The Sunlight Budget of Earth
asimov.press
asimov.press
* Solar energy (on earth) gives about 250 W/m^2 [0]
* Earth has an approximate radius of 6.371 * 10^6 m
* Estimating sunlight on a disk of earth's radius yields ~ 700 * 10^15 (Wh/day) (3.14159 * (6.371 * 10^6)^2 (m^2) * (240 W/m^2) * (24 h/day))
That is, the earth's budget is just under 1 exa (Wh/day).
Earth's population is 8.2 B people and under a very generous energy consumption of 30 (kWh/day), that gives approximately 250 (TWh/day) (8.2 * 10^9 (ppl) * 30 * 10^3 ~ 250 * 10^12 (kWh/day/ppl)).
In other words, we're using about 1/1000 of a (back-of-the-envelope) theoretical upper limit of solar energy available to us on a daily basis.
[0] https://www.solar-electric.com/learning-center/solar-insolat...
Only 70% of the incident sunlight enters the Earth’s energy budget—the rest immediately bounces off of clouds and atmosphere and land without being absorbed. Also, being land creatures, we might consider confining our solar panels to land, occupying 28% of the total globe. Finally, we note that solar photovoltaics and solar thermal plants tend to operate around 15% efficiency. Let’s assume 20% for this calculation. The net effect is about 7,000 TW, about 600 times our current use. Lots of headroom, yes?
When would we run into this limit at a 2.3% growth rate? Recall that we expand by a factor of ten every hundred years, so in 200 years, we operate at 100 times the current level, and we reach 7,000 TW in 275 years. 275 years may seem long on a single human timescale, but it really is not that long for a civilization. And think about the world we have just created: every square meter of land is covered in photovoltaic panels! Where do we grow food?
To very TL;DR things: solar and tidal energy (and their derivatives like wind) are essentially the only sources of energy we can rely on as our energy requirements grow. We are shockingly close (~300 years) to measurably raising the equilibrium temperature of earth's surface through purely thermodynamic effects if energy use trends continue. This is completely independent of greenhouse gases, and assumes that Earth is a perfect blackbody radiator. Once we exhaust our energy budget from these sources, that's it. No magical unobtanium source of energy can solve the fact that producing additional energy on the surface of the Earth will raise its temperature. We will stop increasing our energy use one way or another once we hit this wall.
If we want to continue using more energy we'll need a whole second Earth to do it on. Great, we've colonized mars! What does that get us? Based on a 2.3% growth rate and the Rule of 70[3], we'll use up that second Earth in thirty years. We'll now need two Earths to keep growing for the next thirty years.
[1] https://dothemath.ucsd.edu/2011/07/galactic-scale-energy/#:~...
[2] https://www.youtube.com/watch?v=F-QA2rkpBSY&pp=ygUodGhlIG1vc...
Posts like abetusk's are a great illustration that "the solar budget" is a very generous energy budget. That may seem too obvious to mention, but in 20th century ecology literature (or even as recently as the early 2010s) living within "the solar budget" was often conflated with a low-energy, deindustrialized future. Constant growth fueled by sunlight (or anything else) can't go on indefinitely, but there's also no prospect that a sunlight-fueled world would have less energy available than the old fossil-fueled one.
Globally rather than an exponential curve instead the global quality of life keeps rising as more people enjoy the benefits of modern technology like AC and tablets, but the number of people isn’t continually increasing. Birth rates keep declining so in the short term its populations catching up to increased lifespans.
But then you run into other problems. Can an economy like ours—which is wholly predicated upon unbounded exponential growth—continue indefinitely when energy use is effectively capped? Yes, there are efficiency gains and productivity gains to be made. Yes, our population growth is slowing and fill eventually flatline or even decline. Those will extend the length of time before we fully exhaust Earth's energy budget. Growth will end, and likely on a significantly shorter timescale than recorded history.
https://dothemath.ucsd.edu/2011/07/can-economic-growth-last/
https://dothemath.ucsd.edu/2012/04/economist-meets-physicist...
This is a false assumption. The economy is based on doing what people want. If people suddenly want glow in the dark kitchens someone will ramp up production of glow in the dark paint and take customers from companies that didn’t follow the trend. That’s the feedback mechanism keeping the economy functioning.
Growth at the micro level and growth at the macro level aren’t the same thing.
What you are describing is not economic growth, but a steady-state economy. Money will still change hands, but we'll capped in the total amount of energy that can be expended towards production. That is going to require an enormous change in the way the economy functions.
Overall population growth hides how minimal the per capita growth has actually been. If you assume ~1$/day as subsistence level 2,000 years ago then we’re talking something like 0.2% annual growth with the vast majority of that being very recent. But even that’s overstating things based on how the modern economy values hand crafted goods.
We are talking about the entire planet, as a whole, transitioning to a fully steady-state economy for the entirety of the planet’s future. If you that is anything like what any modern industrialized civilization has experienced or been built around, you are out of your mind.
This is not just idle musing. I encourage you to do some reading about the arguments being made before assuming they can just be hand-waved away.
https://dothemath.ucsd.edu/2012/04/economist-meets-physicist...
No we’re not, just recently COVID saw a global drop. The Great Depression and WWII saw a significant decline in the global economy. The idea that such issues are forever behind humanity is laughably absurd.
I’m aware of people writing about such things, but their arguments are no more accurate than the quite recent worrying about overpopulation.
Energy growth is physically bounded and so must stop (on a surprisingly near timescale). Economic growth can continue for a bit with efficiency improvements and other blood-from-a-stone extraction, but is ultimately bounded by energy growth so also must stop soon thereafter. For quite literally forever.
I'm not sure how to continue this discussion if you cannot understand how fundamentally unlike one-another these two situations are.
However if dips aren’t problematic long term why would stagnation at the peak? What’s being described isn’t a problem but an idealized state impossible to realize.
The economy does not require and is not predicated upon unbounded exponential growth.
Do not confuse economic activity growing where growth is easy, as a mandate for growth to work at all.
Companies that can no longer grow but can keep profits by keeping prices above costs can continue indefinitely. But ultimately the economic represents the activity of human workers aided by technology.
If everyone is working and there's no technological help to be had then the economy can no longer expand... but in this model of the world everyone is gainfully employed and doing something, and that can resolve in many different ways of setting cost (of labor and technology) and price (of goods and services).
Nuclear power, on the other hand, releases energy stored in fissile heavy elements that were produced by the death of previous stars. There is no natural process using incident light from the Sun to create them.
That makes it pretty easy to calculate the thermodynamic equilibrium temperature of earth, given the amount of energy we receive from the sun. Notably this is much less than our actual, observed temperature thanks to greenhouse gases.
Creating energy here adds to this equilibrium calculation. This applies to oil, coal, nuclear, fusion, and any mythical new energy source you want to come up with. If the energy is released here, it shows up as an increase in our equilibrium temperature.
Thankfully right now, the added energy doesn’t add up to much. But in surprisingly short order, using more and more energy will add up and produce a noticeable (and fatal) rise in Earth’s equilibrium temperature. Once we hit that point, our options are to stop using more energy or to boil the oceans. That’s thermodynamically unavoidable.
There’s no way around it either. Capture more solar and beam it here? That just directly contributes to our energy excess. Pump excess heat into objects and launch them into space? That’s literally worse than just not having produced the energy here in the first place.
> If we want to continue using more energy we'll need a whole second Earth to do it on
Or we make like Niven's Puppeteers and move the Earth out into a further orbit with less insolation.
If we want to keep growing energy use past this, we’ll need to inhabit other rocky bodies.
I absolutely would not confine ourselves to land as oceans provide a large area available for solar energy capture and there's no reason to think we might not be able to use it.
Photovoltaics, or whatever technology we use to capture sunlight, will get better but even at a modest 20% still gives us a lot of headroom.
Whether its 2.3% or 2.5% energy growth per year, the calculation gives us a timeline of 200-400 years. For some reason this is used as a countdown to oblivion instead of a rallying cry about where we're headed. Mars is great but there's a *lot of space in space*. Besides pushing solar panels into orbit, either the earth, moon or sun directly, there's also tons of asteroids, ripe for mining.
Forget a second earth, we can make a Dyson swarm. What you take as a countdown timer to a bomb, I take as a timeline for us to go up the Kardashev scale.
These reduction to absurdity arguments about the temperature of the earth assume we're not going to space. I don't understand why you reject this idea outright.
All the above calculations about the energy available to us are from a tiny pin-prick sliver of how much energy the sun deposits in all directions, every day, all day, for the 4 billion years. Once we have access to a significant fraction of the suns energy, going to other stars and repeating is well within feasibility.
Most people just want to stay home, they don't want to go anywhere, let alone space. Regardless of whether some humans or manmade probes go to space, inhabited regions need their growth to be checked in order to maintain a stable, long-term steady state, lest we want to keep shipping billions and billions of people from the center to ever-farther reaches of the galaxy (a logistical and humanitarian nightmare).
Not just going to space, but transitioning to a primarily space-based civilization. We'll need to terraform Mars within 400 years, and even then that will only buy us thirty years of sustained growth. Even if we slow down to 1% growth, once we exhaust Earth, we'll exhaust Earth-2 in just seventy.
Further, this doesn’t actually solve much because all the growth is happening on the outer planets. Earth is definitionally full in this future, so it needs to stay steady-state. Unless we forcibly ship off half the population every doubling period.
Is it going to happen? Maybe. But I'm not sure we should be playing brinksmanship with the one planet we know we've got on the idea that the overwhelming majority of humans will (and must) live in space on every rock in the solar system in the same amount of time it took us to go from the Renaissance to the Internet.
> All the above calculations about the energy available to us are from a tiny pin-prick sliver of how much energy the sun deposits in all directions, every day, all day, for the 4 billion years.
Exponential growth is a bitch. In roughly the same time it took us to go from the dark ages to now, we'll need to be consuming 100% of the sun's total energy output.
I think it's far more likely that either:
a) we won't stop using more energy and literally cook ourselves off the planet, or
b) we'll stop exponential growth of energy use
If this were a simulation or a science article, then yes, don't use frankenunits, but the back-of-the-envelope calculation is meant to build intuition.
It's doing nothing of the kind, because again, it's the same dimensional unit. It's exactly like describing a distance as a speed per day or a distance as acres per mile: both technically work, but it's annoying to do anything with the resulting value except the one specific analogy with.