The value of manned space flight?

[Mustrum_Ridcully]

How I wish!

I've some firsthand knowledge of this process.


That's exactly the point of my comments. It is being funded more for reasons of national prestige than scientific output. That doesn't mean it is a bad thing, or a poor investment--I'm happy the mission is going on! But it is good to know why it's being done.


Let's see. Average ocean, water, iirc, is about 4 C (not surface t; all ocean). Ocean mass is about 1.35 e21 kg. So, you get about 5.4 e26 Joules to boil. For energy consumption, I get about 200,000 Twh. That goes to (3.6 e15 J) * 2e5 = 7.2e20 J yearly. So, you're looking at a million years, solely of the energy use being sent to the oceans.

For reference, solar irradiation is about 240 W/m2. With surface area of 5.1e14 m2, and 3.15e7 seconds in a year, it works out to...4e24 J. So, human energy use is something like a factor of 8,000 smaller than solar irradiation.

Have I made any errors? Always appreciate corrections.

Does the solar irradiation account only for the radiation that is absorbed by Earth,or is it the sum of energy the sun sends our way, without the energy that Earth reflects back?

But what might be missing is that that energy use of humanity has been growing considerably over the last 200 years (starting at at an estimated 5,900 TWh in the year 1800 to 186,000 TWh) , and projecting such exponential growth can lead to extreme effects. Of course, it's not plausible that it would be sustainable at that level.
We probably won't need to worry about boiling oceans killing us (at least not until sun's Red Giant phase) - but we might have to worry about the effects that lead to this level of growth not being sustainable. If we're lucky, it's just a technical limitation, but it might be that the ecosystem no longer supports human life.

 

[briggart]

If all of human energy use is instead pumped directly into the oceans, and none is lost to space, you get to boiling the oceans after about 10,000 years, IIRC.

Looking at the half full glass: well before that we'll pass the point in which global temperatures will rise above 37C, so the problem will correct itself.

Let's see. Average ocean, water, iirc, is about 4 C (not surface t; all ocean). Ocean mass is about 1.35 e21 kg. So, you get about 5.4 e26 Joules to boil. For energy consumption, I get about 200,000 Twh. That goes to (3.6 e15 J) * 2e5 = 7.2e20 J yearly. So, you're looking at a million years, solely of the energy use being sent to the oceans.

For reference, solar irradiation is about 240 W/m2. With surface area of 5.1e14 m2, and 3.15e7 seconds in a year, it works out to...4e24 J. So, human energy use is something like a factor of 8,000 smaller than solar irradiation.

Have I made any errors? Always appreciate corrections.

IIRC, human energy consumption has steadily increased at 1-2% annual rate since the start of the industrial revolution. If we assume that our energy output will keep growing at a similar rate, it will take between 500 to 900 years to dump ~5e26 J of energy into the environment.

 

[The Firebird]

Does the solar irradiation account only for the radiation that is absorbed by Earth,or is it the sum of energy the sun sends our way, without the energy that Earth reflects back?

The number I used (240 w/m2) is only that absorbed. If you include the part that is reflected, it is about 340 w/m2.

But what might be missing is that that energy use of humanity has been growing considerably over the last 200 years (starting at at an estimated 5,900 TWh in the year 1800 to 186,000 TWh) , and projecting such exponential growth can lead to extreme effects. Of course, it's not plausible that it would be sustainable at that level.

What specifically are you imagining?

We probably won't need to worry about boiling oceans killing us (at least not until sun's Red Giant phase) - but we might have to worry about the effects that lead to this level of growth not being sustainable.

The boiling oceans claim is the one I was responding to.

 

[The Firebird]

IIRC, human energy consumption has steadily increased at 1-2% annual rate since the start of the industrial revolution. If we assume that our energy output will keep growing at a similar rate, it will take between 500 to 900 years to dump ~5e26 J of energy into the environment.

If energy is that abundant, the landscape will look very different in terms of possible responses!

 

[Umbran]

fLet's see. Average ocean, water, iirc, is about 4 C (not surface t; all ocean). Ocean mass is about 1.35 e21 kg. o, you get about 5.4 e26 Joules to boil. For energy consumption, I get about 200,000 Twh. That goes to (3.6 e15 J) * 2e5 = 7.2e20 J yearly. So, you're looking at a million years, solely of the energy use being sent to the oceans.
...
Have I made any errors? Always appreciate corrections.

I was quoting memory before. Let me do some math.

Working to order of magnitude, I get...

I find other sources listing world energy consumption in 2023 as being 620 Exajoules (17 x10^4 Twh), so 6.20x10^20 J/year. (That's Primary, rather than Final, consumption, but whatever.)

Ocean volume is about 1x10^24cc. Density of seawater about 1.028 g/cc (varies slightly with pressure, but only to about 1.07 g/cc at the bottom so close enough. So mass of water is about 1x10^24 g.

Specific heat capacity of seawater is about 4 j/gC

We are looking for a temperature change of about 100C (to order of magnitude).

mc(delta-T) = 1x10^24g * 4j/gC* 100 C = 4x10^26 J.

So we get a time = 4x10^26 J / 6.20x10^20 J/year
= 0.6 x10^6 year.
= 600,000 years-ish.

So, my remembered time was off by a factor of ten. Oh, well.

But also, we are BOTH wrong. Why? That's only the time to bring the ocean to a boil! To actually boil the ocean away, we also must provide its latent heat of vaporization!

And that gets beyond simple arithmetic, fast. The heat of vaporization of water will vary with salinity. And as we boil it, it gets saltier, until we are trying to boil almost pure salt!

Either way, it is a long time.

 

[Umbran]

If we assume that our energy output will keep growing at a similar rate...

Ew, no. Misses the point.

I mean, this is where this cartoon model breaks down. This analysis assumes every scrap of energy we produce goes into the ocean as heat. We aren't spending any energy on expanding our energy production capacity. Actually, our assumption is that we aren't spending energy on producing energy, or feeding people and operating a functional society to run the machinery, either, so the whole thing falls over.

The whole thing is just supposed to be demonstrative of how large the task is.

If we want to think practically, about how much heat the human race can produce in the next few centuries, that's actually a much different analysis, in which that energy output increase would need to be interrogated.

 

[Clint_L]

My main reason for continuing to support crewed spaceflight is that it is really sexy PR for science. Which is itself a great thing - we need all the people excited about science that we can get. At a personal level, I think space flight and space science in gereral are exciting and fascinating, even when there is no immediately obviously application.

I wish humans got as excited about, say climate science and vaccine science, but our brains seem wired for novelty, not for slow, incremental, meticulous study and experiment. But at least a few people who get jazzed by science through the Artemis missions will go on to become scientists, and that's a good thing.

 

[Umbran]

Don't forget that the biggest threat isn't the additional energy that we're pumping into the environment, but the byproducts of it that increase the results of solar radiation. Our industrial heat pales by comparison to what the sun does, but our actions can have an impact on how much of the energy hitting the Earth is absorbed/internally reflected.

So, here's where we note: we are all dead long before the oceans boil anyway. We show that heating is a bit ludicrous to get people back to something slightly more practical.

But, since we are here.

Humans sweat to control our internal temperature. Humans stop being able to live (unprotected) on the surface of the planet when we can no longer cool ourselves by sweating - that happens when what's called the "wet bulb temperature" (a combination of air temperature and humidity) rises above about 35C.

Typical models say that point comes globally when average global temperature rises by 12C over pre-industrial levels. Far, far before the oceans boil, there is nowhere on the surface where you don't just die of heat exhaustion.

The Paris Agreement was trying to keep change down below 2C, really hoping for under 1.5C, because by about 3C, the impacts on society start becoming severe enough that our history does not give us a handle on the sociopolitical changes that would follow. Humans would be alive, but the existence of modern social order becomes... dicey?

 

[Ryujin]

So, here's where we note: we are all dead long before the oceans boil anyway. We show that heating is a bit ludicrous to get people back to something slightly more practical.

But, since we are here.

Humans sweat to control our internal temperature. Humans stop being able to live (unprotected) on the surface of the planet when we can no longer cool ourselves by sweating - that happens when what's called the "wet bulb temperature" (a combination of air temperature and humidity) rises above about 35C.

Typical models say that point comes globally when average global temperature rises by 12C over pre-industrial levels. Far, far before the oceans boil, there is nowhere on the surface where you don't just die of heat exhaustion.

The Paris Agreement was trying to keep change down below 2C, really hoping for under 1.5C, because by about 3C, the impacts on society start becoming severe enough that our history does not give us a handle on the sociopolitical changes that would follow. Humans would be alive, but the existence of modern social order becomes... dicey?

I'm certainly not going to argue that the ocean need to boil in order for us to end. I never did. I've had heat exhaustion, if not heat stroke.

But there are ways other than the direct heat that we output, that can cause catastrophic climate change.

 

[briggart]

Ew, no. Misses the point.

I mean, this is where this cartoon model breaks down. This analysis assumes every scrap of energy we produce goes into the ocean as heat. We aren't spending any energy on expanding our energy production capacity. Actually, our assumption is that we aren't spending energy on producing energy, or feeding people and operating a functional society to run the machinery, either, so the whole thing falls over.

The whole thing is just supposed to be demonstrative of how large the task is.

If we want to think practically, about how much heat the human race can produce in the next few centuries, that's actually a much different analysis, in which that energy output increase would need to be interrogated.

I assumed "pumping energy into the oceans" to be a shorthand for "energy produced on Earth is not radiated into space", in which case energy from non-renewable sources eventually ends up in the environment as heat. There may be a lag between when energy gets produced and when it eventually ends up in as waste heat, but even if the lag is of the order of decades or even hundred years, it doesn't significantly change the picture.