[HN Gopher] The Sunlight Budget of Earth
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The Sunlight Budget of Earth
Author : mailyk
Score : 46 points
Date : 2025-08-07 16:15 UTC (6 hours ago)
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| abetusk wrote:
| For context, a back of the envelope calculation is:
|
| * 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...
| stouset wrote:
| From Tom Murphy's _excellent_ Do the Math blog[1].
| 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?
|
| Seriously, if you haven't read his take on things yet, at
| _least_ the first few posts are a must-read. It 's on par with
| the Arithmetic, Population, and Energy lecture at UC Boulder by
| Al Bartlett (popularly titled "The Most Important Video You'll
| Ever See", which is less hyperbole than you might think; the
| lecture is riveting)[2].
|
| 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...
|
| [3] https://en.wikipedia.org/wiki/Rule_of_72
| philipkglass wrote:
| The "Galactic-Scale Energy" post is a great illustration that
| a constant percentage growth rate eventually hits hard limits
| imposed by physics.
|
| 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.
| stouset wrote:
| It can't go on indefinitely, but thanks to exponential
| growth it also can't go on for much longer. 275 years is--
| to me at least--a shockingly short time-frame. Obviously it
| won't be something I live to see, but humanity being faced
| with insurmountable physical limits to growth in a handful
| of generations was eye-opening for me.
| Retric wrote:
| Assuming steady exponential curves over hundreds of years
| always results in nonsense. Going backwards you find out
| energy consumption doesn't fit that model. For example the US
| total electricity consumption over the last 20 years should
| have gone up by 1-1.023^20 = 57.6% when it's almost flat over
| that timeframe. Efficiency gains matter, as we don't want to
| heat our homes to unlimited temperatures jump a comfortable
| 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.
| stouset wrote:
| It is not assuming that steady exponential curves will
| continue. The argument is that exponential growth _cannot_
| and so _will not_ continue.
|
| 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...
| Retric wrote:
| > which is wholly predicated upon unbounded exponential
| growth
|
| 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.
| stouset wrote:
| A series of fads are not responsible for the economic
| growth we've experienced as a civilization for the last
| 2,000 years.
|
| 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.
| Retric wrote:
| Stagnation, growth, and shrinking have been commonplace
| over the last 2,000 years. "Countries" have taken
| hundreds of years to regain economic peaks without being
| destroyed.
|
| 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.
| stouset wrote:
| We are not talking about an individual city or country
| having stagnation.
|
| 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...
| Retric wrote:
| > 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.
|
| 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.
| stouset wrote:
| You are describing events that lasted for a handful of
| years and comparing them with a total and indefinitely-
| sustained global halting of economic growth.
|
| 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.
| Retric wrote:
| Such events aren't singular and lifetimes are finite.
| WWI, the Great Depression, WWII, and the Spanish Flu hit
| one after another.
|
| 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.
| mpyne wrote:
| > which is wholly predicated upon unbounded exponential
| growth
|
| 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).
| kulahan wrote:
| Is there some reason nuclear isn't considered while wind is?
| zahlman wrote:
| Wind energy ultimately results from solar energy: solar
| radiation differentially heats the atmosphere, causing
| atmospheric gases to expand, causing a pressure gradient,
| etc. Similarly, energy from our Sun enables plants and
| animals to grow, producing carbon-rich materials (fossil
| fuels, wood etc.) when they die that can be burned to
| release energy.
|
| 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.
| kulahan wrote:
| That makes complete sense - thank you for explaining.
| It's correct that nuclear would not be in this group
| here.
| stouset wrote:
| Earth is a sphere with a fixed surface area. We have one
| way to shed heat energy off our planet: thermal radiation.
| There's no convecting or conducting into space.
|
| 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.
| eunoia wrote:
| > 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
|
| Or we make like Niven's Puppeteers and move the Earth out
| into a further orbit with less insolation.
| stouset wrote:
| This doesn't solve anything. We get less power from
| sunlight so we need to generate more on Earth. But we're
| still fundamentally capped by the surface area of our rocky
| sphere.
|
| If we want to keep growing energy use past this, we'll need
| to inhabit other rocky bodies.
| abetusk wrote:
| The 250 W/m^2 is measured sunlight, not the amount reflected
| by the atmosphere.
|
| 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.
| breuleux wrote:
| > 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.
|
| 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).
| stouset wrote:
| > 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.
|
| 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
|
| Based on our approach to the economy vs. climate change, I
| think it's pretty clear which of those two choices humanity
| will opt for. If we even make it there, thanks to the
| aforementioned climate change.
| desperate wrote:
| Wow, I've been wanting an article on this topic for a while and
| this one really delivered that and more. Thank you.
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