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.