Guest geology by David Middleton

Way back in the Pleistocene (1976-1980), when I was a young geology student, the notion of CO₂ as a driver of climate change was largely scoffed at…

Suggestion that changing carbon dioxide content of the atmosphere could be a major factor in climate change dates from 1861, when it was proposed by British physicist John Tyndall.

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Unfortunately we cannot estimate accurately changes of past CO₂ content of either atmosphere or oceans, nor is there any firm quantitative basis for estimating the the magnitude of drop in carbon dioxide content necessary to trigger glaciation.  Moreover the entire concept of an atmospheric greenhouse effect is controversial, for the rate of ocean-atmosphere equalization is uncertain.

Dott, Robert H. & Roger L. Batten. Evolution of the Earth. McGraw-Hill, Inc. Second Edition 1976. p. 441.

Sometime after 1980, a new paradigm emerged, suggesting that Phanerozoic Eon climate change had largely been driven by CO₂ (Royer et al., 2004). The model was that the weathering rates of silicate rocks governed the atmospheric concentration of CO₂ (Berner & Kothavala, 2001) and that CO₂ was the “control knob” for temperature. Well, this paradigm may have just taken a bullet to the head.

Rutgers Today > Research
Is Theory on Earth’s Climate in the Last 15 Million Years Wrong?
Rutgers-led study casts doubt on Himalayan rock weathering hypothesis
September 22, 2019

A key theory that attributes the climate evolution of the Earth to the breakdown of Himalayan rocks may not explain the cooling over the past 15 million years, according to a Rutgers-led study.

The study in the journal Nature Geoscience could shed more light on the causes of long-term climate change. It centers on the long-term cooling that occurred before the recent global warming tied to greenhouse gas emissions from humanity.

“The findings of our study, if substantiated, raise more questions than they answered,” said senior author Yair Rosenthal, a distinguished professor in the Department of Marine and Coastal Sciences in the School of Environmental and Biological Sciences at Rutgers University–New Brunswick. “If the cooling is not due to enhanced Himalayan rock weathering, then what processes have been overlooked?”

For decades, the leading hypothesis has been that the collision of the Indian and Asian continents and uplifting of the Himalayas brought fresh rocks to the Earth’s surface, making them more vulnerable to weathering that captured and stored carbon dioxide – a key greenhouse gas. But that hypothesis remains unconfirmed.

Lead author Weimin Si, a former Rutgers doctoral student now at Brown University, and Rosenthal challenge the hypothesis and examined deep-sea sediments rich with calcium carbonate.

Over millions of years, the weathering of rocks captured carbon dioxide and rivers carried it to the ocean as dissolved inorganic carbon, which is used by algae to build their calcium carbonate shells. When algae die, their skeletons fall on the seafloor and get buried, locking carbon from the atmosphere in deep-sea sediments.

If weathering increases, the accumulation of calcium carbonate in the deep sea should increase. But after studying dozens of deep-sea sediment cores through an international ocean drilling program, Si found that calcium carbonate in shells decreased significantly over 15 million years, which suggests that rock weathering may not be responsible for the long-term cooling.

Meanwhile, the scientists – surprisingly – also found that algae called coccolithophores adapted to the carbon dioxide decline over 15 million years by reducing their production of calcium carbonate. This reduction apparently was not taken into account in previous studies.

Many scientists believe that ocean acidification from high carbon dioxide levels will reduce the calcium carbonate in algae, especially in the near future. The data, however, suggest the opposite occurred over the 15 million years before the current global warming spell.

Rosenthal’s lab is now trying to answer these questions by studying the evolution of calcium and other elements in the ocean.

Rutgers Today

Basically, everything is bass-ackwards relative to the CO₂-driven climate paradigm.

As far as press releases go, this one is very good. I would only take serious issue with this bit:

Many scientists believe that ocean acidification from high carbon dioxide levels will reduce the calcium carbonate in algae, especially in the near future. The data, however, suggest the opposite occurred over the 15 million years before the current global warming spell.

The “current global warming spell” is indistinguishable from other Holocene and Pleistocene global warming spells.

We’ve already experienced nearly 1.0 ºC of warming since pre-industrial time.  Another 0.5 to 1.0 ºC between now and the end of the century doesn’t even put us into Eemian climate territory, much less the Miocene. 15 million years ago (MYA) was the middle of the Mid-Miocene Climatic Optimum (MMCO).

Their paper is pay-walled; here is the abstract:

Abstract
The globally averaged calcite compensation depth has deepened by several hundred metres in the past 15 Myr. This deepening has previously been interpreted to reflect increased alkalinity supply to the ocean driven by enhanced continental weathering due to the Himalayan orogeny during the late Neogene period. Here we examine mass accumulation rates of the main marine calcifying groups and show that global accumulation of pelagic carbonates has decreased from the late Miocene epoch to the late Pleistocene epoch even though CaCO₃ preservation has improved, suggesting a decrease in weathering alkalinity input to the ocean, thus opposing expectations from the Himalayan uplift hypothesis. Instead, changes in relative contributions of coccoliths and planktonic foraminifera to the pelagic carbonates in relative shallow sites, where dissolution has not taken its toll, suggest that coccolith production in the euphotic zone decreased concomitantly with the reduction in weathering alkalinity inputs as registered by the decline in pelagic carbonate accumulation. Our work highlights a mechanism whereby, in addition to deep-sea dissolution, changes in marine calcification acted to modulate carbonate compensation in response to reduced weathering linked to the late Neogene cooling and decline in atmospheric partial pressure of carbon dioxide.

Weimin & Rosenthal Nature Geoscience

The assumption has been that the rise of the Himalayan Mountains during the Miocene increased the rate of silicate rock weathering, drawing down atmospheric CO₂ and precipitously cooling the Earth’s atmosphere. While the Neogene cooling did follow the uplift of the Tibetan Plateau, the cool-down from the MMCO trailed the uplift by about 7 million years (Myr).

Part of the problem is that it is unclear if atmospheric CO₂ levels were significantly elevated 15 MYA.

We can see that the range of estimates for 15 MYA range from 250 to 500 ppm. While there is some support for higher CO₂ levels 20-22 MYA, when the Tibetan Uplift was accelerated, it does not coincide with the MMCO at 15 MYA.

We now have clean kills of the MMCO being driven by CO₂ emissions from the Columbia River Basalt Group eruptions and the subsequent cooling being driven by a draw down of atmospheric CO₂. How many clean kills does it take to kill a paradigm?

References

Berner, R.A. and Z. Kothavala, 2001. “GEOCARB III: A Revised Model of Atmospheric CO2 over Phanerozoic Time”.  American Journal of Science, v.301, pp.182-204, February 2001.

Dott, Robert H. & Roger L. Batten.  Evolution of the Earth.  McGraw-Hill, Inc.  Second Edition 1976.  p. 441.

Pagani, Mark, Michael Arthur & Katherine Freeman. (1999). “Miocene evolution of atmospheric carbon dioxide”. Paleoceanography. 14. 273-292. 10.1029/1999PA900006.

Royer, D. L., R. A. Berner, I. P. Montanez, N. J. Tabor and D. J. Beerling. “CO₂ as a primary driver of Phanerozoic climate”.  GSA Today, Vol. 14, No. 3. (2004), pp. 4-10

Tripati, A.K., C.D. Roberts, and R.A. Eagle. 2009.  “Coupling of CO₂ and Ice Sheet Stability Over Major Climate Transitions of the Last 20 Million Years”.  Science, Vol. 326, pp. 1394 1397, 4 December 2009.  DOI: 10.1126/science.1178296

Weimin Si & Yair Rosenthal. Reduced continental weathering and marine calcification linked to late Neogene decline in atmospheric CO₂. Nature Geoscience, 2019 DOI: 10.1038/s41561-019-0450-

Zachos, J. C., Pagani, M., Sloan, L. C., Thomas, E. & Billups, K. “Trends, rhythms, and aberrations in global climate 65 Ma to present”.  Science 292, 686–-693 (2001).