Question

### Archean Atmospheric Composition

During the Archean Eon (approximately $3.8$ to $2.5$ billion years ago), the Sun's energy output was only $70\%$ to $75\%$ of its current value. Under these conditions, without a strong atmospheric greenhouse effect, Earth's surface water would have frozen completely. Yet, geological evidence shows that liquid oceans existed. Two scientists discuss the atmospheric conditions that resolved this "Faint Young Sun Paradox."

Scientist 1
The primary greenhouse gas keeping the Archean Earth warm was biogenic methane ($CH_4$), which was maintained at concentrations above $1,000\text{ ppm}$ by widespread methanogenic archaea. Carbon dioxide ($CO_2$) was not abundant enough to prevent global glaciation. Basaltic rock weathering on the early continents was highly efficient, drawing $CO_2$ out of the atmosphere and mineralizing it as carbonates. This weathering feedback restricted Archean atmospheric $CO_2$ pressure to less than $0.01\text{ bar}$. Because atmospheric methane is unstable and rapidly destroyed by solar ultraviolet radiation (photodissociation), a continuous biological source was required. Without these methanogenic microbes, Earth would have immediately entered a global ice age.

Scientist 2
Methanogenic microbes had not yet evolved during the Archean, so biogenic methane was absent. Instead, Earth was kept warm by extremely high levels of carbon dioxide ($CO_2$)—reaching partial pressures of $1$ to $2\text{ bar}$—supplemented by volcanic hydrogen ($H_2$). Basaltic weathering was negligible because continental landmasses were small and mostly submerged, preventing the drawdown of $CO_2$. Volcanic outgassing continuously supplied $CO_2$ and $H_2$ to the atmosphere. Furthermore, collision-induced absorption between $N_2$, $CO_2$, and $H_2$ significantly boosted the warming effect of these gases. The Archean climate was thus regulated entirely by abiotic, geochemical cycles.

According to the passage, Scientist 1 and Scientist 2 differ in their views regarding which of the following aspects of the Archean Earth?