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 was brought up in the comments, and worth noting that while the stellar luminosity grows in time, it is usually less recognized that 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 have vital implications for atmospheric photochemistry, atmospheric escape rates in the early stages, biology, and the stability of prospective greenhouse gases that may have been present. For example, the original Sagan and Mullen (1972) suggestion for helping to offset a faint sun was that ammonia (which can be a greenhouse gas) concentrations were higher on primordial Earth, however Kuhn and Atreya (1979) later showed that ammonia was photo-chemically unstable at any appreciable concentration, and would become 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 of interrogating the implications of solar changes for climate. In the rest of 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?
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. Even with Neoproterozoic solar insolation the Earth should be fully glaciated at modern-day CO2 levels, and even colder with early Archaean insolation, yet Earth managed to avoid persistent global glaciation for billions of years.
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 known 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 early Earth easily prone to glaciation. Instead, a well-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 enhanced weathering can draw down CO2 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) 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.
The silicate weathering thermostat also serves to extend the orbital range of habitability around our sun (Kasting et al., 1993), since the negative feedback allows for greater flexibility in the conditions where liquid water can persist. 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 how other hypotheses such as feedbacks involving 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). However, 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.
The post-snowball Earth setting provides another opportunity to test how robust the thermostat mechanism is. In the geologic record we see carbonate deposits (cap carbonates) that indicate extreme carbonate supersaturation in the ocean and rapid deposition. This is a main
indicator for Neoproterozoic glaciations and deglaciations involving transitions between completely different states. Cap carbonates are continuous layers of limestone and/or dolostone that overlie Neoproterozoic glacial deposits. Transfer of atmospheric CO2 to the ocean would result in the rapid precipitation of calcium carbonate in warm surface waters, producing cap carbonate rocks.
A Future Outlook
Evidently, the silicate-weathering thermostat does not operate on neighboring planets, either because they lost water (e.g., Venus) or were small enough to lose substantial tectonic activity and forbid release of CO2 back into the air (e.g. Mars). These observations lead us to believe that a thermostat can only operate within a reasonable range of conditions, and these should be understood to explore habitability limits and Earth's future.
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.
On Venus, following the runaway greenhouse it is likely that CO2 was free to accumulate in the atmohere once the weathering was inhibited by water loss, resulting in the ~90 bar CO2 atmosphere we see today.
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.
Posted by Chris Colose on Wednesday, 30 March, 2011
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