According to the Standard Solar Model, stars get hotter as they get older. If we apply this model to our sun when it first began, then its luminosity (total energy output) was about 0.7 L. In other words, 70% of the sun’s current output. Using methods in chapter 5 – Astronomical Circumstances, I figure about 0.71 L at Zero Age Main Sequence. So, both estimates are very close.
In talking terms this means that the sun was about 30% cooler when the solar system was forming 4.5 billion years ago. As time went by, the sun gradually grew more luminous – gradually became hotter.
For Earth, its surface temperature is largely a consequence of radiation from the sun. If the sun gets hotter, so does Earth. If the sun gets cooler, so does Earth. So, it’s reasonable to predict that 4 billion years ago, the Earth must have been much cooler than it is today – because the sun was much cooler.
We can do some simple estimates. Today, under our current sun, Earth’s average temperature is 288 degrees Kelvin. This converts to 15 degrees Celsius, and 59 degrees Fahrenheit. Taking what we know about the Standard Solar Model and the sun’s evolving temperature, we can predict what the sun’s temperature was when it first formed. That works out to be about 0.7 – or 70% of its current output. So, let’s run some numbers.
0.7 x 288 degrees K = 202 degrees K, which converts to -71 degrees C, which is -96 degrees F. So, when the Earth first formed, and after its surface cooled, it was a very cold world. Any water on the surface would be frozen solid. The below image visualizes this early frigid Earth.
A visualization of early frozen Earth. Image created with Far Cry 5 Arcade Editor by Ubisoft Entertainment.
There would be no liquid surface water, no oceans or rivers – possibly for billions of years. But as the sun grew warmer, the ice would melt, the hydrologic cycle would become established, the oceans would fill, and the planet would start to resemble the world we know today.
But, there is a little snag in this simple scenario. Despite the cooler sun of this early era, liquid water did flow. According to geologic records, the first evidence of flowing, liquid surface water is found in some rare sedimentary rocks dated 3.9 billion years old. The key point about sedimentary rocks is that they are formed as a result of the deposition of sediments on a lake bottom or shallow ocean bottom. Liquid surface water erodes the sediments. Liquid surface water transports the sediments, carrying them to a body of liquid surface water (like a lake). There they settle to the bottom in distinctive layers that reflect the temporal patterns of deposition. Over time, the sediments are compressed by the weight of later sediments – solidifying into… sedimentary rock.
Example of sedimentary rocks. Note the obvious layering. Image by Tom Morris / PlanetaryBiology.com
The evidence of liquid surface water gives us a good indication of the Earth’s surface temperature from 3.9 Billion years ago to the present time. Normally at one atmosphere of pressure, water stays in a liquid state between 0 degrees C (32 degrees F) and 100 degrees C (212 degrees F). The presence of sedimentary rocks and, therefore, liquid water, upsets our thermal history of planet Earth. Because what it means is this – As the sun grew hotter, planet Earth stayed about the same temperature – which was not too cold for liquid surface water, and not too hot for liquid surface water.
This logical conundrum is known as the Faint Young Sun Paradox. How can the sun grow hotter, but the Earth not?
One clever approach to this paradox rewords the problem to make it more general, giving more flexibility in finding a solution. For example, “As the sun changed, so did the Earth.” The change that happened to the sun had to do with its radiation output. We are fairly certain that Earth’s temperature didn’t change. So, what DID change? Earth’s atmosphere.
This solution goes something like this. When the sun was young and much cooler, the Earth was covered by a thick layer of greenhouse gases like CO2, CO, and methane. These gases let in sunlight but trap infrared radiation coming from the planet’s surface. . As the sun grew hotter, these greenhouse gases were removed from the atmosphere. That kept the planet from overheating.
Imagined early Earth with weaker sun but thick atmosphere of greenhouse gases. Image created with Far Cry 5 Arcade Editor by Ubisoft Entertainment.
Think of your car’s interior temperature when you leave it parked in the sun on a hot summer day. All that time sunlight is entering through the glass and warming up your car’s interior, which re-radiates that absorbed energy as infrared radiation (radiant heat). But when the infrared tries to pass through the glass, the glass blocks its escape. So, the infrared energy accumulates inside the car. As a result, the car’s interior temperature is much hotter than the outside air temperature.
In the car, the glass is the gatekeeper. It lets in sunlight, but traps escaping infrared. On a planet, greenhouse gases are the gatekeepers. They let in sunlight, but trap outgoing infrared. In general, the more greenhouse gases in a planet’s atmosphere, the warmer the planet.
As time went by and as the sun grew hotter, most of the greenhouse gases were removed from the atmosphere. This kept the planet from overheating.
Present day Earth, warmer sun but low in greenhouse gases. Image created with Far Cry 5 Arcade Editor by Ubisoft Entertainment.
This is a very ingenious solution to the faint young sun paradox. But where is the evidence for massive CO2 removal? And given such evidence, what are the processes that produce this result?
More about that in a later post.
You can watch a video I made on this topic.