In 2007 Bill McKibben asked me whether 450 ppm would be a reasonable target for atmospheric CO2,
if we wished to maintain a hospitable climate for future generations.
450 ppm seemed clearly too high, based on several lines of evidence, but
especially based on paleoclimate data for climate change.
We were just then working on a paper,[1] Climate sensitivity, sea level
and atmospheric carbon dioxide, that included consideration of CO2
changes over the Cenozoic era,[2] the past 65 million years, since the
end of the Cretaceous era. The natural source of atmospheric CO2
is volcanic emissions that occur mainly at continental margins due to
plate tectonics (“continental drift”). The natural sink is the
weathering process, which ultimately deposits carbon on the ocean floor
as limestone.
David Beerling, an expert in trace gas biogeochemistry at the University
of Sheffield, was an organizer of the meeting at which I presented the
above referenced paper. Soon thereafter I enlisted David and other
paleoclimate experts to help answer Bill’s question about a safe level
of atmospheric CO2, which we concluded was somewhere south of 350 ppm.[3]
Since then, my group at Climate Science, Awareness and Solutions (CSAS),
has continued to cooperate with David’s group at the University of
Sheffield. One of the objectives is to investigate the potential for
drawdown of atmospheric CO2 by speeding up the weathering
process. In the most recent paper, which will be published in Nature
tomorrow, David and co-authors report on progress in testing the
potential for large-scale CO2 removal via application of rock dust on croplands.
A press release on the paper is available here.
Weathering, nature’s process of removing CO2 from the air,
can be sped up by grinding silicate materials into fine dust and
spreading it on soils that can otherwise benefit from the addition. Many
farmers are accustomed to liming their fields, and have equipment for
such purpose. The silicate particles will dissolve slowly, react with
CO2, forming carbonates. Much of this carbonate will
eventually find its way to the ocean, ending up as limestone on the
ocean floor.
In order for enhanced weathering to play an important role in CO2
drawdown it will be necessary to demonstrate that it provides a
significant benefit in increased soil fertility and crop yield. If
governments also provide a financial incentive via a carbon market, the
chances of obtaining large-scale by-in by farmers will be much
increased.
Other CO2 drawdown approaches, such as reforestation, are
important, but require management to assure that the carbon sink is
maintained. We will need the combination of reforestation, enhanced
weathering, and other techniques to draw down atmospheric CO2 to a safe level.
Of course, the most important action required to avoid dangerous climate
change is to phase over to carbon-free energies as rapidly as is
economically justified. In that event, it should be possible to bring
down atmospheric greenhouse gas levels before slow amplifying climate
feedbacks occur, the most dangerous climate impacts need never occur,
and we can bend the climate curve back toward the climate within which
that humanity and nature lived during the Holocene.
[1] Hansen, J., M. Sato, P. Kharecha, G. Russell, D.W. Lea, and M. Siddall: Climate change and trace gases. Phil. Trans. Royal. Soc. A, 365, 1925-1954, 2007.
[2] Volcanic CO2 emissions were large in the
early Cenozoic as the Indian plate subducted carbon-rich ocean crust
while moving north through the present Indian Ocean and colliding with
Asia. Since then, over the past 50 million years, CO2 has
declined and Earth has been in a long-term cooling trend, albeit with
many climate oscillations, especially climate change associated with
perturbations of Earth’s orbit.
[3] Hansen, J., M. Sato, P. Kharecha, D. Beerling, R.
Berner, V. Masson-Delmotte, M. Pagani, M. Raymo, D. Royer, and J.C.
Zachos: Target atmospheric CO2: Where should humanity aim? Open Atmos. Sci. J., 2, 217-231, 2008.
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