2011-03-28 07:51:24Part 2 on solar evolution: Planets
Chris Colose


Looking for feedback....


Part 2

 In Part 1, we outlined some general characteristics of stellar evolution.  Notably, as the star converts Hydrogen into Helium in the core, its luminosity gradually goes up in time, and will eventually leave the main-sequence phase.  For our own sun, it will then spend a relatively short time as a red giant with a more quickly evolving spectrum, and eventually collapse into a fading white dwarf. 

 It is worth noting (and is perhaps lesser known) that while the stellar luminosity grows in time, the flux at very short wavelengths (in the extreme ultraviolet, ~ 0.01-0.12 μm) was likely several orders of magnitude higher in the early stages of Earth’s history, and at least 2-3 times the present value at 2.5 billion years ago (e.g., Ribas et al., 2005). The energetic tail of the solar flux is dominated by the emission from high temperature plasma in the chromospheres and corona, not blackbody radiation produced within the stellar interior.  These wavelengths are unimportant for planetary energy balance, but must have had vital implications for atmospheric photochemistry, atmospheric escape rates in the early stages, biology, and the stability of various greenhouse gases that might be proposed to exist on early planets.  For example, the original Sagan and Mullen (1972) suggestion for helping to offset a faint sun was that ammonia concentrations were higher (which can be a greenhouse gas), however Kuhn and Atreya (1979) later showed that ammonia was photo-chemically unstable at any appreciable concentration, becoming rapidly photolyzed into H2 and N2.

 Because the irradiation from the parent star is, by far, the most important source of energy in planetary atmospheres, we will take up the task in this post of interrogating the implications of solar changes for climate. In this post, we will tackle two questions: what sort of mechanism might exist to help offset a fainter sun and keep early Earth or Mars capable of supporting liquid water? Secondly, what will happen to Earth (or other bodies) in the future as the sun continues to evolve?


Searching for a Thermostat


Earth has been subject to many large climate changes in the past; however, even with a substantially brighter sun later in Earth’s history, it has generally always been conducive to life.  With the exception of a few brief snowball events recorded in the geologic record (which we managed to get out of, despite being very hard to do so), Earth has always supported vast amounts of liquid water.   Indeed, even at Neoproterozoic solar insolation, the Earth should be fully glaciated at modern CO2 levels, and even colder with early Archaean insolation.  These observations lead us to believe some sort of thermostat that contains a negative feedback may help to offset the early sun.  A lower albedo is one candidate, but early Earth would have to be almost completely black just to get you back to modern conditions with the same greenhouse effect.  There’s also no plausible mechanism to adjust the albedo in such a way to keep a stable climate over geologic time, and the surface ice-albedo and water vapor feedback would lead to glaciation being very easy on early Earth.  The accepted candidate is a greenhouse stabilizer known as the silicate-weathering feedback.  How does it work? 

Our planet is continually re-supplied with CO2 to the atmosphere through volcanism, which could double CO2 over just several thousands of years if operating on its own.  CO2 is also removed over long timescales by weathering reactions.  Atmospheric CO2 reacts with water to form a weak carbonic acid, which can then dissolve silicate rocks.  The byproducts are calcium and bicarbonate ions along with dissolved silica, which can be carried by rivers and streams into the ocean.  In the ocean, organisms use these to make shells of calcium carbonate (CaCO3) or silica; these organisms eventually die and settle to the ocean floor.  Due to plate tectonic processes, these materials are then processed into Earth’s interior and eventually emitted back into the air through eruptions to complete the cycle (shown below). 




Figure 1: Schematic of the silicate-weathering thermostat.  Note “metamorphosis” should read “metamorphism”.  From Prof James Kasting's web page (under research --> habitable zones around stars)


To make this into a thermostat, we can note that volcanoes do not really listen to the climate, but on the other hand weathering rates depend on temperature and precipitation (Walker et al., 1981), such that CO2 can be drawn down by enhanced weathering in warm/wet climates.  In colder climates, weathering would be reduced, allowing CO2 to build up.  The thermostat is estimated to be strong enough to reach equilibrium within a few hundred thousand years, so that shorter-lived fluctuations such as those over glacial-interglacial cycles are easily sustained for some time.  Zeebe and Caldeira (2008) however showed that carbon fluxes into and out of the Earth’s atmosphere have mostly been in balance over the long-term mean during the last 650,000 years, giving additional credibility to the thermostat.  During a snowball Earth event, weathering would virtually shut off allowing CO2 to build up to rather high values.

The silicate weathering thermostat also serves to extend the orbital range of habitability around our sun (Kasting et al., 1993).  There are still many twists in the thermostat, and it is quite evident that solid Earth sources and sinks of CO2 are not, in general, balanced at any given time; during times of unusual plate tectonic activity or mountain-uplift, the carbon imbalance can be large.  There is still considerable work needed to be done, as well as debate within the community concerning the importance of the silicate thermostat, how to test it, and how other hypotheses such as feedbacks involving subcomponents like organic carbon burial, or the “uplift mountain hypothesis” of Raymo and others, fit into the geologic evolution of the Earth (e.g., Raymo et al., 1988; Raymo and Ruddiman, 1992; Edmond and Huh, 2003; see link for one brief summary and above references).  The silicate weathering thermostat helps put into perspective that CO2 plays a fundamental role in the evolution of Earth’s climate.

Increased weatherability also plays a role in understanding the Ordovician climate, a past period that skeptics have abused as evidence that CO2 has little effect on climate.    Young et al (2009) proposed that there was enhanced basaltic weathering beginning in the mid-Ordovician that continued through the end of the Ordovician, also a time period with increased volcanism in North America.  By the Upper Ordovician, volcanism returned to normal conditions but weathering remained high, such that CO2 concentrations were drawn down and the familiar Hirnantian glaciation was initiated.


A Future Outlook

Evidently, the silicate-weathering thermostat does not operate on neighboring planets, either because they lost water (Venus) or were small enough to lose substantial tectonic activity and forbidding release of CO2 back into the air (Mars).  As the sun brightens in time, Earth will eventually get quite hot and allow for significant loss of water to space.  Kasting (1988), following on previous work (Ingersoll, 1969), determined that Earth will get to a point in which even the stratosphere is rather wet and substantial amount of water can photodissociate and be lost.  Substantial water loss occurs at ~10% increase in solar luminosity above today's value. At 140% of today’s solar luminosity, a full-fledged runaway greenhouse is possible, in which liquid water is incompatible on Earth’s surface.  This occurs because the longwave emission of planetary atmospheres that contain a condensable absorbing gas in the infrared, which is in equilibrium with its liquid phase at the surface, can exhibit an upper bound.  Pushing the absorbed shortwave radiation over this threshold makes a new radiative equilibrium impossible, at least until the oceans are depleted or the planet gets hot enough to start losing a lot of radiation in visible wavelengths.

As the sun evolves, all solar system objects should get hotter, and the potential for habitability may also be pushed outwards. In the red giant phase, the surface of the sun should actually cool (diminishing the UV flux as well), but the luminosity will increase as its radius does. Interestingly, Lorenz et al (1997) found a brief window of a few hundred million years, about 6 billion years from now, in which Saturn’s moon Titan will be compatible with liquid water-ammonia at the surface.  By then, Earth will be incinerated.

2011-03-28 09:02:01
Alex C


>>>with a much faster evolving spectrum

Is faster supposed to modify evolving or spectrum?  If the former, perhaps you could phrase it as "with a more quickly evolving spectrum."


>>> It is also worth noting as a digression (and perhaps lesser known)

Better phrased:  "It is also worth noting as a digression (and is perhaps lesser known)"


You then go on to discuss the ultraviolet intensity, though your time frames appear to be jumbled.  What it sounds like is "As it progresses, this was stronger then."  The prepositional phrase does not agree with the main subject.  Also, make sure that if you're talking about the past, you use "were" and "was" accordingly - you used "is" instead at, e.g., "(in the extreme ultraviolet, ~ 0.01-0.12 μm) is likely several..."


>>>ammonia concentrations were higher (which can be a greenhouse gas)

Better phrased: "ammonia (which can be a greenhouse gas) concentrations were higher..."


I think that since the topic above is related to the impacts of solar output on planetary atmospheres, it is not necessarily something you should lead into with "as a digression..."  Perhaps "To start out with," or some other opener.  Also, if I recall correctly, was the ammonia issue not something that came up in the comments section?  If so, why not just say so?  "To clarify a point made by/here by [this commenter]..."  If the issue is large enough so that you have to bring it up, then say why you're bringing it up and lead in with some other phrasing.  It doesn't seem to fit otherwise.


>>>Earth has been subject to many large climate changes in the past, however even with...

Better phrased: "Earth has been subject to many large climate changes in the past; however, even with..."


>>>(which we managed to get out of, despite being very hard to do so)

Eh, it's not that the wording is wrong here, but I think using "we" is odd.


>>>Indeed, even at Neoproterozoic solar insolation, the Earth should be fully glaciated at modern CO2 levels, and even colder with early Archaean insolation.

Sounds like you're saying the Earth should be fully glaciated now.  I think the placement of "even" is critical here - right now the message is "even with this extra variable, the Earth should be glaciated with our levels of CO2" - and what, without the variable it should still be glaciated?  That's my point - "even" ought to go before "at modern CO2 levels," not before "at Neoproterozoic solar insolation."  The second "even" is ok.


>>>These observations lead us to believe some sort of thermostat that contains a negative feedback may help to offset the early sun.

Better phrased: "These observations lead us to believe some sort of thermostat that contains a negative feedback may have helped to offset the early sun."

2011-03-28 09:15:44
Chris Colose


Fixed some stuff up...

2011-03-28 09:23:55Continued...
Alex C


I think a link to Prof Kasting's page might be good.  Simplify things for the reader.


>>>warm./wet climates

Extra period after "warm" here.


>>>Zeebe and Caldeira (2008)

Hyperlink here, or no?


>>>and how other hypotheses such feedbacks involving subcomponents...

Correction: "and how other hypotheses such as feedbacks involving subcomponents..."


>>>burial, or the “uplift mountain hypothesis” of Raymo and others

In context with the rest of the sentence, there should be a comma after "others" here.


>>>Edmond and Huh, 2003, see link for one...

"Edmond and Huh, 2003; see link for one...


As a side comment, didn't Mars also lose a good deal of its water due to its lower gravitational hold?  Or do I have that wrong?  It's not very important either way to the article, I don't think.  Mere curiosity on my part.


>>>At 40% of today’s solar luminosity, a full-fledged runaway greenhouse is possible, in which liquid water is incompatible on Earth’s surface.

Is this supposed to be 40% of current luminosity or an extra 40%?  Taken with the following sentences, it seems like it should be 40% more.


Sorry about the length of my responses.  I thought the post is very interesting - I did not know that silicate weathering contributed to extending our habitable range.

2011-03-28 14:45:22
Glenn Tamblyn




You might consider extending this by explaining that it is not all rock types that support the weathering so uplift or vulcanism that changes the amount exposed rock of the right types can also affect the rate of the reactiion and sequestration. As a result some geological processes can temporarily increase the sequestration rate without temp change, driving a collapse in CO2.

This has been put forward as the explanation for the Ordovician Ice Age. Vulcanism along what is now the Eastern US and perhaps extending all the way down through South America caused the creation of what are the modern day Appalachians during the early Ordovician. Later, during the later Ordovician the Vulcanism seems to have shut down. CO2 emissions would have increased with the eruptions but so too new rock formation increasing weathering, keeping them roughly in balance. Then the vulcanism switched off and the weathering continued, crashing CO2 levels and triggering the OIA. http://en.wikipedia.org/wiki/Ordovician%E2%80%93Silurian_extinction_event#Volcanism_and_Weathering

This would allow you to link to the rebuttals 45 & 103, linking everything together. That fills in the picture that CO2 is essential to explaining the past geological record given solar evolution all the way back to the origin of the Earth and also shoots down arguments like 'its weak' or 'its saturated'.

The coherent picture given by these interlocking domains of science is compelling if people can get their heads around it.

2011-03-28 18:11:38
Chris Colose


Fixed some more stuff...sorry for the grammar issues.  Will work in some Ordovician tomorrow...

2011-03-28 18:19:06
Glenn Tamblyn


Chris, you might like to look at this over on General Chat

'Plot of past CO2 vs solar level over geological time scales'


2011-03-28 22:14:27I wouldn't feel obliged to include anything from the CO2 vs solar thread
John Cook

I'll probably do a follow up post down the track, referencing Chris' two posts. Nice way to reinforce the message.
2011-03-29 09:38:35
Chris Colose


Added one more paragraph

2011-03-29 15:46:15Thanks Chris
John Cook


Have cued this up for tomorrow in the schedule:


Have enjoyed these posts - the CO2 thermostat is one of my fav subjects in climate, remember it being a revelation when I first learned of it in Richard Alley's online video lecture.

2011-03-29 20:27:18
Rob Painting

Nice work Chris. Great reading.