From Cosmos to Carbonate: Contemporary Carbon Cycling in Comprehensive Chemical Context

Scott Denning

seminar
Oct. 18, 2022

11:00 – 11:45 am MDT

Main seminar room or YouTube

Webcast

Main content

We study today's climate crisis and its changing carbon cycle by peering deeply at a single frame in an epic movie spanning billions of years. After primordial hydrogen and helium, carbon and oxygen are the most abundant elements in the universe. The galactic disk is filthy with carbonaceous soot and organic ice whose reverse greenhouse effect promotes wildly complex chemistry in the cryogenic interiors of interstellar clouds. Recent results from radio spectrometry and meteorite samples have shown that planetary origins and development are intertwined with prebiotic chemistry across the galaxy.

Earth’s composition, climate, and biosphere have evolved through distillation, separation, and condensation of volatiles that regulate its radiative balance, both stabilizing and perturbing conditions over geologic time. Volatile carbon compounds erupt via seafloor spreading and are converted to carbonate sediments through orogeny, weathering, and deposition. Tiny imbalances in these fluxes have swung climate repeatedly from snowball to hothouse to icehouse and back again. Carbon cycle catastrophes have repeatedly punctuated paleoclimate with both sudden and slow-motion disturbances that reset biological evolution. During the Cenozoic, tectonic forces consumed CO2 and plunged Earth’s climate into a glacial epoch for the first time since Trilobites were dominant.

The sudden explosive release of fossil CO2 has reversed 50 million years of cooling, threatening a 6th mass extinction and interrupting 5 centuries of economic development. The contemporary carbon cycle is wildly out of balance, with fluxes among Earth’s crust, atmosphere, biosphere, and oceans hundreds of times what they’ve been in the geologic past.

Every 1000 Gt of carbon transferred from the rock reservoir to the labile reservoirs in the air and sea raises Earth's temperature by about 0.5 Kelvin. When emissions stop, warming will stop but only negative emissions can reverse the warming and carbon dioxide removal is likely to be only 50% effective on century timescales.

After fossil mining ceases, anthropogenic CO2 will come into chemical equilibrium with the deep ocean over several millennia. Depending on the size of the fossil pulse, this may dramatically change marine chemistry and biology. If the pulse is large enough, a return to the Cenozoic trend may take half a million years as mountains dissolve and carbonate sediments accumulate.

In the distant future, the coevolution of Earth’s carbon, climate, and biota will return to the slow pace dictated by mantle convection.

Scott Denning

Colorado State University

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