Faint Young Sun Paradox Resolved
For decades, astronomers and geologists have worried about a paradox. Stellar evolution theory claims sunlight on the early earth would have been 20-30% dimmer than it is today, but geology shows the oceans were liquid in the earliest (Archean) rocks. For that matter, so does the book of Genesis, but that record is not usually allowed in scientific discussions. Anyway, how could the earth remained warm enough under a dim sun to keep the oceans from freezing? This has been called the “faint young sun paradox.” A new answer came from researchers at the Tokyo Institute of Technology and University of Copenhagen’s Department of Chemistry, published this month in PNAS.1
The solution involves carbonyl sulfide produced by volcanoes. Matthew Johnson of the Copenhagen group explained the scenario in the University’s press release, “The greenhouse gas that saved the world.” (See also Science Daily.) Carbonyl sulfide (OCS) is the perfect greenhouse gas, he said. “We estimate that a blanket of Carbonyl Sulphate [sic; sulfide] would have provided about 30 percent extra energy to the surface of the planet. And that would have compensated for what was lacking from the sun.” Very convenient. But why, then, is OCS not a problem today, with all our concern about greenhouse gases and global warming? Because oxidizing conditions (free oxygen of carbon dioxide in the atmosphere) destroy OCS. In other words, OCS was plentiful when it was needed to warm the early earth, but gradually was depleted as life pumped oxygen into the atmosphere. Instead of producing the warming OCS, it would have produced cooling sulfates. This could have led to a proposed “snowball earth” period before the sun became warm enough to melt the oceans again. Johnson tied this in to modern political fears about global warming: “Our research indicates that the distribution and composition of atmospheric gasses swung the planet from a state of life supporting warmth to a planet-wide ice-age spanning millions of years,” he said. “I can think of no better reason to be extremely cautious about the amounts of greenhouse gasses we are currently emitting to the atmosphere.”
The story is messier inside the scientific paper. The authors needed to thread a needle getting the right balance of factors and assumptions to make this work. Here’s a taste of it (the scientific jargon can be overlooked to see the amount of hedging and special pleading going on):
When the column density [atmospheric gas] increases, three types of behavior are seen. First, an increase in CO2, H2O, NH3, CS2, or O2 concentration produces a similar negative shift of 34[epsilon] and 33E (Fig. 2), because these gases generally attenuate wavelengths shorter than 202 nm (Fig. 1). In contrast, O3 and OCS shielding shift 34[epsilon] and 33E toward more positive values because they attenuate wavelengths longer than 202 nm. In contrast, an increase of O3 or OCS has the opposite effect. Finally, SO2 self-shielding produces increasing 34[epsilon] and decreasing 33E. The previous estimate of isotope effects in SO2 photolysis from self-shielding found an increasing trend in 33E (12) that is opposite to our result. The reason for the difference may be because the spectra of the isotopically substituted species were approximated by shifting the absorption peaks of the natural abundance SO2 spectrum using a set of isotope-dependent frequency shifts. These shifts were based on vibrational wavefunctions calculated using a single ab initio SO2 excited potential energy surface. However, SO2 has a score of electronic states in the relevant energy region with multiple curve crossings, giving rise to the complicated pattern seen in the experimental results. For example, three vibronic peaks of the three isotopologues are found between 200 and 205 nm (Fig. S1). First 33SO2, then 34SO2, and then 32SO2 has the highest peak intensity. Whereas the isotopologues’ peak positions shift linearly with distance from the band origin, the peak intensities, widths, and profiles of the vibrational structure change in a complex “mass independent” manner. A theoretical description of the origin of these isotope effects awaits further study….
It appears clear there is much not well understood about the molecules themselves, let alone their complex, interacting effects on global warming.
Further down in the paper, they compared other greenhouse gas candidates. Carbon dioxide doesn’t work, because it would require too much of it – 30%, far more than present today – to produce the 30% warming needed, and it also would ruin the isotopic sulfur signal in the geological record. Ammonia doesn’t work, because it would photolyze too rapidly. Methane doesn’t work, because the haze would have cooled rather than heated the planet. OCS (carbonyl sulfide) seemed the only candidate left standing. It would allow higher concentrations of ammonia (given that the early atmosphere was reducing), or of methane, if OCS were present in concentrations of 10 parts per million or more. Even so, the model called for more special pleading and future research:
The atmospheric models presented here are an initial attempt at predicting the [delta]33S value of aerosol sulfate for a set of atmospheric shielding scenarios. Further model studies are needed to evaluate the relative contributions of the greenhouse gases. Nonetheless, a CO-rich reducing atmosphere would have resulted in OCS-rich conditions when volcanic sulfur input was high enough. Moreover, such an atmosphere is so far the only one that can explain both the preservation of MIF [mass-independent fractionation of sulfur isotopes] and the negative [delta]33S values of Archean sulfate deposits. Hence, UV-shielding and the greenhouse effect of OCS should be considered for any model of the Archean atmosphere. These results are qualitative and remain to be confirmed by more advanced models of the Archean atmosphere and further laboratory studies.
The tone of these paragraphs sounds much more reserved and tentative than the press release that triumphantly pronounced carbonyl sulfide as “the greenhouse gas that saved the world.”
1. Ueno, Johnson et al, “Geological sulfur isotopes indicate elevated OCS in the Archean atmosphere, solving faint young sun paradox,” Proceedings of the National Academy of Sciences USA, online August 17, 2009, doi: 10.1073/pnas.0903518106.
Once again, we have revealed to you the huge discrepancy between the hedging and fudging found in scientific papers and the victory speeches in the press. Beware of bluffing from scientists. Always look at the data they base their conclusions on. But even the paper announced in the title that this model solves the faint young sun paradox. It did no such thing. It was a like a string of if’s followed by a then maybe. One of the authors stood on that quicksand and preached to us about global warming. Well, if the earth goddess was smart enough to save the world when our star was weak, why isn’t she smart enough to protect the earth from carbon-belching humans? Oh, but of course. It wasn’t Gaia; it was Lady Luck.
This model, with speculations interleaved between assumptions like a giant theoretical Dagwood sandwich, never questioned the presumed age of the earth and the sun, or stellar evolution theory, or the snowball earth hypothesis, or the amount of ancient volcanism, or the sulfur content of outgassing in ancient times, or the plausibility of a reducing atmosphere, or the origin of life, or the interpretation of sulfur isotopes in rocks. Its links to scientific evidence were weak at best: a few known properties about UV shielding of certain gases, the output of modern volcanoes, and measurements of sulfur isotopes in certain rocks. That’s it. The rest is nebulous fluff. The model is like tying clouds together with rope (or with silly string). The early history of the earth is one cloud, and the early history of the sun is another cloud. Even with multiple strands of silly string trying to hold them together, the clouds will most likely do what they do: drift apart, oblivious to these scientists’ heroic attempts to lasso them together.