Fossil Raindrops and the Faint Young Sun
It is accepted that the ancient Sun was considerably cooler than our local star is today, so much so that Earth a few billion years ago should have been a lifeless frozen ball. But scientists have also shown that the planet was not frozen—shallow seas warmer than any modern ocean abounded with microbial life. A recent study, detailed in the journal Nature, is a good example of the sometimes convoluted, even improbable reasoning is used to get a handle on earthly climates during eons long vanished. Using the fossilized impact dimples from rain drops that fell 2.7 billion years ago, researchers have calculated new limits on the density of Earth's atmosphere. This, in turn, has implications on the development of the ancient atmosphere and what role greenhouse gases may have played in warming the young Earth.
The paradox of the Faint Young Sun has vexed scientists for decades, having first been raised by astronomers Carl Sagan and George Mullen in 1972. Applying the observation made by astrophysicists, that main sequence starts like the Sun grow steadily warmer as they age, and working back in time from current day conditions they posed an awkward question: why did the ancient Earth not freeze solid billions of years ago? In their new paper, “Air density 2.7 billion years ago limited to less than twice modern levels by fossil raindrop imprints,” the problem is framed this way by Sanjay M. Som et al.:
According to the ‘Faint Young Sun’ paradox, during the late Archaean eon a Sun approximately 20% dimmer warmed the early Earth such that it had liquid water and a clement climate1. Explanations for this phenomenon have invoked a denser atmosphere that provided warmth by nitrogen pressure broadening or enhanced greenhouse gas concentrations. Such solutions are allowed by geochemical studies and numerical investigations that place approximate concentration limits on Archaean atmospheric gases, including methane, carbon dioxide and oxygen. But no field data constraining ground-level air density and barometric pressure have been reported, leaving the plausibility of these various hypotheses in doubt.
Two main explanations have been proposed: that Earth's atmosphere retained heat more efficiently in the past than it does now, possibly because of increased concentrations of greenhouse gases; or that the albedo of Earth was lower in the past, perhaps because there were fewer clouds and/or less ice. Arguments have raged with multiple proponents on either side. In 2010, Minik T. Rosing et al. claimed the mystery was easily solved by lower albedo. Writing in “No climate paradox under the faint early Sun,” they argue against high levels of greenhouse gases:
It has been inferred that the greenhouse effect of atmospheric CO2 and/or CH4 compensated for the lower solar luminosity and dictated an Archaean climate in which liquid water was stable in the hydrosphere. Here we demonstrate, however, that the mineralogy of Archaean sediments, particularly the ubiquitous presence of mixed-valence Fe(II–III) oxides (magnetite) in banded iron formations is inconsistent with such high concentrations of greenhouse gases and the metabolic constraints of extant methanogens.
Then, just last year, Colin Goldblatt and Kevin J. Zahnle wrote in “Faint young Sun paradox remains”:
Rosing et al. claim that the paradox can be resolved by making the early Earth’s clouds and surface less reflective. We show that, even with the strongest plausible assumptions, reducing cloud and surface albedos falls short by a factor of two of resolving the paradox. A temperate Archean climate cannot be reconciled with the low level of CO2 suggested by Rosing et al.; a stronger greenhouse effect is needed.
That's science speak for “it's on, albedo fanboys!” As you can see, this argument has continued back and forth in the pages of Nature and other journals to this day. It seems reasonable to assume that knowing more about how Earth's atmosphere and oceans were heated in the distant past may help shed some light, as it were, on the current global warming kerfuffle. However, a reasonable person might well ask “how do they know what was going on back then?” This was before the first primitive animals crawled onto land, before the first fish swam in the sea, even before cyanobacteria managed to “poison” the global atmosphere with deadly oxygen, setting the stage for all that was to come. Enter the fossil raindrops.
Liquid water abounded 2.7 billion years ago.
While it may initially appear daft that such a thing as fossilized raindrops (more accurately, the impact craters made by the drops striking the ground) could exist, let alone reveal the mysteries of deep time climate, the idea is not a new one. It was originally proposed by Charles Lyell, based on observations he reported in 1851. Sir Charles was the foremost geologist of his day, an influential friend of Charles Darwin and not unaccustomed to scientific controversy in his day, as we described in The Resilient Earth. After Lyell's prescient report, analysis of fossil raindrop imprints languished for more than a century and a half.
Of course, scientific knowledge has expanded and methods of observation and measurement have improved somewhat since the mid-19th century. Falling raindrops flatten and fragment when the total aerodynamic forces exceed the combination of surface tension and hydrostatic forces (see the article for the pertinent equations and explanation). On the ancient Earth, maximum raindrop diameters should have been essentially identical to today’s, because the maximum size beyond which raindrops disintegrate at terminal velocity is independent of air density. What air density does impact is terminal velocity.
The size distribution for raindrops under modern conditions is well known and the effects of their impact on various soils was studied. “The relationship between drop impact momentum and corresponding imprint area was obtained from experiments in which we released water drops of known mass from an indoor height sufficient to guarantee that they reached terminal velocity onto ash substrates analogous to the Archaean tuff,” report Som et al. Moreover, as improbable as it may sound, fossilized impact patterns have been found in a number of locations dating back to the mid Archean.
2.7-billion-year-old fossil raindrop imprints in tuff at Omdraaivlei, South Africa.
Som et al. conclude that the atmospheric density 2.7 billion years ago was between 50 and 105% of that present today. “This finding immediately calls into question solutions to the faint young Sun paradox that invoke elevated concentrations of greenhouse gases, unless small increases of greenhouse-gas concentration were able to exert a large warming effect,” state William S. Cassata and Paul R. Renne in an accompanying article in the same issue of Nature. “It is also unlikely that higher concentrations of greenhouse-enhancing nitrogen could have caused the paradox, because concentrations of twice or more the present atmospheric abundance would have been required to provide sufficient warming in the presence of a modest increase in carbon dioxide. Under such conditions, the atmospheric density would have been greater than that predicted by the authors,” they conclude.
Anyone with a modicum of scientific knowledge can see what a tenuous link to past conditions this new proxy data provides—the conditions must have been just right to capture such a fleeting pattern made by falling raindrops. Of course, those conditions only needed to occur a few times anywhere on the planet over a period of hundreds of millions of years. More troubling is interpreting the preserved rain dimples accurately. Cassata and Renne detail the some of the variables involved:
Although raindrop size distributions associated with typical storms are well known, it is possible — albeit unlikely — that the ancient raindrops responsible for the fossilized imprints were unusually large. Small errors in the inferred size of the raindrops would result in significant errors in the atmospheric pressure predicted by Som and colleagues' method. The accuracy of the method is further limited by lack of information about factors (such as moisture content) that would have affected the cohesiveness of the ash in which the fossil imprints were made; the cohesiveness can affect the morphology of impact craters.
The bottom line implication of the Som et al. work is that what kept the young Earth from turning into a permanent snowball was not CO2 or other GHGs, the atmosphere just was not dense enough for the greenhouse effect to do the job. This is even with broadening of the infrared absorption bands of those gases due to higher atmospheric pressure as high as twice modern levels. So, despite the uncertainties involved, the science behind interpreting fossil raindrop data reinforces two things: the power the Sun has over Earth's climate and the ineffectual nature of greenhouse gases. If solar output varies significantly, all the CO2 generated by humans and other members of the planetary biota acts only at the margins to change climate.
The other lesson here is that people are constantly bombarded by “science” reports that are used by one side or the other in the climate change debate. Even if the reports are based on actual science—not computer model hand waving—the public remains unaware of the abstract and often tortuous methodology behind the pronouncements. The actual work is reported by the investigators, interpreted by other scientists and journalists and eventually so dumbed down that neither the voracity nor the importance of any single statement can be ascertained. Add to that questionable chain of evidence the fact that, in these hyper-charged political times, everyone seems to have an agenda, from the investigators to the media talking heads.
So should we care about the implications of measuring fossil raindrops from several billion years ago? Certainly, this is a fascinating paper that highlights the lengths to which scientists go in trying to discover Earth's past climate. True, the results are highly uncertain and open to differing interpretations, but that is true of all climate science today. Good science with interesting results often does not rise to the level of a clarion call for action.
Still, those whose reputations and livelihood depend on government money aimed at “fighting global warming” will become ever more strident in their warnings as we near the release of yet another IPCC report next year. Should you be worried about human CO2 emissions ruining Earth's fragile climate? Let me put it this way—I just bought a new SUV.
Be safe, enjoy the interglacial and stay skeptical.