A POSSIBLE ARCHEAN CLIMATE FEEDBACK MECHANISM

The absence of evidence for major glaciation during the Archean, when solar luminosity was ~20% lower than today, suggests the greenhouse effect was much stronger then than it is now. Rye et al. (1995) showed that CO₂ levels at 2.75 Ga were at least a factor of ten lower than that required by 1-D climate models using a simple CO₂-H₂O greenhouse. The radiative deficit left by the CO₂-H₂O system was probably filled by methane.

Biogenic methane production could have been rapid enough to provide 100s ppm atmospheric CH₄ even in the absence of a UV screen. At such high CH₄ levels Earth would have remained glacier free. The residence time of CH₄ in such an atmosphere would have been decades. Thus, sudden increases in CH₄ production might have led to runaway greenhouse conditions. However, if CH₄ accumulates until CH₄/CO₂ > 1 an aerosol haze will form. This haze probably will be a strong UV screen. Ammonia, which is highly UV labile, will begin to accumulate in the atmosphere, quickly driving rain pH above 7 and dramatically slowing chemical weathering on the continents. Without weathering little or no phosphate will be delivered to the oceans. Phosphate is necessary for life. Its ocean residence time is ~10,000 years. Thus, over a time scale of ~10,000 years primary productivity will drop dramatically without phosphate input. Biogenic methane production, the end of the food chain, will probably suffer most. With little or no CH₄ production CH₄/CO₂ goes to < 1. The UV screen and atmospheric NH₃ disappear in a few years. Rain pH drops. Weathering restarts and biological productivity recovers.

Today the CO₂-H₂O Walker feedback is thought to be largely responsible for long-term climate stability. This proposed feedback may have operated for over a billion years, prior to the rise of oxygen. It responds on a time scale of ~10,000 years, and is, therefore, slightly faster than the Walker feedback.