Quick recap.
Stars get hotter as they get older. Including the sun. We would expect Earth to get warmer as the sun got warmer over billions of years. But apparently, that was not the case.
On Earth, sedimentary rocks started to form 3.8 billion years ago. They continued to form from that point in time forward through present times. This means the planet had abundant LIQUID surface water from 3.8 billion years ago, throughout the remainder of its history, including present times. The presence of LIQUID surface water on Earth is a proxy for global temperature. It means that the Earth’s average temperature was never hotter than 100° C, and never colder than 0° C. So, the sun has gotten hotter and hotter, but the Earth has not. It has never been too hot, or too cold for liquid surface water. How is that possible? This is a paradox. The Faint Young Sun paradox.
The greenhouse gas solution
One popular solution to the Faint Young Sun paradox imagines early Earth with a very dense atmosphere rich with greenhouse gases like CO2 and methane. Greenhouse gases have the effect of trapping infrared radiation, causing the atmosphere to become warmer. A thick greenhouse atmosphere could have kept the planet warm during the early times when the sun was much cooler.
Then, over billions of years as the sun got warmer, those greenhouse gases were removed. Their removal would prevent the planet from overheating as the sun got hotter.
Evidence for greenhouse gas removal
Calcium carbonate deposits
All around the planet, there are numerous massive rock formations composed of calcium carbonate (CaCO3). Mainly limestone. Examples include the White Cliffs of Dover, certain geologic layers in the Grand Canyon, and the Pancake Rocks in New Zealand. The carbonate (CO3) part of this mineral came from the atmosphere as CO2 gas. Rocks containing calcium carbonate make up 10-20% of all the sedimentary rocks on Earth. It’s an enormous mass. These rock formations represent ancient burial grounds for atmospheric CO2.
White Cliffs of Dover
Grand Canyon. Image by Tom Morris / PlanetaryBiology.com
Pancake Rocks, New Zealand
Process leading to the deposition of calcium carbonate
Rock weathering
This is a chemical process where gases in the atmosphere react with mineral sin the rocks and make new chemical salts. For example, O2 reacts with the iron in steel and makes a new salt called iron oxide, otherwise known as rust.
For our purposes, CO2 reacts with calcium silicate rocks in the crust and makes calcium carbonate salts (CaCO3).
Goat Rocks Wilderness, Washington state. Exposed rocks like this are prime reaction sites for rock weathering.
Calcium carbonate solidification and precipitation
These salts get washed into the sea. There, the dissolved calcium carbonate is solidified by marine organisms to make their shells. Example organisms include plankton. When they die, they sink to the sea bottom and accumulate. After millions of years, the settled masses of plankton shells get compressed into limestone rock. This process represents a one-way movement of CO2 out of the atmosphere and into the crust.
Neogloboquadrina, a kind of plankton that makes its shell out of calcium carbonate
This process is ongoing across the globe and is immense. For example, plankton blooms happen frequently throughout the year in all the world’s oceans. Although each plankter is tiny, an isolated bloom can contain trillions of individuals. Certain kinds of plankton (like coccolithophores and foraminifera) make their shells out of calcium carbonate. Blooms may last several weeks. Afterwards, the plankton dies and settle to the sea bottom, taking their calcium carbonate shells with them.
If this cycle repeats over millions of years immense quantities of calcium carbonate will be buried in the crust.
In this way, rock weathering and the burial of calcium carbonate represent a one-way movement of CO2 out of the atmosphere and into the crust.
Plankton bloom south of Plymouth, UK
Fixed carbon deposits
Fixed carbon deposits represent the undecomposed organic remains of living things. Like the undecomposed remains of a tree or the organic body parts of a plankton.
Process leading to the deposition of fixed carbon
Carbon fixation and the burial of fixed carbon
In chemical/environmental terms, fixation is a chemical process where a gas is transformed to a solid. Photosynthesis is an example process. In photosynthesis, CO2 is removed from the atmosphere and its carbon atoms are linked to other carbon atoms making a three-carbon molecule that is a solid – not a gas. That new molecule is contained within the body of the photosynthesizing organism. It is later used to make larger molecules that contribute to the body’s operations and mass.
In the ocean, there are plankton that have the ability to photosynthesize. These are phytoplankton. They remove dissolved CO2 from the water and use it as a building material to make fixed carbon molecules. The dissolved CO2 removed from the water for this process is replaced by new atmospheric CO2 dissolving into the sea, working toward spontaneous equilibrium. As we saw above, plankton blooms also mean bulk consumption of environmental resources. In this case, CO2. When the plankton die, they settle to the bottom of the sea, taking their organic remains with them. Depending on environmental conditions, if this process repeats over millions of years, the undecomposed organic remains of these former plankton eventually turn into petroleum deposits.
Petroleum sample.
Human beings search for those deposits and drill deep holes to extract them from the Earth.
Oil field, Lost Hills, CA. Image by Tom Morris / PlanetaryBiology.com
There are massive petroleum deposits on every continent.
In this way, photosynthesis in the ocean represents a one-way movement of CO2 from the atmosphere into the crust.
On land, plants are the main photosynthesizers. Plants remove CO2 from the atmosphere and use it to make fixed carbon. They use this fixed carbon to build themselves. A mature tree represents a large reservoir of carbon. The carbon came from CO2 in the atmosphere. After the tree dies, it falls over and is washed into a nearby lake.
Lake. A depositional environment for trees.
There it becomes waterlogged and sinks to the lake bottom. If the lake is deep enough, the deepest waters will be cold and oxygen free. Under these conditions, the settled tree will not decompose. If this process continues for millions of years, the undecomposed remains plants will turn into coal deposits.
Open pit coal mine
There are massive coal deposits on most continents.
In this way, photosynthesis on land and the burial of fixed carbon represents a one-way movement of CO2 out of the atmosphere and into the crust.
Conclusion
Based on the evidence in the rocks (large deposits of limestone and fixed carbon), and the existence of CO2-removing processes, the greenhouse solution to the Faint Young Sun paradox is reasonable and possible.
This story reveals the potentially powerful influence that life can have in transforming the planetary surface environment of Earth -- another example of the Planetary Biology perspective.
You can watch a video I made on this topic.