In the past, long before man populated the earth, volcanoes emitted enormous amounts of CO2 into the earth's atmosphere. These quantities were unmatched by industrial activity, and undoubtedly contributed to increasing the greenhouse effect that regulates the earth's temperature. Despite these emissions, the Earth has not become a catastrophic furnace in which life forms have been transformed, or perished. What is the reason for this? What phenomenon regulates the greenhouse effect and causes it to correct itself?
Susan Brantley, Evan Pugh University professor and Barnes Professor of Geosciences at Penn State University, helped us better understand the thermostat that regulates the Earth's temperature.
Siliceous rocks
Part of the answer is known to be found in siliceous rocks or acid rocks. "The idea is that silicate rock weathering is this thermostat, but no one has ever really agreed on its temperature sensitivity."
To gain a better understanding of the role of these rocks, Brantley analyzed soil samples from 45 places around the globe.
When you do experiments in the laboratory versus taking samples from soil or a river, you get different values. So what we tried to do in this research is look across those different spatial scales and figure out how we can make sense of all this data geochemists around the world have been accumulating about weathering on the planet. And this study is a model for how we can do that.
The Earth's thermostat
How does the Earth's thermostat, which counteracts the massive emission of CO2 into the atmosphere, work? Rain washes some of that CO2 to the ground. It creates a weak acid that wears away the silicate rocks on the surface. The by-products are carried away by currents (rivers and subway streams) to the ocean. All that carbon dioxide is trapped in the deep sea, and the Earth's thermal system is balanced. This process is called weathering.
Brantley explains how this process works:
It has long been hypothesized that the balance between carbon dioxide entering the atmosphere from volcanoes and being pulled out by weathering over millions of years holds the temperature of the planet relatively constant. The key is when there is more carbon dioxide in the atmosphere and the planet gets hotter, weathering goes faster and pulls more carbon dioxide out. And when the planet is cooler, weathering slows down.
The main objective of the study is to figure out the extent to which weathering corrects the increase in CO2, but it is insurmountably difficult to gather this information just from experiments:
In a soil profile, you are seeing a picture of soil where the camera shutter was open for sometimes a million years - there are integrated processes happening for a million years, and you're trying to compare that with a two-year flask experiment.
It's only when you start crossing spatial and time scales that you start seeing what's really important. Surface area is really important. You can measure all the rate constants you want for that solution in the lab, but until you can tell me how surface area forms out there in the natural system, you are never going to be able to predict the real system.
