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.

Are we forgetting water vapor?

You state, Doug, that Minik T. Rosing et al argued in their 2010 paper that the warm climate in the early eons of the earth’s existence was due to lower albedo, and not high levels of greenhouse gases. The fossilized rain drops study seemed to point to an atmospheric density less than double of what it is now. Perhaps both of these observations are misleading. Even the Kyoto documents admit that CO2 is a minor GHG compared to water vapor; however the IPCC sponsored computer models use CO2 as a forcing function to incorrectly pump up the concentrations of water vapor in the atmosphere.

In your blog, it is pointed out that water vapor concentrations currently average 20,000 ppm. Perhaps evaporation was much more pronounced in the early eons of the earth’s existence. Large shallow water bodies open 365 days a year to partially cloudy skies would have been conducive to high rates of evaporation. Water vapor, a much lighter gas than CO2 and CH4, would not be a factor in atmospheric density estimates. While working in northern latitudes in the Yukon, I experienced as many as five daily intense rain storms under close to cloudless skies, each with a duration of about five to fifteen minutes.

Don Farley

Gatineau, Quebec, Canada

I don't know

I have seen some hypotheses that atmospheric density was likely about 5x today judging from the wings of various "flying" reptiles and from what it takes to pump blood to the heads of some of the dinosaurs. 2x wouldn't be dense enough to allow those reptiles to get off the ground and if they couldn't get off the ground, their wings wouldn't have been as well-developed.

And how do you tell a fossilized raindrop impact from a fossilized hailstone impact?

some links

Neither do they

You raise some interesting points, and that is part of my motivation for highlighting this paper. The news media has a tendency to report on science "facts" that fit their preconceived notions, totally ignoring how tentative the conclusions or shaky the supporting evidence really is. A directly observed result from a repeatable experiment ranks just a high as a wild guess based on questionable proxy data (or, even worse, computer modeling). People forget that there is a lot of guesswork in science.

How exactly do they know the Sun was faint

As well as the atmosphere probably being of a different density the Sun may not be what some think it is. I'm no nuclear physicists, but have some difficulty with the whole scenario of the Sun being a star that formed from gas and became dense enough for fussion to start, but at the same time having rocky planets form that contain lots of heavy elements that we are told formed in Super Nova.

Nowhere is this explained convincingly, leaving me to feel that many of the hypnosis's about the faint sun may be wrong.

As for the atmosphere, I think that conditions on earth maybe owe more to the moon than is given credit. There has to be more than just the Goldilocks distance from the sun creating the right conditions for life. The constant recycling of CO2 via volcanoes must have played a part, and perhaps we as humans only exists today because of a very quiet period in plate tectonics as our atmosphere is very CO2 deficient. Does the action of the moon keep our core molten providing the energy for this, or is there perhaps other forces at work? So many questions and no answers in the above work. It right to highlight how deficient it is and how accepting of tripe are our press and political class.

That inconsistent Sun

The theory that predicts how main sequence stars behave over their lifetime is called the Standard Solar Model and it has been around for a while. The SSM relates the luminosity, radius, age and composition of a star like the Sun. As the name suggests, the model uses our Sun as its standard, primarily because the luminosity, radius, age and composition of the Sun are well determined. A good discussion of the assumptions and calculations can be found here. It, like all other theories, is not perfect but it is considered a very successful theory in astrophysics.

Basically, for a star to be stable the force of gravity must be counteracted by energy from the thermonuclear reactions in its core. Those reactions turn hydrogen into helium, changing the star's composition over time. Taking all of the variables into account, any star, including our Sun, changes slowly over time until it eventually exhausts its supply of hydrogen and turns into a red giant (briefly). One of the things that falls out of the set of equations is that the Sun will slowly brighten over time. Indeed, it is predicted that long before the Sun turns into a red giant (~6 billion years from now) it will become hot enough to end life on Earth.

Raindrops, Hailstones and Mudballs

Anonymous's question is a good one. I have on several occasions seen the impact craters from hailstones. The craters are steep-walled due to the imbedding of the hailstone in the soil and then melting. I imagine that raindrop impressions would not have that characteristic.
As to the issue of atmospheric density, raindrop size and terminal velocity, those are variables which make interpretation of the impressions very difficult. Allow me to posit another scenario which, if valid, would call into question any interpretation of the impressions as a proxy for atmospheric conditions.
1. The substrate for the impressions is loose, undisturbed volcanic ash so a nearby volcanic eruption is a given condition.
2. The impressions were preserved by a second ashfall which was likely of slightly different composition that allowed the upper layer to separate or erode to reveal the impression layer to the geologists.
3. Large raindrops can be carried laterally several miles from an active storm cell.
4. If laterally transported rain fell through an ash cloud, the raindrops would pick up ash on their descent thus forming mud drops. The surface tension of a mudball would allow a much higher terminal velocity than pure water.
5. The specific gravity of a mudball could easily be more than twice that of water. With a higher terminal velocity and its increased density, a mudball of a given size could have the same impact effect of a raindrop two or three times its size.
6. The impact impression of the mudball would be similar to that of a raindrop.