Back in 2137 AD, as the people of that golden age would call it, a team of engineers and scientists worked on developing the first light speed capable spacecraft drive (which would have massively increased the capability of the human race to travel to other planets. As it stood at the time, only small colonies on Mars and the Moon had been successfully made.). The initial test with an actual, full-size prototype was done in the Salt Flats of Nevada. They had failed to correctly handle the fuel source (a proprietary mix of compounds unknown to me or the public… especially since I live hundreds of years after said event and no one alive could remember the event in such detail at all if there weren’t these videos) which caused a failure of the fuel pump regulation, pumping the drive full of energy-rich fuel, resulting in an explosion upon drive startup that literally vaporized several hundred miles, removing the team [and their drive] from existence. There were two major effects that occurred when this drive failed. The first is that time was stretched in the surrounding area. To this day, one can visit this location and an hour will pass in the mind of a human when only a minute or two, in reality, is traversed. Secondly, the initial explosion did actually create an accelerative force near that of the speed of light, causing a tiny warp bubble to be generated around the engine and then disappear as quickly as it appeared. That singular point had pushed against earth’s rotational force just for a moment, but it was enough. The residual effects caused the earth to start its deceleration (something that lasted several decades to reach completion) and brought forth tsunamis, earthquakes, volcanic eruptions, and super strong winds at such a level that over half of the world was forever changed. The 300 billion people on earth at the time was cut to less than a third, with several billion more being injured beyond full repair. The destructive weather that followed caused many billions more to fall from life due to food growth being stressed and temperatures tending towards greater and greater extremes.
Scientists of the time theorized that the earth would eventually speed up again, even if it was after rotation hit zero. They were wrong on that front; it never got past zero rotation. Either way, governments that weren’t totally shattered gathered together their might and quickly built underground cities and travel-ways that would allow them to escape the dangerous weather above. What was not predicted by those trumpeting this move was that the continents and oceans would shift. Because of the slowly increasing dramatic change from the high and low temperatures of the ever-expanding day, the oceans and lakes more quickly evaporated and froze over. The earthquakes from the blast had opened up many crevices around the world that allowed for the ocean to lower their levels to fill in these new zones. But as the earth continued to slow, the continents started to shift on their molten supports, shifting more central on the equator and towards the poles. This, combined with the lower water levels, created ring-like continents, connected only by small strips of land that followed the newly located underwater mountain ranges. These shifts caused many of these underground cities to collapse or be cut off of from their supply lines or travel-ways. That being said, it was soon after all the shifting stopped that rotation of the earth also reached zero and the underground realms stopped any communication with those still on the surface of the planet. Even to today, I know of no one who has heard from those below, and I have spoken with every leader of every major caravan on the surface that is not warlike in nature. Today, there are about four hundred million people in the caravans on Earth, which is several billion less than when I was born.
- Unnamed man of fifty-seven earth crossings.

My first thought when I saw your thread title was "what if Earth stopped spinning relative to what?"

The idea of the Earth's rotation stopping, other than gradually over millennial timescales or longer, "feels" wrong to me, but I can't really explain why.

I thought I remembered some similar question coming up in the "XKCD What If?" articles I "binge-read" awhile back, so I Googled and found a slightly similar question (that I plainly hadn't read since it wasn't on that site but was a "bonus" one released to promote his book ...), actually more "what if Earth stopped spinning immediately": link.

I also thought to suggest that you read Four-Day Planet by H. Beam Piper, which is set on a world that rotates only four times in each of its years.

What if the Earth stopped spinning relative to its axis?

I had planned for a few decades being taken (if not a century) by the slowing process, with havoc being increasingly swathed across the whole of the planet.

Would the described effects come to pass anywhere near as severely til the later years of the slow-down process? I realize that it would still be crazy extreme weather and being even remotely safe is unlikely.

I just finished reading the whole thing. I utterly loved the book - 'twas a beautiful story. This book made me think of the long hours day and night would last if my described scenario took place... (4383 hours) Dang, that is a seriously long time to have a "grand day at the beach"! Not that anyone would live to tell the tale if they weren't geared up for crazy extreme heat.

No, no, that's not what I meant. In a parallel case, the Moon is tidally locked to the Earth, rotating once per revolution; it's reasonable to say that, relative to the Earth, it does not rotate at all ... but, relative to the solar system, it certainly does rotate. Do you mean that the Earth would become tidally locked to the Sun, or stop rotating from the perspective of some other frame of reference?

That still feels (to my instinct ... I'd strongly recommend doing the calculation of the acceleration that would have to be applied to bring anything from the current speed of the Earth's rotation to "zero" in however long you have in mind, and then investigate what effects that much acceleration would have) several orders of magnitude too fast.

Things wouldn't be as extreme as that article describes if the change were more gradual. But it entirely depends on the acceleration curve of whatever you come up with to cause this: if the deceleration is slow at first and then "snowballs" somehow (which doesn't seem plausible to me if it's a "short sharp shock"), then yes, the effects would go from bad to worse. If it's the reverse, then effects would be very bad and then moderate somewhat.

...The big thing is, if it's really supposed to stop and not start spinning the other way, the amount of acceleration can't be too large, since the forces trying to slow the rotation down "naturally" (the equivalent, on that scale, of friction) only do so at a rate closer to seconds per epoch than minutes per millennium, let alone two dozen hours in a century.

I once read a bit of a Star Wars novel set on a planet that was almost tidally locked, which only had an area a few (or maybe as much as a hundred) miles across that was habitable, on the border between the "day side" and the "night side" of the planet.

What's that? You're relocating to the relatively stable (though still awfully cold) polar regions? Bad move. They're deep underwater. In fact, the boundaries between ocean and land on a spin-free Earth would look nothing like they do today. Because the Earth rotates, centrifugal force causes the planet to bulge along the equator. No rotation, no bulge. Without that bulge, all of the extra water held in place along the equator would go rushing back toward the poles. Esri, a company that develops geography-focused technology, modeled the world's land and oceans after its equatorial bulge subsided and found that the Earth would have a band of land -- one giant supercontinent -- that circles the equator and separates two massive oceans to the north and the south.

...Because of the slowly increasing dramatic change from the high and low temperatures of the ever-expanding day, the oceans and lakes more quickly evaporated and froze over. The earthquakes from the blast had opened up many crevices around the world that allowed for the ocean to lower their levels to fill in these new zones. But as the earth continued to slow, the continents started to shift on their molten supports, shifting more central on the equator and towards the poles. This, combined with the lower water levels, created ring-like continents, connected only by small strips of land that followed the newly located underwater mountain ranges

Back in 2137 AD, as the golden age peoples would call it, a team of engineers and scientists worked on developing the first light speed capable spacecraft drive (which would have massively increased the capability of the human race to travel to other planets. As it stood at the time, only small colonies on Mars and the Moon had been successfully made.). The initial test with an actual, full-size prototype was done in the Salt Flats of Nevada. They had failed to correctly handle the fuel source (a proprietary mix of compounds unknown to me or the public… especially since I live hundreds of years after said event and no one alive could remember the event in such detail at all if there weren’t these videos) which caused a failure of the fuel pump regulation, pumping the drive full of energy-rich fuel, resulting in an explosion upon drive startup that literally vaporized several hundred miles, removing the team [and their drive] from existence. There were two major effects that occurred when this drive failed. The first is that time was stretched in the surrounding area. To this day, one can visit this location and an hour will pass in the mind of a human when only a minute or two, in reality, is traversed. Secondly, the initial explosion did actually create an accelerative force near that of the speed of light, causing a tiny “warp bubble” to be generated around the engine and then disappear as quickly as it appeared. The “bubble” may have disappeared quickly, but it caused a long-term frictional force on the whole of the planet, causing Earth to expedite the slowing of its rotation about its own axis. The residual effects brought forth tsunamis, earthquakes, volcanic eruptions, and super strong winds at such a level that over half of the world was forever changed. The 300 billion people on earth at the time was cut in half, with several billion more being injured beyond full repair. The destructive weather that followed caused many billions more to fall from life due to food growth failing and temperatures tending towards greater and greater extremes.
Scientists of the time theorized that the earth would eventually speed up again, even if it was after rotation hit zero. They were wrong on that front; it never got past zero rotation. Either way, governments that weren’t totally shattered gathered together their might and quickly built underground cities and travel-ways that would allow them to escape the dangerous weather above. Because the world’s rotational velocity was dropping steadily, the centrifugal forces causing the oceans to group [more so] around the equator also lessened, causing the oceans to increase their standing upon the poles. As the velocities and forces halted to a stop, what was left of the continents was a simple loop, several thousand miles in width. These shifts caused many of these underground cities to collapse or be cut off of from their supply lines or travel-ways. It was soon after all the shifting stopped that the underground realms stopped any communication with those still on the surface of the planet. Even to today, I know of no one who has heard from those below, and I have spoken with every leader of every major caravan on the surface that is not warlike in nature. Today, there are about two million people in the caravans on Earth, which is several billion less than when I was born.
- Unnamed man of fifty-seven earth crossings.