Question

### The Faint Young Sun Paradox

Astronomical models indicate that $4.0$ billion years ago, the Sun's energy output was only about $70\%$ of its current level. Under these conditions, liquid water on Earth's surface should have frozen, yet geological evidence shows that liquid water and early life existed. Two scientists discuss how early Earth maintained temperatures warm enough to support liquid water.

Scientist 1
Early Earth's atmosphere was kept warm primarily by a dense greenhouse atmosphere dominated by carbon dioxide ($CO_2$) and water vapor ($H_2O$) released through intense volcanic outgassing. Because the early Earth lacked continental landmasses to weather and remove $CO_2$ from the atmosphere, $CO_2$ levels remained extremely high—hundreds of times greater than modern levels. This massive reservoir of $CO_2$ was sufficient to trap enough heat to prevent Earth from freezing. Biogenic activity was not required to maintain warm conditions, and methane ($CH_4$) played no significant role because it would have been rapidly destroyed by chemical reactions with oxygen-containing radicals.

Scientist 2
Volcanic outgassing alone could not have supplied enough carbon dioxide ($CO_2$) to prevent global glaciation. Instead, the primary warming was driven by methane ($CH_4$) and ammonia ($NH_3$). These gases are much more potent greenhouse gases than $CO_2$. They were continuously produced and replenished by early anaerobic microbes (methanogens) living in the oceans. Although solar ultraviolet (UV) radiation normally breaks down methane and ammonia, a thick photochemical organic haze formed in the upper atmosphere, blocking the UV light and protecting these biogenic gases from decomposition, thereby maintaining a stable greenhouse effect.

Based on the passage, Scientist 1 and Scientist 2 disagree on which of the following points?