I was thinking about Mars and how it lost its water, and that, for obvious reasons, made me think about Earth. Now Earth has an advantage over Mars due to the larger gravity and the still molten and spinning iron core. But… a lot of the same processes still have to be at work. Water is pretty volatile, that’s kinda what “humid” means. Hell, that’s why we have clouds. Now, you get some of those water molecules really high up and they are likely to break due to solar radiation. Hydrogen spins off into space, no more water molecule. Now, I don’t think this is something we need to be worried about, but as an intellectual exercise, how much is lost? It may not be much, but it’s got to be non zero, right?
Follow up, is there enough incoming water from icy meteorites to offset this?
Subducted water could come back out again, if the rate of outgassing increased for some reason. Water has been going in and out of the crust and mantle for billions of years
The long-term problem is water loss at the top of the atmosphere. This link suggests we’ve got about a billion years until the Sun heats up enough to boil the oceans; a bit more optimistic than other recent estimates I’ve read.
A billion years doesn’t really cause me much existential angst.
Yep, I think the worry is the opposite - that the lack of water could slow down or stop tectonic movements, because water below the crust encourages vulcanism and thus the churning of the mantle. Which would lead to the rest of the water (and carbon, etc.) in the earth being locked up.
In the other direction is new water being created and added to the hydrological cycle by the burning of fossil fuels. (This is very different than the water consumption of fuel production which while removed from lakes and aquifers is still within the hydrological cycle, mostly used up as evaporative losses and into vapor, and in recent years some relatively sequestered injected deep during fracturing and deep well drilling.) To think of it pretty simplistically the oxidation of fossil fuel produces both new CO2 and new water from the hydrocarbon and oxygen:
CH4[g] + 2 O2[g] -> CO2[g] + 2 H2O[g] + energy.
Given that we know we are adding significant amounts of CO2 by the process, it follows that we are also adding water as well. (It just isn’t where we want it.) Real world fuel is of course not all methane and has variable amounts of sulfur and nitrogen but academics have done the math for us:
The subduction calculations are highly limited not only by the fact noted in the article, that how much water is released by volcanic activity is not very well understood, but also by the fact that vulcanism (and its associated release of sequestered water) may not be a consistent item over geological epochs.
Also armedmonkey you should likely also highlight that your linked article suggests almost all of that loss was early in the earth’s history, slowed down dramatically since the atmosphere increased its oxygen concentration.
Thank you DSeid, but, and don’t take this the wrong way, I am well aware of how combustion works. I don’t need the methane stoichiometry. I also know that my link was to a an article that summarizes an only sorta peer reviewed study by some Danish scientists regarding stuff that happened a long, long time ago. But the question still holds. The Earth’s atmosphere just has to be outgassing, at least a little. Simple gravity and the magnetosphere keeps the vast majority of it in, but it can’t stop the “solar wind” completely, can it? I mean, some of it has to be stripped off, right?
Hey, I found this This leads me to believe, which seems obvious in retrospect, that we are losing water vapor at roughly the same rate as everything else. But… and it’s a fine point, but this is a thread about negligible but non-zero loss, I wonder if the loss of water is faster than that of say, Nitrogen. H2O is just an ionic bond - it doesn’t take a whole lot of energy to split that thing. And Hydrogen is very light, it should be the stuff more able to be stripped off by all those fast moving alpha particles from the sun. I think. But I don’t really know. That’s why I posted this.
Not sure what you are defensive about. My comment was merely how changes of the Anthropocene are huge and fast compared to other periods of our history.
Anyway the actual meat of your most recent citation is interesting.
First it answers that yes hydrogen leaves the atmosphere preferentially.
But it isn’t completely stripped away. Lots apparently comes back.
A lecturing cosmologist proclaimed that Sol will go nova in 4 billion years, destroying Earth. A man rose in the audience and yelled, “What?!? How long do we have?” The lecturer repeated, “Four billion years.” The audience member sighed with relief. “Oh, BILLION! I though you said MILLION! Whew!”
If humanity hasn’t left Terra and conquered the universe by then, we deserve extinction. Meanwhile, to stem H2 and H2O loss, merely slam small icy comets into the Pacific.
Water loss (actually hydrogen loss) will continue at a slowly increasing rate for the next billion years, and during this time we could lose another small but significant fraction of our water. Ten, twenty percent over a gigayear? We will still have deep oceans at this point, however.
But at around a billion years from now the oceans at the equator will start to boil, and this will release vast amounts of water vapour into the atmosphere- water vapour is a greenhouse gas, so the temperature of the Earth will skyrocket, and we’ll lose most of our hydrogen and become like Venus. We might be lucky and retain some deuterium, though.
The second part of your question was if enough new water was coming in to offset the losses (you specifically asking about icy meteorites). Annually the amount of new water coming into the system from the burning of fossil fuels more than offsets the losses, by, if I calculated at all near correctly, three to four orders of magnitude. Icy meteorites need not apply.
It is a shame we only have a limited amount of fossil fuel to burn, in that case. The hydrogen in the hydrocarbons is the important factor; when we run out of hydrocarbons to burn, there won’t be any new hydrogen entering the hydrosphere from that source any more, and we’ll still be losing hydrogen at the top of the atmosphere at a rate of 95,000 tonnes per year.
