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I’m under the impression that we’re still in the quaternary ice age. What makes a snowball impossible now? The division of the ocean in two that restricts convection and the large polar continent that provides a convenient place for ice sheets to increase the Earth’s albedo both suggest, to my admittedly limited understanding, that a severe glaciation isn’t out of the cards. I ask not to challenge but to learn.

Edit: I can’t remember the name right now, but there are also a bunch of solar cycles that affected past glaciations. I’d be grateful for information on that too.

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We are still in an icehouse period, yes! However, a few things may make it a bit harder for that to develop into a full snowball, currently:

1) While having continents at the poles makes it easier to have icesheets at all, it also makes it harder for them to grow into a full snowball. This is because

a) covering the continents with ice shuts down silicate weathering (and silicate weathering consumes CO2, so that's a stabilizing negative feedback)

b) the difference in albedo between water and sea ice is greater than the difference in albedo between land and ice. So if you can get cold enough to start making sea ice at the poles, you should get a stronger positive feedback of cooling -> higher albedo -> more cooling

During the Neoproterozoic, most or all of the continents seem to have been near the equator, so silicate weathering could keep going until sea ice reached the "point of no return" of the sea ice-albedo feedback [e.g. 1]

2) The biosphere is pretty different today than it was last time we had a snowball, and there is some reason to think that evolutionary developments like land plants and pelagic calcifiers may make the climate system more stable than it was 700 million years ago.

None of that is to say it's impossible though! The solid earth acts slowly, but it's a big lever, so hypothetically if you could somehow crank silicate weathering up high enough and volcanic degassing down low enough, you could probably still in principle reach the tipping point again.

For your last question, you are probably thinking of Milankovitch cycles [2] -- those are definitely going strong as well, though generally not strong enough to get us into or out of a snowball state.

[1] https://doi.org/10.5194/cp-8-2079-2012

[2] https://en.wikipedia.org/wiki/Milankovitch_cycles

>a few things may make it a bit harder for that to develop into a full snowball

Are you making a point by leaving human emissions of CO2 out of your list?

That certainly doesn't help either! To some extent though, anthropogenic emissions are dangerous more for their rate than their absolute magnitude; in the long run, once we stop emitting, silicate weathering will take back over "soon enough" -- it's just that "soon enough" in this case means ~5 myr and probably a mass extinction later.

The other one I forgot to mention is that the sun is a bit brighter now than it was 700 Myr ago (by perhaps a few percent). Go back another two or three billion years to the Archean and the difference would have been bigger -- to the point that we have some trouble explaining why there weren't a lot more snowballs back then [1]

[1] https://en.wikipedia.org/wiki/Faint_young_Sun_paradox

I have to say I was thinking to myself this person identified as a geologist, so an event that started 200 years ago probably isn’t a major concern. In the sense of geologic timescales I mean.

I perceive this as a little patronizing.

Up thread, there seems to be a question about human civilizations being erased by the ice.

A billion years ago.

And there was a polite reply, from said geologist, not a comment about timescales.

Is it possible from geological evidence to confirm that Sun was dimmer billions years ago? I am asking as stability of Earth orbit cannot be taken for granted for such periods. For example, we could underestimate the effects of solar wind in past that could have pushed Earth, or the could be an interaction with passing close stars.

Ah, so there are many things that it is hard to be absolutely certain of in geology, but changing Earth's orbit is at least very very hard; even the kinetic energy from things like the Chixulub impact are far too small to have a significant effect. The "moon-forming impact" in the most common model of the origin of the moon might be more on the right order of magnitude, but there don't seem to have been any of those more recently than about 4.51 Ga. An astronomer could say more, but solar luminosity is also relatively well understood from studying other main-sequence stars of various ages.

The most common solutions involve high concentrations of organic greenhouse gases like methane as well as high CO2, but it's always possible there are other possibilities that have not yet been considered.

We do not have good models of the rate Sun has been losing hydrogen especially on scale of billions of years. So from that we do not have a precise answer how heavier was Sun in past. But heavier Sun implies that Earth was closer compensating for the dimmer younger Sun.

As I understand according to the current estimates this not enough to avoid the cold Earth problem, but there are way too much uncertainty. But if we do not have way to read the brightness from geology alone, that can be an answer.

Thank you for all the great information. And yes it was Milankovitch cycles I was thinking of. I really appreciate your taking the time to educate me.

Most of what you ask is discussed and explained in the 8 episodes of Earth Story documentary [1] and the other links of my comments here.

[1] https://www.youtube.com/watch?v=UFcKEcyWhGQ

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