The Goldilocks principle: New hypothesis explains Earth's continued habitability

Mar 19, 2014
Researcher Josh West treks through a valley in Peru in search of evidence of chemical weathering of rocks as they erode. Credit: Mark Torres / USC

Researchers from USC and Nanjing University in China have documented evidence suggesting that part of the reason that the Earth has become neither sweltering like Venus nor frigid like Mars lies with a built-in atmospheric carbon dioxide regulator – the geologic cycles that churn up the planet's rocky surface.

Scientists have long known that "fresh" rock pushed to the surface via mountain formation effectively acts as a kind of sponge, soaking up the greenhouse gas CO2. Left unchecked, however, that process would simply deplete atmospheric CO2 levels to a point that would plunge the Earth into an eternal winter within a few million years during the formation of large mountain ranges like the Himalayas – which has clearly not happened.

And while volcanoes have long been pointed to as a source of carbon dioxide, alone they cannot balance out the excess uptake of carbon dioxide by large mountain ranges. Instead, it turns out that "fresh" rock exposed by uplift also emits carbon through a chemical weathering process, which replenishes the dioxide at a comparable rate.

"Our presence on Earth is dependent upon this carbon cycle. This is why life is able to survive," said Mark Torres, lead author of a study disclosing the findings that appears in Nature on March 20. Torres, a doctoral fellow at the USC Dornsife College of Letters, Arts and Sciences, and a fellow at the Center for Dark Energy Biosphere Investigations (C-DEBI), collaborated with Joshua West, professor of Earth Sciences at USC Dornsife, and Gaojun Li of Nanjing University in China.

While human-made atmospheric carbon dioxide increases are currently driving significant changes in the Earth's climate, the geologic system has kept things balanced for million of years.

"The Earth is a bit like a big, natural recycler," West said. Torres and West studied rocks taken from the Andes mountain range in Peru and found that weathering processes affecting rocks released far more carbon than previously estimated, which motivated them to consider the global implications of CO2 release during mountain formation.

The researchers noted that rapid erosion in the Andes unearths abundant pyrite—the shiny mineral known as "fool's gold" because of its deceptive appearance—and its chemical breakdown produces acids that release CO2 from other minerals. These observations motivated them to consider the global implications of CO2 release during .

Like many other large mountain ranges, such as the great Himalayas, the Andes began to form during the Cenozoic period, which began about 60 million years ago and happened to coincide with a major perturbation in the cycling of atmospheric . Using marine records of the long-term , Torres, West, and Li reconstructed the balance between CO2 release and uptake caused by the uplift of large and found that the release of CO2 release by rock weathering may have played a large, but thus far unrecognized, role in regulating the concentration of over the last roughly 60 million years.

Explore further: Ancient forests stabilized Earth's CO2 and climate

More information: Sulphide oxidation and carbonate dissolution as a source of CO2 over geological timescales, Nature, DOI: 10.1038/nature13030

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Rute
3.5 / 5 (4) Mar 19, 2014
There is a clear trend where CO2 levels have been going down during Earth's billions of years of existence, which has counteracted the increased solar irradiance (see stellar evolution) to keep the Earth's mean temperature quite stable. That temperature regulation is a sign of homeostatic system (as is mammalian warm-bloodedness for example) and it seems pretty absurd to think that it can be explained by abiotic factors alone.

James Lovelock wrote about this subject in the 70s and he was wise to understand that the cybernetic, self-regulating system on Earth needs a biotic component in order for the regulation based on negative feedbacks to work properly.

The abiotic nature is full of positive feedback systems such as the formation of planet-wide dust storms on Mars and runaway greenhouse effect on Venus. I would bet over any sum of money that had not life existed on Earth, our atmosphere would have reached a devastating chemical equilibrium similar to those seen on Mars or Venus.
Torbjorn_Larsson_OM
5 / 5 (1) Mar 20, 2014
And pyrite is in turn produced by sulfur bacteria, I think.

These large recyclings making controlling negative feedback loops are nice, but there are many of them. If CO2 wasn't regulated hugely by geology, we wouldn't have complex multicellular life for very long is all.

If CO2 wasn't regulated smallish by other erosion, prokaryotes would do it on some scale as they eat through the rocks. The planet would still be habitable for prokaryotes, setting up their own negative feedbacks however minute. Exponential population growth ensures that will eventually happen.

@Rute: Very few astrobiologists take notice of Lovelock's ideas, since they have been massively rejected. None of his ideas has worked out, what I hear. Examples of mainly abiotic negative feedback: the water/ice border cloud cover that regulated the Archean climate and prevented a snowball with CO2 atmosphere, or the erosion vs CO2 that regulated against snowball with O2 (snowball -> less erosion -> more CO2 -> thaw).

Torbjorn_Larsson_OM
4 / 5 (1) Mar 20, 2014
@Rute: Well, I found I was both right and wrong by checking for refs. The description I found is that Lovelock's ideas got criticism, had to be modified, has "highly scientifically respected detractors" and is as of yet untested.

After "long debate and much criticism" the idea "has found acceptance among _ecologists_, apparently without testing. But not among astrobiologists. [ http://books.goog...;f=false ]

I would take that bet. =D
Torbjorn_Larsson_OM
not rated yet Mar 20, 2014
Edit fail: "has found acceptance".
Benni
1 / 5 (5) Mar 20, 2014
The Martian atmosphere is 95% CO2, by contrast the Earth's atmosphere is 0.04 % CO2. If this gas were such a driver of warming atmospheric temperatures you'd expect to see it a lot warmer on Mars than it is.
Rute
4.4 / 5 (7) Mar 20, 2014
Benni: The greenhouse effect is dependable on both the relative abundances of gases as well as atmospheric density. The atm pressure on the surface of Mars is about 0,6% of the pressure on Earth's surface which results in almost negligible greenhouse effect. In the other extreme we have Venus whose atm pressure on surface is about 92 times that of Earth's and CO2 concentration is at 96,5%, creating a very powerful greenhouse effect. Because of the greenhouse effect, Venus is a lot hotter planet than Mercury (which is closest to the sun), although it receives only 25% of the solar irradiance compared to Mercury.
Rute
3.3 / 5 (3) Mar 20, 2014
@Torbjorn Larsson

I am not entirely sure what you are referring to when you mention water/ice border cloud cover. However, the ice-albedo feedback is yet another example of positive feedback in purely abiotic conditions. As cooling increases ice cover, the albedo increases as well, which leads to more light reflected to space, more cooling, and results in ice cover expansion.

In today's world the ice ages are regulated by the expansion of boreal forests in warm periods which are significant oxygen net producers and CO2 sinks, and accumulation of CO2 in ice ages because of an inverse situation. (The tropical forests consume almost all of the oxygen they produce due to the high decomposer activity.)
Rute
2.3 / 5 (3) Mar 20, 2014
@Torbjorn Larsson - continued

True, snowball Earth did accumulate CO2 because CO2-consuming cyanobacteria were robbed of light due to the ice cover, among other reasons. It is estimated that the thawing must have taken about 350 times the CO2 concentration compared to today's, but it was counteracted by a cyanobacteria population explosion that then consumed the CO2. I'm not sure what the CO2 concentration would be on Earth without life altogether, but from what I remember, most models estimate it to be over 90%.
Benni
1 / 5 (4) Mar 20, 2014
Orbital Variations:

Changes in orbital eccentricity affect the Earth-sun distance. Currently, a difference of only 3 percent (5 million kilometers) exists between closest approach (perihelion), which occurs on or about January 3, and furthest departure (aphelion), which occurs on or about July 4. This difference in distance amounts to about a 6 percent increase in incoming solar radiation from July to January. The shape of the Earth's orbit changes from being elliptical (high eccentricity) to being nearly circular (low eccentricity) in a cycle that takes between 90-100k yrs. When the orbit is highly elliptical, the amount of insolation received at perihelion would be on the order of 20 to 30 percent greater than at aphelion, resulting in a substantially different climate from what we experience today.

@Rute: Ice ages are not governed by expansion, or contraction, of boreal forests. Ice ages are governed by the tilt of the Earth's axis which is cyclical & above orbital periods.
Benni
1.3 / 5 (4) Mar 20, 2014
Obliquity (change in axial tilt)

As the axial tilt increases, the seasonal contrast increases so that winters are colder and summers are warmer in both hemispheres. Today, the Earth's axis is tilted 23.5 degrees from the plane of its orbit around the sun. But this tilt changes. During a cycle that averages about 40,000 years, the tilt of the axis varies between 22.1 and 24.5 degrees. Because this tilt changes, the seasons as we know them can become exaggerated. More tilt means more severe seasons—warmer summers and colder winters; less tilt means less severe seasons—cooler summers and milder winters. It's the cool summers that are thought to allow snow and ice to last from year-to-year in high latitudes, eventually building up into massive ice sheets. There are positive feedbacks in the climate system as well, because an Earth covered with more snow reflects more of the sun's energy into space, causing additional cooling.

Boreal forest expansion cannot cause tilting of Earths axis.
Benni
1 / 5 (4) Mar 20, 2014
CO2 in ppm levels in Earths' atmosphere are now the highest levels they've been for the past century, or longer, yet we've been experiencing a steady global cooling for the past 16 years & we are now below the moving average of the past several decades.

I should remind you too that the atmospheres of Venus & Mars are 95% CO2, yet we see no correlation of CO2 levels with regard to "warming", or lack of it, on those two planets, meanwhile Earth's atmosphere is only 0.04% CO2, so much for boreal forests having anything to do with climate change, warming or cooling.

Modernmystic
4 / 5 (1) Mar 20, 2014
So there's another variable in the mix. You have to have a certain amount of carbon and it has to cycle in either exactly the same way it does here or in a very similar way or you end up with with a feedback spiral to either an ice ball or a magma ball.

Those who think we don't live in a fairly rare condition here on Earth are running out of room for their ideas of universal mediocrity with respect to life bearing planets.
Benni
1 / 5 (4) Mar 20, 2014
Benni: The greenhouse effect is dependable on both the relative abundances of gases as well as atmospheric density. The atm pressure on the surface of Mars is about 0,6% of the pressure on Earth's surface which results in almost negligible greenhouse effect. In the other extreme we have Venus whose atm pressure on surface is about 92 times that of Earth's and CO2 concentration is at 96,5%, creating a very powerful greenhouse effect. Because of the greenhouse effect, Venus is a lot hotter planet than Mercury (which is closest to the sun), although it receives only 25% of the solar irradiance compared to Mercury.


No kidding, Venus is closer to the sun & that causes the planet to be warmer? Why I'd have never guessed.....what a revelation.....tomorrow night I'm gonna make a presentation to our astronomy club about that, maybe they have never known Mercury is cooler than Venus either.

Perhaps you can come up with a "rate of reaction equation" explaining your version of AGW?
Maggnus
4.3 / 5 (6) Mar 20, 2014
No kidding, Venus is closer to the sun & that causes the planet to be warmer? Why I'd have never guessed.....what a revelation.....tomorrow night I'm gonna make a presentation to our astronomy club about that, maybe they have never known Mercury is cooler than Venus either.
Hey there Mr Engineer, perhaps you should climb off your high horse long enough to actually read what the poster said! If you weren't so focussed on proving how much of an ass you are, you might have noticed he said that Venus is hotter than Mercury.

Perhaps you, Mr Engineer, can explain how CO2 loading of the atmosphere can avoid warming the planet? You being an Engineer and all, it shouldn't present a problem for you!
Maggnus
4.3 / 5 (6) Mar 20, 2014
The Martian atmosphere is 95% CO2, by contrast the Earth's atmosphere is 0.04 % CO2. If this gas were such a driver of warming atmospheric temperatures you'd expect to see it a lot warmer on Mars than it is.


Is that an Engineering thing? Cause you being an Engineer and all, one would expect that you would understanding how a large difference in atmospheric pressure can have an effect on the ability of an atmosphere to hold heat, right?

Maybe you can impress all us not Engineers and show, using your Engineering ways, how you might expect a denser atmosphere to not act differently than a less dense atmosphere? I mean, you being an Engineer and all, that shouldn't be too difficult right?
Maggnus
4.3 / 5 (6) Mar 20, 2014
True, snowball Earth did accumulate CO2 because CO2-consuming cyanobacteria were robbed of light due to the ice cover, among other reasons.
I think the "other reasons" has more traction in this case, although certainly the inability of cyanobacteria to consume CO2 will have exacerbated the problem. The accumulation was the result of geologic processes, not biotic ones.
Whydening Gyre
not rated yet Mar 20, 2014
So there's another variable in the mix. You have to have a certain amount of carbon and it has to cycle in either exactly the same way it does here or in a very similar way or you end up with with a feedback spiral to either an ice ball or a magma ball.

Those who think we don't live in a fairly rare condition here on Earth are running out of room for their ideas of universal mediocrity with respect to life bearing planets.

MM - it's the variability (mediocrity, if you will) that keeps the whole shebang runnin'...
An' one thing leads to another...
Benni
1 / 5 (5) Mar 20, 2014
Is that an Engineering thing? Cause you being an Engineer and all, one would expect that you would understanding how a large difference in atmospheric pressure can have an effect on the ability of an atmosphere to hold heat, right?

Maybe you can impress all us not Engineers and show, using your Engineering ways, how you might expect a denser atmosphere to not act differently than a less dense atmosphere? I mean, you being an Engineer and all, that shouldn't be too difficult right?


All the climatologists I've ever met never saw a differential equation they could solve......or even a much simpler "rate of reaction equation" not requiring calculus.
Benni
1 / 5 (5) Mar 20, 2014
@Mag: You know guy, you just don't get it do you. All the literature with which we are continually inundated point to the % CO2 of Venus' atmosphere as the model we're faced with.

The AGW geniuses never mention anything about the more dense atmosphere of Venus. Why not? Because they don't want the issue being diverted from % CO2 content to THE MORE DENSE ATMOSPHERE.....!!!!!!!

All the modeling data that's been produced trying to correlate CO2 with "warming" has actually been showing the inverse effect. Doubling of ppm of CO2 since the 1998 Hockey Stick has not produced one iota of warmth, this is & has been measurable.

Now AGW guy, maybe you can come up with some advanced grade school math to provide a linear relationship between CO2 & atmospheric temperature, you know, do a "swindell" for us here. Or better yet give us your best "rate of reaction equation" showing us how all those atmospheric particles & gases mix to produce the reaction for your vaunted warming apocalypse.
Rute
3.7 / 5 (3) Mar 21, 2014
The message in which I stated that most models predict that Earth without life would result in an atmosphere with over 90% CO2, was given a rating of 1 star. So I did some googling to back up my statement. I tried to find the source of multiple estimates that I had read but for the life of me I cannot find it.

The only one I found was the original estimate by Lovelock & Sillén in which it is calculated, based on equilibrium chemistry, that the CO2 concentration would be at 98% in an abiological Earth. It is originally presented in the book Gaia: A New Look At Life on Earth (1979). The estimate is also referred to in some university level text books such as Environmental Microbiology (Maier et al, 2009, p. 288).

If someone can find some additional estimates, It'd be nice to see them. My googling skills failed me.
Modernmystic
not rated yet Mar 21, 2014
MM - it's the variability (mediocrity, if you will) that keeps the whole shebang runnin'...
An' one thing leads to another...


Indeed not. It's the sheer numbers of variables that have to be met to support life as we know it that completely obliterates the notion of mediocrity in relation to it. The universe is generally inimical to life. The anthropic principle is no where in sight out there, and neither is the principle of mediocrity.