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Carbon dioxide emissions from Deccan volcanism and a K/T boundary greenhouse effect

Ken Caldeira and Michael R. Rampino · Geophysical Research Letters 17, 1299-1302 · 1990

Key finding. The Deccan Traps eruptions released an estimated 6 to 20 × 10^16 moles of CO2 over several hundred thousand years, which would have raised atmospheric CO2 by less than 75 ppm and produced less than 1 °C of global warming — too weak for the direct climate effects of that CO2 to have been an important factor in the end-Cretaceous mass extinctions.

A line plot with the increase in atmospheric partial pressure of carbon dioxide, from 0 to 70 parts per million, on the left axis and the corresponding global mean temperature increase, from 0 to 0.7 degrees Celsius, on the right; time runs from 0 to 2 million years. Four curves show different combinations of total carbon dioxide released and eruption duration. Each rises to a peak within the first half-million years and decays back toward zero by about 1.5 million years. The highest, a dotted curve for 20 times 10 to the 16 moles released over 100,000 years, peaks near 63 parts per million; the lowest peaks near 11.
Even the most extreme combination modelled — the largest plausible eruption volume released over the shortest plausible duration — raises atmospheric carbon dioxide by only about 65 parts per million and global mean temperature by less than 0.8 degrees Celsius, because the release is slow enough that the ocean and the carbonate-silicate cycle absorb most of it as it arrives. Figure 1 from Caldeira and Rampino (1990), Geophysical Research Letters 17, 1299-1302. Reproduced under author reuse rights. Extracted from the published PDF and resized for web display.

What question did this research address?

The Deccan Traps eruptions coincide with the end-Cretaceous mass extinction, and a greenhouse warming driven by volcanic CO2 had been proposed as the cause of the extinctions on land and in the sea.

This paper asked whether the quantities work. How much CO2 did the eruptions actually release, and how much warming would that amount of carbon produce once the ocean and the carbon cycle are accounted for?

What did we find?

Total eruptive and non-eruptive CO2 output was estimated at 6 to 20 × 10^16 moles, built up from the CO2 weight fraction of the original basalts and basaltic melts, the fraction of that CO2 actually degassed, and the erupted volume of the Deccan Traps.

A model of the effect of added CO2 on climate and ocean chemistry indicates the resulting rise in atmospheric partial pressure of CO2 would have been under 75 ppm.

The corresponding global warming is less than 1 °C, spread across several hundred thousand years rather than delivered as a pulse.

The reason the effect is so small is the timescale. Released slowly enough, CO2 is substantially absorbed by the ocean and by weathering as it accumulates, so a very large total release produces only a modest standing increase in concentration.

Why does it matter?

The result removes one candidate explanation for the end-Cretaceous extinctions on quantitative grounds rather than on argument, narrowing the field of hypotheses that remain viable.

It also demonstrates a principle that carries directly into the present. The climate consequence of a carbon release depends on the rate at which it is delivered, not only on the total amount — which is why an emission that would be absorbed harmlessly across hundreds of thousands of years is not harmless across two centuries.

Citation

Ken Caldeira and Michael R. Rampino (1990). Carbon dioxide emissions from Deccan volcanism and a K/T boundary greenhouse effect. Geophysical Research Letters 17, 1299-1302.

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