The single most important assumption in this paper is that energy consumption will increase by 2% per year. This kind of exponential growth leads to outlandish estimates for the amount of tidal energy that society will demand.
Energy consumption has decoupled from population growth rates and economic growth.
How much energy will we consume in 1,000 years? Most projections of the population have it stabilizing at around 15 billion. But continuing at its current growth rate (an optimistic assumption I think), gets us to about 150 trillion humans in 1,000 years.
And at 2% growth rate, each of those humans will consume 20,000 times more energy than a circa 2023 human.
Now state of the art technology wastes about 80% of the energy consumed, so this is equivalent to 100,000 times more useful energy consumed per human.
So the physics in this page is a good examination of the surprisingly large compounding effects of unchecked exponential growth.
> The single most important assumption in this paper is that energy consumption will increase by 2% per year.
There's a 2nd big assumption:
That tidal energy extracted is additional Earth's rotational energy loss above what Earth does by itself.
According to the paper, tidal energy is dissipated through friction between ocean water & the seafloor. This dissipated energy subtracts from Earth's rotational energy. And some rotational energy is transferred to the moon (which makes the moon move further out). Ok so far.
Author's 2nd assumption is that as tidal energy is tapped, this is extra energy that subtracts from Earth's rotation.
But is it? It might also be that tidal energy extracted by humans, comes out of some fixed 'budget', and the remainder is dissipated naturally. More tidal energy extracted by humans -> less tidal energy dissipated through ocean vs. seafloor friction.
Kind of like solar influx: it's a huge but (apart from fluctuations) fixed amount. We can tap some % of that potential, but what's available doesn't increase. And what humans don't tap, gets absorbed / radiated out by other natural processes.
I won't even hazard a guess. But it would be interesting to figure out which of those applies.
> The single most important assumption in this paper is that energy consumption will increase by 2% per year. […] How much energy will we consume in 1,000 years
I keep posting this link here on HN but, once again, it seems very appropriate:
> The upshot is that at a 2.3% growth rate (conveniently chosen to represent a 10× increase every century), we would reach boiling temperature in about 400 years.
The lesson that constant percentage (that is, exponential) growth cannot continue endlessly is the fundamental message behind the concept of Limits to Growth. That is, there exist intractable limits to growth, and that no matter how convenient it may be to pretend otherwise, humans ignore this fact at their extreme peril.
Long-term ongoing economic growth, expressed as a constant percentage, is baked in to most current orthodox economics and economic policy. Even apparent mavericks such as Thomas Piketty assume that growth will continue interminably (noted in Capital in the Twenty-First Century).
Rather than being a critique of Liu, you've actually written a criticism of those he himself is generally addressing.
20000X the energy of a current human shows how absurd it would be for growth and energy use to not decouple. I can’t even imagine what I could do on Earth using that much power. Fly my own 737 to a new city every day?
I suppose spacefaring humans might use that kind of power but if they are living in space they are no longer part of the biosphere of Earth.
With this analysis no energy source is renewable, and we'd even run out of sunlight (or surface area that can be dedicated to solar power anyways, or see demand exceed solar power were we to put solar panels on every square inch of the planet's surface).
In reality we're already certain to hit a population maximum in just two+ decades, and not only that but the world is very likely to be on a negative population growth curve after that for some time. And we're going to need some new tech revolution to drive energy demand in developed countries up much more than it is, and the developing world's per-capita demand will not likely exceed the developed world's once fully developed.
I.e., we're not growing forever, the end of population growth is around the corner, and the end of energy demand growth is not much further.
I'm not sure the author realized this, but they're actually making a statement about how crazy exponential growth is. Not about the sustainability of tidal power. This becomes obvious once you look closer at what that 2% growth rate (as assumed in the post) implies.
Our global energy consumption in 2008 was estimated to be 474 exajoules. The total energy received by the earth from the sun during a year is about 5 million exajoules, a fraction of which reaches the surface. 5 million is much more than 474. But at a seemingly modest 2% per year growth rate (as it was between 1980 and 2006), our energy consumption will match those 5 million exajoules in less than 500 years!
Think about that: if energy consumption growth continues at the current pace, then in 500 years we'll either be using ALL solar energy received by the earth (leaving none for the biosphere), or we'll have figured out some magic technology to produce 5 million exajoules of energy per year. Assuming the magic technology, where are we going to get rid of all that extra heat? It would effectively be like having a second sun on earth, cooking us in place.
edit: I copied the numbers above from a post I wrote in 2010, so it may be a bit out of date. But Sabine Hossenfelder recently made a video where she talked about a similar timescale, i.e. boiling oceans in 400 years: https://www.youtube.com/watch?v=9vRtA7STvH4
Humanity needing that amount of energy seems obviously absurd, but then I suppose a CPU with 100 billion transistors seemed clearly impossible in 1960.
Maybe a few hundred years from now, Internet archaeologists will find your comment as one of the first harbingers of the coming World Energy Crisis, much as we see the 1912 Rodney & Otamatea Times "Coal Consumption Affecting Climate" snippet today.[0]
Human energy use isn't going to continue to grow exponentially for the next 500 years.
The world population is predicted to peak about 30% above where it is now and then fall back -- maybe to 7 billion people sustained. We seem to need about 200 GJ per person to be really happy [1], so let's assume 300 GJ per person.
We should be able to get by, happily, on 2000 EJ per year sustained. By your figures, that's less than 1% of what the sun provides.
> Our global energy consumption in 2008 was estimated to be 474 exajoules.
If my maths is right, that's about 40,000 kcal per person per day, or about 15 times the energy consumption of an average adult.
I'm trying to figure out if that's a lot or not. On the one hand, not: this ratio is not strictly bounded in any way, and the human energy consumption is an arbitrary denominator. Still, a very rough interpretation is to say, we consume daily, on average, the fruit of work of 15 people.
Of course, that is very skewed. I wonder what the ratio looks like for an average American or European - must be much more.
It goes to show what a gilded life we live (on average!!!). Before industrial revolution, all energy basically was muscle power, and it's like for every 1 person alive, we have 15 servants turning the generator for us. I suppose some of this number is literal service providers, fed by food produced by powered agriculture.
Is there a logical upper bound to this number? Is there some energy amount we can't use given sufficient supply? Ultimately, all energy we produce is spent on humans so our energy consumption is a yardstick for human energy needs.
But that's not quite right either; a lot of energy is wasted. I wonder how much of this number is clothes that go straight from factory to landfill, AC left overnight, inefficient engines and energy storage.
I don't really have a point, other than I think this is an interesting fraction to look at.
Any sort of straight line extrapolation is useless. Look at the US energy consumed per capita, flat or declining for the last 50 years. Other parts of the world will follow as they develop. Likely the same for population growth.
“the rotation of the Earth would lock to the Moon in about 1000 years.” — based on unlimited exponential growth. That’s not worthy of a high school term paper, let alone the work of a PhD. Thanks for saving me the read.
I was super skeptical of this article, because I was trying to imagine how we'd have to arrange extractors in the oceans to pull any meaningful amount of energy out of the rotation of the earth based on tides, and only at the end of the article did I see that assumption of 2% compounding multiplied out as far as needed...ugh.
The implausibly is staggering, we'd run out of resources to build extractors way before causing any meaningful shift in the rotation: if the monstrous land barriers of the continents take millions of years to drain a small percentage of the energy, we have zero hope of achieving much more with the solid mass available to us on Earth, short of a Dyson sphere-level breakthrough in "moving mass around", oh, and by the way it has to happen deep underwater as well, and we need to channel all the energy somewhere useful! Way harder than just making a ginormous but thin solar array in space, which is almost certainly what we'd do instead from a "energy cost to arrange matter" optimization POV.
While this is true, a lot of economists build their models assuming unlimited exponential growth. It does seem that we're going to hit a brick wall in growth in a few hundred years if that doesn't stop. There seems to be some idea that we can continue exponential growth if we can just figure out renewable energy and we can't. There is _no such thing_ as renewable energy in a regime with unlimited exponential economic growth.
Human population growth is set to reach a peak in just two+ decades, and then will start declining for some time. Where we've seen fertility drop below replacement rate we've also seen that drop be persistent for decades, which means that population declines will accelerate before they arrest (if they arrest).
So this is a bit of much ado about nothing. We're not going to exceed low single digits percentage points of the energy reaching the planet from the Sun.
Heat dissipation is relatively easy if we use 1% equivalent of Solar energy on Earth: the water cycle can speed up in response to positive forcings and slow down in response to negative forcings. This is why the planet's climate has been so stable in the sense of being a random walk centered on an optimum that supports the sort of life that we have on the planet. Similar observations explain Mars' and Venus' inhospitability: there's not enough atmosphere, much less liquid water on Mars to drive a similar thermostatic system, while Venus' climate is pegged at a maximum, with albedo as high as it can reasonably go, and it can't cool itself significantly with anything like a water cycle.
We, or those of us who by chance survive the worst case scenarios, will just end up learning that maybe huge amounts of socially useless investment isn't compatible with life on earth. It is just not possible unless we carefully plan and ration energy - something the market simply won't do.
Suppose we put solar panels in space. Then we would be entirely unconstrained by the amount of energy received by the earth. Heat dissipation would also no longer affect earth. Given our current trajectory with regards to spaceflight, it seems plausible we will be doing large projects of that scale in space in say 300 years, enough time to meet such exponential growth targets.
"Think about that: if energy consumption growth continues at the current pace, then in 500 years we'll either be using ALL solar energy received by the earth (leaving none for the biosphere), or we'll have figured out some magic technology to produce 5 million exajoules of energy per year."
Sci-fi idea: binary star system is really the remnants of an advanced civilization, originally orbiting a single (much larger) star on their own planet, that collapsed in on itself from the consequences of its energy generation technology.
> So, the 2% growth rate for world energy consumption should be a conservative assumption.
An important caveat: this article assumes that energy consumption will continue to increase exponentially to get the 1000 year timeline of draining the rotational energy of the Earth.
One thing I notice is the assumption of unlimited exponential growth by 2% per year. That's a huge fallacy. Quick check, yeah, 1.02^1031 = 735,829,316. I'm pretty sure we will be using magnitudes more energy than today because we're more people and hopefully with better living standards for everyone. But even then that's a lot. And on that scale I'm not entirely sure where the whole energy should go to... Maybe produce mass/objects out of it?
If we reach peaked our energy consumption in merely 250 years, that's less than 150 times our current consumption. I didn't do the math, but would date to suggest this gives us a few years more time on this planet.
But don't the tides naturally dissipate a lot of that energy anyway? E.g. whenever you go to the beach and see waves crash upon the shore, that's tidal energy being dissipated as heat. If you stick a turbine in the mix to extract useful work before it turns into heat, isn't it still turned into heat regardless?
> Based on the average pace of world energy consumption over the last 50 years, if we were to extract the rotational energy just to supply 1% of the world's energy consumption, the rotation of the Earth would lock to the Moon in about 1000 years.
This is bananas. I stopped right there and closed it. I see somewhere else in this thread that they tried to do that by extrapolating an exponential growth curve through an outlier (the industrial revolution!) for a thousand years. Maybe that explains it.
But... that's not an error. That's just bananas. Absolutely insane.
Some quick googling, FWIW, gives the earth's rotational kinetic energy as a quite plausible 2.1e29 J (though a little of this will not be extractable tidally, as the earth will lock to the moon at a few percent of rotation speed), and the total world energy consumption as 22.8 TWh/year. So the back of my envelope says that at current consumption we have a hair over... two trillion years.
Tons of comments here highlighting that 2% annual growth rate in energy consumption is ludicrous.
But so what if we made a more reasonable assumption that annual energy usage will stabilize at, say, 5X of what it is currently, and the (unreasonable) assumption that we get 100% of that energy from tides.
Then how much of a rotational slowdown do we get after 1000 years?
This is mostly based on the assumption of a 2% growth rate in overall energy consumption, with tidal power as an ever-growing 1% of that. The effect of covering 1% of our current energy usage would be minimal. And, likely, the ability to exploit tidal power will plateau at some (fairly small) amount, due to geographic constraints.
Issues with exponential growth set aside, does wind power suffer from the same problem? If I understand correctly, wind mostly caused by the coriolis force, which is a consequence of the earth's rotation. Would building too many wind turbines slow down earth's rotation?
The time limit of 1000 years is the worst part of this paper. If the energy consumption literally grows 2% every year, in 3013, the humanity will be consuming 3.98x10^8 times more energy than now. If it's really the case, the dissipation heat from the use of such a large amount of energy alone will kill us all, long before the tidal lock b/w the earth and the moon.
Then technically neither is wind, since it slows down surface air currents and widens the effective boundary layer of the atmosphere.
Technically-technically, no forms of energy "generation" (technically just conversion) are 100% efficient so something is always lost to heat. I guess the important question is, what is the net effect in changing that kind of energy into purely thermal energy?
Let me nitpick: The headline alone is right: Tidal energy cannot be renewed. But that is true for all renewables like wind and solar, because "renewable" is a misnomer. Real renewal would violate thermodynamic's 2nd law.
What the author actually seems to doubt is the ABUNDANCE of tidal energy (but supports that with questionable assumptions (cf. other comments here))
I think out of embarrassment, the student also decided not to showcase his proof that in 1000 years, the mass of human beings will outweigh the Earth.
Eh, who am I to criticize? They say that your early grad school years are a time to publish large amounts of papers that you don't think are likely to stick. This is a little bit out there even by this standard though.
The author makes the point that the slow-down of the day is non-linear, and then he uses a linear equation to estimate the time until the Earth is locked to the Moon!
His doomsday scenario of total tidal locking would never occur -- as energy is removed from the rotation of the Earth, the maximum power level that can be extracted would decrease. Also, the cost-efficiency would also drop.
There would be a point where the day is "merely" longer, the Earth is not yet tidally locked to the Moon, but extracting more tidal energy is no longer worth the trouble.
The original point however is still valid. Even if the rotation was slowed to just 1/2 of what it is now, the Earth would have a 48-hour day and that would obviously cause absolute havoc with the environment.
[+] [-] blake1|2 years ago|reply
Energy consumption has decoupled from population growth rates and economic growth.
How much energy will we consume in 1,000 years? Most projections of the population have it stabilizing at around 15 billion. But continuing at its current growth rate (an optimistic assumption I think), gets us to about 150 trillion humans in 1,000 years.
And at 2% growth rate, each of those humans will consume 20,000 times more energy than a circa 2023 human.
Now state of the art technology wastes about 80% of the energy consumed, so this is equivalent to 100,000 times more useful energy consumed per human.
So the physics in this page is a good examination of the surprisingly large compounding effects of unchecked exponential growth.
[+] [-] RetroTechie|2 years ago|reply
There's a 2nd big assumption:
That tidal energy extracted is additional Earth's rotational energy loss above what Earth does by itself.
According to the paper, tidal energy is dissipated through friction between ocean water & the seafloor. This dissipated energy subtracts from Earth's rotational energy. And some rotational energy is transferred to the moon (which makes the moon move further out). Ok so far.
Author's 2nd assumption is that as tidal energy is tapped, this is extra energy that subtracts from Earth's rotation.
But is it? It might also be that tidal energy extracted by humans, comes out of some fixed 'budget', and the remainder is dissipated naturally. More tidal energy extracted by humans -> less tidal energy dissipated through ocean vs. seafloor friction.
Kind of like solar influx: it's a huge but (apart from fluctuations) fixed amount. We can tap some % of that potential, but what's available doesn't increase. And what humans don't tap, gets absorbed / radiated out by other natural processes.
I won't even hazard a guess. But it would be interesting to figure out which of those applies.
[+] [-] codethief|2 years ago|reply
I keep posting this link here on HN but, once again, it seems very appropriate:
> The upshot is that at a 2.3% growth rate (conveniently chosen to represent a 10× increase every century), we would reach boiling temperature in about 400 years.
https://dothemath.ucsd.edu/2012/04/economist-meets-physicist...
[+] [-] dredmorbius|2 years ago|reply
Long-term ongoing economic growth, expressed as a constant percentage, is baked in to most current orthodox economics and economic policy. Even apparent mavericks such as Thomas Piketty assume that growth will continue interminably (noted in Capital in the Twenty-First Century).
Rather than being a critique of Liu, you've actually written a criticism of those he himself is generally addressing.
[+] [-] bobthepanda|2 years ago|reply
[+] [-] api|2 years ago|reply
I suppose spacefaring humans might use that kind of power but if they are living in space they are no longer part of the biosphere of Earth.
[+] [-] cryptonector|2 years ago|reply
In reality we're already certain to hit a population maximum in just two+ decades, and not only that but the world is very likely to be on a negative population growth curve after that for some time. And we're going to need some new tech revolution to drive energy demand in developed countries up much more than it is, and the developing world's per-capita demand will not likely exceed the developed world's once fully developed.
I.e., we're not growing forever, the end of population growth is around the corner, and the end of energy demand growth is not much further.
[+] [-] wcoenen|2 years ago|reply
Our global energy consumption in 2008 was estimated to be 474 exajoules. The total energy received by the earth from the sun during a year is about 5 million exajoules, a fraction of which reaches the surface. 5 million is much more than 474. But at a seemingly modest 2% per year growth rate (as it was between 1980 and 2006), our energy consumption will match those 5 million exajoules in less than 500 years!
Think about that: if energy consumption growth continues at the current pace, then in 500 years we'll either be using ALL solar energy received by the earth (leaving none for the biosphere), or we'll have figured out some magic technology to produce 5 million exajoules of energy per year. Assuming the magic technology, where are we going to get rid of all that extra heat? It would effectively be like having a second sun on earth, cooking us in place.
edit: I copied the numbers above from a post I wrote in 2010, so it may be a bit out of date. But Sabine Hossenfelder recently made a video where she talked about a similar timescale, i.e. boiling oceans in 400 years: https://www.youtube.com/watch?v=9vRtA7STvH4
[+] [-] Nition|2 years ago|reply
Maybe a few hundred years from now, Internet archaeologists will find your comment as one of the first harbingers of the coming World Energy Crisis, much as we see the 1912 Rodney & Otamatea Times "Coal Consumption Affecting Climate" snippet today.[0]
Then they'll find this comment...
----
[0] https://paperspast.natlib.govt.nz/newspapers/rodney-and-otam...
[+] [-] gnfargbl|2 years ago|reply
The world population is predicted to peak about 30% above where it is now and then fall back -- maybe to 7 billion people sustained. We seem to need about 200 GJ per person to be really happy [1], so let's assume 300 GJ per person.
We should be able to get by, happily, on 2000 EJ per year sustained. By your figures, that's less than 1% of what the sun provides.
[1] https://esajournals.onlinelibrary.wiley.com/doi/10.1002/ecs2...
[+] [-] rich_sasha|2 years ago|reply
If my maths is right, that's about 40,000 kcal per person per day, or about 15 times the energy consumption of an average adult.
I'm trying to figure out if that's a lot or not. On the one hand, not: this ratio is not strictly bounded in any way, and the human energy consumption is an arbitrary denominator. Still, a very rough interpretation is to say, we consume daily, on average, the fruit of work of 15 people.
Of course, that is very skewed. I wonder what the ratio looks like for an average American or European - must be much more.
It goes to show what a gilded life we live (on average!!!). Before industrial revolution, all energy basically was muscle power, and it's like for every 1 person alive, we have 15 servants turning the generator for us. I suppose some of this number is literal service providers, fed by food produced by powered agriculture.
Is there a logical upper bound to this number? Is there some energy amount we can't use given sufficient supply? Ultimately, all energy we produce is spent on humans so our energy consumption is a yardstick for human energy needs.
But that's not quite right either; a lot of energy is wasted. I wonder how much of this number is clothes that go straight from factory to landfill, AC left overnight, inefficient engines and energy storage.
I don't really have a point, other than I think this is an interesting fraction to look at.
[+] [-] WillPostForFood|2 years ago|reply
http://insideenergy.org/wp-content/uploads//2017/01/historic...
from:
https://insideenergy.org/2017/01/12/energy-explained/
[+] [-] gcanyon|2 years ago|reply
[+] [-] cornel_io|2 years ago|reply
The implausibly is staggering, we'd run out of resources to build extractors way before causing any meaningful shift in the rotation: if the monstrous land barriers of the continents take millions of years to drain a small percentage of the energy, we have zero hope of achieving much more with the solid mass available to us on Earth, short of a Dyson sphere-level breakthrough in "moving mass around", oh, and by the way it has to happen deep underwater as well, and we need to channel all the energy somewhere useful! Way harder than just making a ginormous but thin solar array in space, which is almost certainly what we'd do instead from a "energy cost to arrange matter" optimization POV.
[+] [-] Aerroon|2 years ago|reply
Earth's mass is 10^24 kg.
E_earth = mc^2 = 9 * 10^40 J
E_current * 1.02^x = E_earth
474 * 10^18 * 1.02^x = 9 * 10^40
x = 2357.91 years
At a modest 2% growth we would be using up the entire Earth's mass in energy per year in only 2358 years.
[+] [-] PUSH_AX|2 years ago|reply
[+] [-] empath75|2 years ago|reply
[+] [-] cryptonector|2 years ago|reply
So this is a bit of much ado about nothing. We're not going to exceed low single digits percentage points of the energy reaching the planet from the Sun.
Heat dissipation is relatively easy if we use 1% equivalent of Solar energy on Earth: the water cycle can speed up in response to positive forcings and slow down in response to negative forcings. This is why the planet's climate has been so stable in the sense of being a random walk centered on an optimum that supports the sort of life that we have on the planet. Similar observations explain Mars' and Venus' inhospitability: there's not enough atmosphere, much less liquid water on Mars to drive a similar thermostatic system, while Venus' climate is pegged at a maximum, with albedo as high as it can reasonably go, and it can't cool itself significantly with anything like a water cycle.
[+] [-] unknown|2 years ago|reply
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[+] [-] giantg2|2 years ago|reply
So nuclear?
[+] [-] arnaudsm|2 years ago|reply
[1] https://en.wikipedia.org/wiki/Kardashev_scale
[+] [-] uoaei|2 years ago|reply
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[+] [-] obastani|2 years ago|reply
An important caveat: this article assumes that energy consumption will continue to increase exponentially to get the 1000 year timeline of draining the rotational energy of the Earth.
[+] [-] archi42|2 years ago|reply
If we reach peaked our energy consumption in merely 250 years, that's less than 150 times our current consumption. I didn't do the math, but would date to suggest this gives us a few years more time on this planet.
[+] [-] hn_throwaway_99|2 years ago|reply
[+] [-] FriedPickles|2 years ago|reply
[+] [-] sudhirj|2 years ago|reply
[+] [-] ajross|2 years ago|reply
This is bananas. I stopped right there and closed it. I see somewhere else in this thread that they tried to do that by extrapolating an exponential growth curve through an outlier (the industrial revolution!) for a thousand years. Maybe that explains it.
But... that's not an error. That's just bananas. Absolutely insane.
Some quick googling, FWIW, gives the earth's rotational kinetic energy as a quite plausible 2.1e29 J (though a little of this will not be extractable tidally, as the earth will lock to the moon at a few percent of rotation speed), and the total world energy consumption as 22.8 TWh/year. So the back of my envelope says that at current consumption we have a hair over... two trillion years.
[+] [-] crazygringo|2 years ago|reply
But so what if we made a more reasonable assumption that annual energy usage will stabilize at, say, 5X of what it is currently, and the (unreasonable) assumption that we get 100% of that energy from tides.
Then how much of a rotational slowdown do we get after 1000 years?
[+] [-] almenon|2 years ago|reply
This theory is very interesting, although the author presents it with too much confidence for such big claims.
[+] [-] idlewords|2 years ago|reply
[+] [-] jccooper|2 years ago|reply
[+] [-] joelthelion|2 years ago|reply
[+] [-] esjeon|2 years ago|reply
[+] [-] uoaei|2 years ago|reply
Technically-technically, no forms of energy "generation" (technically just conversion) are 100% efficient so something is always lost to heat. I guess the important question is, what is the net effect in changing that kind of energy into purely thermal energy?
[+] [-] giblfiz|2 years ago|reply
[+] [-] rwoerz|2 years ago|reply
[+] [-] haskmell|2 years ago|reply
[+] [-] kepler1|2 years ago|reply
Eh, who am I to criticize? They say that your early grad school years are a time to publish large amounts of papers that you don't think are likely to stick. This is a little bit out there even by this standard though.
[+] [-] jiggawatts|2 years ago|reply
His doomsday scenario of total tidal locking would never occur -- as energy is removed from the rotation of the Earth, the maximum power level that can be extracted would decrease. Also, the cost-efficiency would also drop.
There would be a point where the day is "merely" longer, the Earth is not yet tidally locked to the Moon, but extracting more tidal energy is no longer worth the trouble.
The original point however is still valid. Even if the rotation was slowed to just 1/2 of what it is now, the Earth would have a 48-hour day and that would obviously cause absolute havoc with the environment.