Apr 2012

Raindrops fossils = a barometer inside a DeLorean


I’ve been meaning to write about friend and colleague Sanjoy Som’s paper for over a week now. I’m finally getting around to it this evening. I submit to you that this paper is both unbelievably cool in the “I didn’t know that was possible” sort of way yet also very important in the “we really needed that dataset” sort of way.

First, the cool. Check out the title: “Air density 2.7 billion years ago limited to less than twice modern levels by fossil raindrop imprints.” What the authors are telling us is that ~2,700,000,000 years ago it rained in what is now South Africa. The raindrops left behind tiny craters that were preserved until today. Based on the sizes of those preserved craters, the authors were able to tell what the atmospheric pressure was at the time that it rained. Think of it as using fossilized raindrop imprints as a barometer stuck inside a time machine. The ingenuity of our species continues to amaze me.

So why is this important? Let’s start with what the “Faint Young Sun Paradox” f=”http://www.sciencemag.org/content/177/4043/52.short”>explained by Carl Sagan and George Mullen 40 years ago. Stars get gradually brighter as they age. Over the course of the 4.6 billions years of Earth’s existence, the Sun has brightened considerably. That means if you go back in time to 2.7 billion years ago, the Sun should have been 20% dimmer than today… and the Earth should have only received about 80% of the solar energy it currently receives. Yet, there is lots of evidence of liquid water at the surface and a lack of glaciations for most of Earth history prior to 2.5 billion years ago (with one known exception). This tells us that despite the dimmer Sun, the Earth was at least as warm as it is today… and probably at least a little bit warmer. Just how warm it was is a matter of debate: some (not me) argue, based on the composition of rocks from the time, for a “hot early Earth” that had average surface temperatures above 120F!

Atmospheric modelers (including yours truly) have tried to show how the early Earth would have stayed warm despite the “faint young Sun” problem. Originally, many solutions turned to increased concentrations of greenhouse gases such as ammonia, methane, and carbon dioxide. However, all of these have issues: ammonia is rapidly destroyed in the atmosphere, too much methane creates an organic haze that blocks even more sunlight, and there are limits on carbon dioxide concentrations from other geochemical data. One proposed solution to the paradox was recently put forth by another friend/colleague, Colin Goldblatt. He suggested that the atmosphere may have had a higher pressure… and that the higher pressure would have caused greenhouse gases to be more efficient. Similarly, if we want to reproduce the temperatures proposed by those arguing for a “hot early Earth” we have to both throw out the limits on carbon dioxide and invoke an atmosphere (full of carbon dioxide) with many times the pressure of the Earth.

Enter Som et al.’s data set. They constrain the pressure 2.7 billion years ago to be less than twice that on modern-day Earth. If they’re right, this would significantly constrain the amount of “extra warming” we’ll be able to squeeze out of greenhouse gases by increasing atmospheric pressure. It would also make it a “hot early Earth” nearly impossible.

Before this study, we had no knowledge of the pressure history of the Earth prior to the invention of the barometer in the 1600′s. Now we have an estimate for 2.7 billion years ago. I point this out for two reasons: 1. it re-emphasizes the “WOW factor” of this study, and 2. it shows that this is new work. And because it’s a new method with important implications, it needs to be corroborated by other metrics. Until that happens, we should not just throw out the “hot early Earth” claims or high-pressure solutions to the “faint young Sun paradox.” In the meantime, while I await such confirmation I will marvel at the ingenuity of the work of Som et al.

S. Domagal-Goldman Posted by
S. Domagal-Goldman
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