Passage VI

Four billion years ago (4 BYA), the Sun was only 70% as bright as it is today. With sunlight that faint, Earth's average surface temperature should have been -15°C and all surface water would have been frozen. However, isotope evidence from certain sedimentary rocks (rocks made of sediments deposited in liquid water) formed around that time indicates that Earth's surface temperature was well above freezing.

Scientists believe that 1 of 2 greenhouse gases, either ammonia (NH₃) or carbon dioxide (CO₂), was present in Earth's atmosphere at a concentration well above that of the respective gas in the atmosphere today. This gas trapped enough heat radiating from Earth's surface to keep the surface temperature above freezing.

Two scientists' present their viewpoints.

Scientist 1

Four BYA, large quantities of NH₃ continuously entered Earth's atmosphere from ocean-floor hydrothermal vents and by other processes. NH₃ is a more efficient greenhouse gas than is CO₂, so a lower concentration of NH₃ is required to warm the atmosphere a given amount. Any NH₃ molecule entering the atmosphere would have been broken down in 5-10 days by UV radiation. However, methane (CH₄), another gas that was abundant in the atmosphere 4 BYA, combined with molecular nitrogen gas to produce organic compounds that blocked much of the incoming UV radiation and kept NH₃ molecules intact for much longer than 10 days. Continuous removal of CO₂ from the atmosphere by ocean water prevented CO₂ from accumulating in the atmosphere.

Scientist 2

Two carbon isotope found in sedimentary rocks indicate a 75% higher concentration of CO₂ in Earth's atmosphere 4 BYA than the 360 parts per million (ppm) present today. Four BYA, large quantities of CO₂ continuously entered the atmosphere through widespread, long-term volcanic activity. Today, as in the past, much CO₂ is removed from the atmosphere as CO₂ becomes part of carbonates in rock and sediment on land. However, 4 BYA, there was 80% less dry land area on which to form and store carbonates, so CO₂ accumulated in the atmosphere.

The vast amounts of CH₄ that would be required to help preserve atmospheric NH₃ did not exist on Earth 4 BYA. Thus, atmospheric NH₃ was too short-lived to have been an effective greenhouse gas.