Understanding long-term climate and CO2 change

Paleoceanographic records tell us that the climate has experienced a long-term cooling trend and declines in atmospheric carbon dioxide over the past several tens of millions of years, interrupted by some shorter intervals of ups and downs, according to research by Louis Derry, a professor in Cornell’s Department of Atmospheric and Earth Sciences.

The coldest climates of the past 100 million years or more have been the repeated ice ages in the last million years.

“A long-standing question in the Earth sciences is what caused this long-term cooling,” said Derry. “It is associated with an overall decline in atmospheric CO2 levels, which ice core measurements tell us varied between about 180 and 280 parts per million (ppm) over the last several hundred thousand years but was higher before that. Reconstruction of (partial pressure of carbon dioxide (pCO2) in the deeper past relies on carbon and boron isotope-based reconstructions and is less precise, but the overall picture is one of declining pCO2 from 450-900 ppm since the early Miocene epoch, around 23 million years ago.”

Previous hypotheses for the overall long-term trend of declining pCO2 and cooling climate since the early Miocene have been based on the notion that there is a delicate balance between geologic production of CO2 largely from volcanic sources and the oxidation of ancient sedimentary organic carbon, and geologic consumption of CO2 by weathering reactions and burial of carbon in marine sediments.

“On these long-time scales, we need to consider both atmospheric CO2 and the much larger amount of CO2 dissolved in the oceans, as they act in concert for times longer than about a thousand years,” Derry said. “Previous models propose that the production and consumption of CO2 became imbalanced so that consumption exceeded supply, leading to an overall decrease in the amount of CO2 in the ocean and atmosphere.”

According to Derry, proposed mechanisms include decreased volcanism from slower sea floor spreading, increased weathering rates associated with uplift of major mountain ranges, and/or increases in net organic carbon burial over time.

The new work—published recently in Earth & Planetary Science Letters—develops a different approach to the problem of changing atmospheric CO2 and climate over the last 23 million years. It shows how changes in ocean chemistry, principally a decrease in dissolved calcium, create conditions where CO2 becomes more soluble in the oceans, such that less CO2 is in the atmosphere, and more is dissolved in the oceans over time. The paper shows that this effect can cause a 200-250 ppm decline in atmospheric CO2, all while the total amount of CO2 in the ocean and atmosphere system remains virtually unchanged. This mechanism of redistribution of CO2 does not require an imbalance in supply vs sink, and represents a new view of the processes leading to long-term climate change since at least the early Miocene

While paleoceanographic records indicate declining ocean calcium ion (Ca++) levels which can in turn drive decreasing pCO2, the causes of the decline in Ca++ are not yet fully understood. A likely explanation is that the rate of exchange between seawater and the basaltic oceanic crust has declined which could explain the observations of lower calcium (Ca) and higher magnesium (Mg) over time. One contributor to this slower rate of exchange could be cooling temperatures themselves, because the reactions that exchange Ca and Mg are strongly sensitive to the temperature of the deep sea. Deep sea temperatures have fallen by about 7˚C since the early Miocene, consistent with this explanation.

“An as yet untested but intriguing corollary hypothesis is that an initial cooling may have impacted ocean chemistry, increasing CO2 solubility and thus lowering atmospheric pCO2,” said Derry. “This would lead to additional cooling, and further reduction in rates of seawater-basalt exchange, possibly setting up a positive feedback between cooling temperatures, lower ocean Ca++, and lower pCO2. In that scenario the cold temperatures of the last ice ages are a sort of culmination of this feedback system.”