Life Under A Faint Sun – Phenomena: The Loom
Hydrogen-Nitrogen Greenhouse Warming in Earth's Early Atmosphere
[1204.4449] The faint young Sun problem

It's long been known from stellar-evolution calculations that the Sun has been gradually brightening over the 4.5 billion years of its existence.

Here's an approximate formula:
L = L0/(1 + (2/5)(1 - t/t0))

L = luminosity
t = time
L0 = present luminosity = 3.86*1026 watts -- about 4 trillion trillion 100-watt light bulbs.
t0 = present age = 4.57*109 years

Time | Luminosity | Temperature
0.0 | 1.00 | 15.0
0.5 | 0.96 | 11.9
1.0 | 0.92 | 9.0
1.5 | 0.88 | 6.2
2.0 | 0.85 | 3.6
2.5 | 0.82 | 1.1
3.0 | 0.79 | -1.3
3.5 | 0.77 | -3.6
4.0 | 0.74 | -5.8
4.5 | 0.72 | -8.0

Time = billion years before present
Luminosity (* present luminosity)
Temperature = deg C using SB's law and similar surface and atmosphere behavior

So with its present atmosphere, the Earth would have been frozen over in its first few billion years. But it wasn't. So there must have been more of a greenhouse effect back then, and the two current favorites are carbon dioxide and methane.

Those two molecules are good infrared absorbers and emitters because they have vibration modes that can give them nonzero electric-dipole moments. Their vibrations then make them act like tiny radio antennas.

Two other common ones with this property are water and ammonia.

But nitrogen, oxygen, and hydrogen molecules do not have this property, and are thus poor infrared absorbers / emitters, and thus poor greenhouse gases.

But hydrogen and nitrogen molecules can collide with each other, and while colliding, they can be better infrared absorbers / emitters. They thus become good greenhouse gases in that way.


It's difficult to get good numbers for the atmosphere's composition and the Earth's average surface temperature for its first few billion years. But with suitable amounts of various greenhouse gases, the Earth could easily have stayed at close to its current temperature for most of its existence.


An interesting circumstance mentioned in Feulner's faint-young-sun review was the growth of the Earth's continents. They grew to about 60% of their present volume by 3 billion years ago, then more slowly to the present -- and approximately linearly. More ocean area would mean better absorption of sunlight -- but it is not good enough to keep the Earth from freezing in its early days.

The Earth's obliquity appears to have been stable around its present value for at least the last 2.5 billion years, the Earth was rotating, and the Moon was closer back then. That would have meant bigger tides and less equator-to-pole circulation. However, that review did not present any numbers.


Near the end, the review showed a diagram of the Earth's ice-line latitude as a function of the Sun's luminosity. The curve:

Stable ice line at 0d (iced over) from 0 to 1.25* present luminosity
Unstable ice line to 30d and 0.9 * present luminosity
Stable ice line ice line to 65d and present luminosity (where we are at now)
Weakly unstable ice line to 90d (ice-free) and a little less than present luminosity
Stablie ice line at 90d (ice-free) for more than present luminosity

Assuming the same amount of greenhouse effect in the Earth's atmosphere.

The Earth had gone though some "snowball Earth" phases in the early and late Proterozoic, and it got out of those phases by CO2 accumulating in its atmosphere and causing enough greenhouse effect to melt that ice.


The future? The Sun will continue to heat up, and about 5 billion years from now, it should be 2/3 or 3/4 more bright than it is now. The Earth will have gotten a Venus-like runaway greenhouse effect by then.

lpetrich wrote:
The future? The Sun will continue to heat up, and about 5 billion years from now, it should be 2/3 or 3/4 more bright than it is now. The Earth will have gotten a Venus-like runaway greenhouse effect by then.

As interesting as this may be, I don't think any of us are too concerned about what's going to happen several billion years from today, we're worried about the next nine or ten decades, even the next two or three decades, when the climate shit is going to really hit the climate fan and bring us an awfully lot of disruption and chaos to deal with, perhaps more than we can deal with.

Yunno?

FACT-MAN-2 wrote:

lpetrich wrote:
The future? The Sun will continue to heat up, and about 5 billion years from now, it should be 2/3 or 3/4 more bright than it is now. The Earth will have gotten a Venus-like runaway greenhouse effect by then.

As interesting as this may be, I don't think any of us are too concerned about what's going to happen several billion years from today, we're worried about the next nine or ten decades, even the next two or three decades, when the climate shit is going to really hit the climate fan and bring us an awfully lot of disruption and chaos to deal with, perhaps more than we can deal with.

Yunno?

Tbh, I don't see as why that stops us considering what else would/could happen in the future, even if that future doesn't include us. For example, we'll happily discuss the theoretical final phases of our star, even though it's an absolute certainty that we - our species - will no longer exist by then.

But what could we do about it, that's what I want to know.

Maybe we could just plant lots of white daisies?
http://library.thinkquest.org/C003763/flash/gaia1.htm

ABE: here's a better daisyworld simulator
http://gingerbooth.com/flash/daisyball/DaisyBall.html

Just move the Earth further from the Sun