According to the Standard Solar Model, the Sun’s brightness steadily increases because helium ash slowly builds up in its core. The introduction of heavier elements like helium forces the Sun to fuse hydrogen faster in order to prevent gravitational collapse, so it shines brighter as it ages. The Sun was ~25% dimmer 4 Billion Years Ago compared to now.

Liquid oceans had already formed 4 Billion Years ago, so Earth’s temperature must have been above the freezing point of water. A Faint Young Sun presents a paradox: how could a 25% dimmer Sun warm the Earth enough to develop liquid oceans?

First, note that the relationship between solar brightness and Earth’s temperature isn’t simple. A 25% fainter Sun only cools Earth by ~7% because Earth’s surface temperature is a balance between energy input in the form of sunlight and energy output from the Earth in the form of blackbody radiation. Further complications include feedback mechanisms such as Greenhouse Gas effects and changes in Earth’s albedo. Having said that, a 7% cooler Earth is enough to make the tropics as cold as the present-day Arctic which makes the evidence for a liquid ocean difficult to reconcile with the standard solar model.

I first heard about this paradox last December at an AGU talk given by Dr. Goldblatt. He pointed out that the early CO₂ partial pressure was ~25 times its current value (anyone have references for this?), but even this increase isn’t sufficient to guarantee the formation of liquid oceans. However, the total pressure of the atmosphere 4 billion years ago is unknown, so Dr. Goldblatt performed simulations with varying amounts of nitrogen.

Nitrogen isn’t a greenhouse gas, but its presence in the atmosphere causes CO₂ to act as a more effective greenhouse gas via pressure broadening. Dr. Goldblatt found that if the early atmosphere contained roughly twice as much nitrogen as it does today, this would raise the Earth’s surface temperature above the freezing point of water.

After his talk was finished, I asked: “You said that evidence constrains the early CO₂ concentration, but is there any experimental evidence which constrains the early nitrogen pressure?” He replied that there wasn’t, and then someone else in the crowd said that ongoing research is attempting to constrain the nitrogen budget through “raindrops.”

I’d like to find out what he was talking about. Do we have fossilized raindrop patterns in ancient rocks? If so, how do they tell us anything about the early nitrogen pressure?