO2 gas wells used to make Dry Ice and as fluid inclusions in olivine xenoliths from Hawaiian volcanos.

Oiver K. Manuel
3 / 5 (2) Nov 11, 2010
Different isotopes of carbon are incorporated into these molecules depending on the environmental conditions under which algae grow.

I have severe reservations about that statement.

I would like to see some reference to studies which prove that any significant change in the rate of take-up of different isotopes of a chemical by living plants can take place just because of "environmental conditions".
What's more, that statement implies that one isotope will be involved in a chemical reaction, but another, chemically indistinguishable, isotope will not!
Is that just bad reporting? I hope??
not rated yet Nov 12, 2010
"implies that one isotope will be involved in a chemical reaction, but another, chemically indistinguishable, isotope will not"

Not exactly how it works. We can observe today, at any water/air interface that as temperature changes, the balance of gasses and their isotopes will change accordingly. The living organisms just absorb whatever isotope is available.

The real problem with all studies like this one lies in the proxy method. Several key factors are based on theory that can't be proven, since there is no 100% sure way to know if what they observe in fosiles actually means what they assume it does. When they say CO2 doubled, that sounds like a lot, but they are actually dealing with very very fine variations in terms of parts per million. It's almost a foregone certainty that the estimates are off by at least a small amount. Small deviations can multiply many times when feedback assumptions are made as well.
not rated yet Nov 12, 2010
The problem with this study in particular (as opposed to proxy studies in general) is that samples from one location may only show regional conditions, or latitude-specific conditions. Incomplete work here. They need enough samples from geologically different areas formed under different conditions from the same time period before they can make these kinds of claims. What if this location was down-wind from a massive volcano, or near the outlet of a huge river, or in the path of an ocean current that makes it atypical of conditions anywhere else in the world. We certainly can find plenty of unique places today. Even the two poles are vastly different in terms of climate processes today. They need more sample locations or this is less than useless data.
not rated yet Nov 12, 2010
Ah, yes.
Thanks GSwift7. I looked up what was expected to be going on and found that it was not a difference chemical reaction rates that was being touted here (next time I'll look before I leap!).
Nevertheless, I share you doubts about this study.
I shudder when I see the number of assumptions made in biogeochemistry about conditions pertaining eons ago. We have enough trouble modelling current systems!

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