Plants Save the Earth from an Icy Doom (w/ Podcast)

July 1, 2009,
As plants become starved for CO2, rock weathering diminishes. Credit: David Beerling

( -- Fifty million years ago, the North and South Poles were ice-free and crocodiles roamed the Arctic. Since then, a long-term decrease in the amount of CO2 in the atmosphere has cooled the Earth. Researchers at Yale University, the Carnegie Institution of Washington and the University of Sheffield now show that land plants saved the Earth from a deep frozen fate by buffering the removal of atmospheric CO2 over the past 24 million years.

While the upper limit for atmospheric CO2 levels has been a focus for discussions of global warming and the quality of life on Earth, this study points to the dynamics that maintain the lower sustainable limits of atmospheric CO2.

Volcanic gases naturally add CO2 to the , and over millions of years CO2 is removed by the weathering of silica-based rocks like granite and then locked up in carbonates on the floor of the world's oceans. The more these rocks are weathered, the more CO2 is removed from the atmosphere.

"Mountain building in places like Tibet and South America during the past 25 million years created conditions that should have sucked nearly all the CO2 out of the atmosphere, throwing the Earth into a deep freeze," said senior author Mark Pagani, associate professor of geology and geophysics and a member of the Yale Climate and Energy Institute's executive committee. "But as the CO2 concentration of Earth's atmosphere decreased to about 200 to 250 parts per million, CO2 levels stabilized."

The study, published in the XX issue of Nature, looked for a possible explanation They used simulations of the global carbon cycle and observations from plant growth experiments to show that as atmospheric CO2 concentrations began to drop towards near-starvation levels for land plants, the capacity of plants and vegetation to weather silicate rocks greatly diminished, slowing the draw-down of atmospheric CO2.

"When CO2 levels become suffocatingly low, plant growth is compromised and the health of forest ecosystems suffer," said Pagani. "When this happens, plants can no longer help remove CO2 from the atmosphere faster than volcanoes and other sources can supply it."

"Ultimately, we owe another large debt to plants" said co-author Ken Caldeira from the Carnegie Institution of Washington at Stanford University. "Aside from providing zesty dishes like eggplant parmesan, plants have also stabilized Earth's climate by inhibiting critically low levels of CO2 that would have thrown spinning into space like a frozen ice ball."

Co-author David Beerling from Sheffield University adds, "Our research supports the emerging view that should be recognized as a geologic force of nature, with important consequences for all ."

More information: Nature, (doi:10.1038/nature08133)

Source: Yale University (news : web)

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5 / 5 (1) Jul 01, 2009
Did the authors take into consideration that C4 plants were widely distributed in tropical areas 25 mm yrs ago. These can readily prosper at less than 200 ppm CO2 whereas the more common C3 plants can't. Did the authors take this into consideration in their simulations?
1 / 5 (1) Jul 01, 2009
Possibly the low CO2 levels were a major factor that favored C4 plants in the first place. Otherwise, they're at a disadvantage due to their higher energy requirements.
1 / 5 (1) Jul 01, 2009
"Ultimately, we owe another large debt to plants ... plants have ... stabilized Earth's climate by inhibiting critically low levels of CO2 that would have thrown Earth spinning into space like a frozen ice ball."

So... we owe plants a debt of gratitude for nearly dying of CO2 starvation? Odd choice of words. But that is not all...

I was surprised to find out that 250 - 200 ppm was "near starvation" for plants! The current level of 350 ppm is often compared is 280 ppm in 1850.

...we were NOT far from CO2 "starvation" as recently as 1850!

Consider this quote:
"When CO2 levels become suffocatingly low, plant growth is compromised and the health of forest ecosystems suffer, .... When this happens, plants can no longer help remove CO2 from the atmosphere faster than volcanoes and other sources can supply it."

How rapidly does "weathering" remove CO2?

I had assumed the ONLY natural process to "sequester" CO2 was photosynthesis in green plants. This article implies that weathering is a significant process compared to photosynthesis since the starvation of plants ended weathering, not what I thought would have been more significant: the end of photosynthesis!

This suggests to me that the natural increase in plant life as C02 rises will lead to greater "weathering" and more rapid, natural sequestration of CO2.

How rapidly does this work? Has it been taken into account in models of Global Warming?
1 / 5 (1) Jul 02, 2009
Of course! ...higher CO2 levels -> higher levels of carbolic acid in rain -> greater weathering of stones into carbonates -> reduced CO2

And ...higher CO2 -> greater plant growth -> more cracking of stones and turnover of soil via root action -> more exposure to carbolic acid -> reduced CO2

Thus "photosynthesis" and "weathering" are complementary ways of reducing CO2 levels by virtue of plant growth!

Question: is this symbiotic relationship linear with CO2 levels? ...or exponential? ...or what?

Obsrvation: possibly another unintended consequence of human activity.... when we practice erosion control, we inhibit weathering, and thereby contribute to higher CO2 levels? ...what is the carbon footprint of erosion control?!

Ah... what tangled webs we weave!
1 / 5 (2) Jul 03, 2009
The earths surface is mostly, 75%, ocean. These simple models say nothing about this.

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