If you’re even passingly familiar with apocalyptic conspiracy theories, you’ve probably heard of “pole shift”, the idea that the Earth’s rotational axis will soon flip or suddenly change and cause some sort of catastrophe. This, of course, is not happening in real life…but there is a kernel of truth in as much as the rotational axes of planets can and do shift over time. It’s called “true polar wander” in scientific literature, and is well-described!
Over the past 200 million years (roughly since the Age of the Dinosaurs) a cumulative shift of 30 degrees is thought to have occurred in Earth’s rotational axis; characteristically, Earth’s axis tends to shift by less than one degree per million years. This is the typical case, so far as we know: very slow, gradual movements. If you’re even passingly familiar with geophysics the very idea that rotational axes can shift might be somewhat surprising, since angular momentum left over from the planet’s formation is conserved; to wit, rapidly changing the spin rate of a planet without a catastrophic impact or some other form of energy release or input that would also be catastrophic. But planets can change orientation much less catastrophically, since, like any spinning object in space, only minimal energy input or output is required.
Rather, all that’s required, mechanically, is for the mass balance within the planet to shift enough: like a gyroscope or, more domestically, a washing machine, if enough mass within the planet shifts from the equator to the poles, the tendency will be for the planet’s orientation to shift so that the “lump” of mass is around the equator. This might sound weird, but a terrestrial planet deep inside is rather fluid: the mantle is hot and convective, and over geological time, rock moves and flows. In principle, this can cause true polar wander of a more abrupt sort, and it is suspected this might have occurred in the Precambrian on Earth. “Abrupt” here means a substantial shift over, say, a million years, which would not be a sudden catastrophe on a human timescale, but would involve a substantial movement of climate zones and hence ecologies across the planet as the latitudes of the continents change much more rapidly than they normally do via continental drift.
In principle we can suppose an even more rapid shift. Consider that there are other sorts of masses that could cause an imbalance besides mantle convection: even today we see lands in North America and Europe rising (to the tune of new islands routinely appearing in the Baltic off the coast of Finland) due to “post-glacial rebound”, the movement of land upward from being depressed by the sheer weight of ice sheets in the last glacial period. Ice caps store and concentrate a lot of mass. Not enough to have caused a pole shift on Earth, obviously, but in principle if you had a very thick ice sheet (Antarctica’s ice even today is up to several miles thick and even thicker ice sheets can be envisaged if accumulation were more vigorous during its formation for whatever reason) that could cause a mass concentration that could tip the balance, especially if the mantle was already configured in such a way as to “prime” the planet to shift if more mass is added to a polar continent.
Let’s take a version of Antarctica (an ice-covered polar continent) that’s the size of Asia. As mass is accreted on top of the land, more and more mass is added to the poles, together with our hypothetical concentration in the mantle leading to the planet reorienting itself. In real geological history it’s thought that even a “sudden” true polar wander would still be very gradual on a human timescale, but there’s no reason in principle that the planet couldn’t reorient itself much faster. By which I mean over the course of a century or so: noticeable from year to year, yet the disruptions to the planet would in this case manifest in the form of an upsurge in volcanism and so forth, since the rock and the seas would have ample time to adjust, albeit unusually rapidly and “catastrophe”-like in terms of the geology it would produce.
Normally this would mean an icy continent reorienting so it’s in the tropics, near the equator, which on an Earth-analogue planet would mean the strong sunlight melts off the ice cap and that’s that. No more pole shifts. However, suppose that the pole shift places another continent of roughly equal size in the same polar position as the previous continent. As ice sheets rapidly accrete onto that continent, the same mass imbalance recurs, depending on the exact configuration of fluid rock in the mantle, and then after a short geological time the ice sheets grow enough to where the rotational axis shifts again…back to where ice sheets accrete onto the original continent! So you can in principle get this oscillator, where rapid pole shifts occur as a regular feature of the planet’s existence.
Notice here that we’ve constructed a planet that by all the measures we typically care about is a perfect analogue of Earth, yet in its fine geological details is utterly alien to our experience. Ice sheets rapidly accreting and then ablating would lead to sea levels cyclically rising and then falling with each cycle…potentially to a much greater degree than in Earth’s recent glacial cycles. Already since last glacial maximum our oceans have risen 400 feet; if all the remaining ice melted the oceans would rise another 200 feet.
Now suppose that you’re starting from an ice-free baseline but you have an Antarctic-style continent with an ice sheet perhaps two to three times as thick and about the same size as Asia. 1000 feet of sea level drop would be conservative; you could easily be looking at 2000 feet or more of sea level recession.
Which sounds extreme, but on Earth, if you peel the seas back, once you’ve reached 600 feet or so down you’ve already exposed virtually all of the continental shelves. Below that elevation you’re mostly dealing with very steep slope down to the abyssal plains that dominate the oceans, which are very deep down below the surface: on Earth you don’t hit them until 10,000 feet below sea level. Even mid-ocean ridges wouldn’t be exposed until you drop sea levels by 8000 feet or so, bar exceptions like Iceland that are far higher up (yes, Iceland is a component of the Mid-Atlantic Ridge, hence the weird “fissure” volcanism that you usually would only see underwater).
On an alien planet we of course can tweak it: even on Earth ocean depths have been known to vary; during times of vigorous “spreading”, mid-ocean ridges rose higher and were much more common, and so water was displaced upward (after the breakup of the supercontinent Pangea this was prevalent, hence why sea levels rose so high and so many continental shelves were drowned during the Mesozoic…by now ocean spreading has calmed down, the sea depths are deeper, and ice sheets are much larger, so most of our continental crust is above water now). But hey, let’s run with it.
Once you’re dealing with thousands of feet of cyclical sea level rise and fall it barely matters anyway; consider that glaciation and deglaciation can in principle occur very rapidly. We think of ice sheet formation as being very gradual, but even today some of our mountains that face the Pacific routinely receive 1000-inch snowfall seasons (they’re legendary for skiing…). What does that much snow do when temperatures cool down so much that summer no longer is warm enough to melt snow? It builds up glaciers. 1000 inches of snow is already 80 feet per year, so after a decade even with compaction you’re dealing with an ice mass that’s measured in fractions of a mile deep, and it would start to flow down toward the lowlands…
And 1000 inches is nowhere near the meteorological limit. Individual days of course have been legendary when a storm sets up just right against a high mountain slope, but what’s not as well known is that even entire months can dump huge quantities of snow compared to what we’d think of as “normal”. Tamarack, California holds the US record for the snowiest month: 390 inches in February 1911. No doubt courtesy of an “atmospheric river”, a parade of storms that hit the same area and transported vigorous quantities of moisture into arctic air masses. Scale that up over an entire year: 390 inches a month sustained over a year is 400 feet of snow. With compaction the resulting ice layer would be shallower, but over a decade of such a weather pattern remaining persistent, you’ve already built an alpine glacier that’s half a mile deep.
And Tamarack, California’s February is probably nowhere near the actual ceiling of what atmospheric moisture transport can do. Consider that a rapid true polar wander will cause solar geometry to change year by year, which would cause land temperatures to respond rapidly, but the oceans are an enormous reservoir of heat. Their circulation takes time to reorganize, and tropical-hot water that finds itself at higher latitudes year by year will express itself in the form of storms, as it collides with incipient arctic air masses at its new latitude. Warmer water of course gives more moisture, and so in a world of this nature, during a pole shift transition we can expect atmospheric rivers that make California’s most legendary storm parades look like a damp squib by comparison.
Tamarack’s record 390 inches in a month might sound insane, but it averages out to “only” 13 inches per day. In a storm parade like what this planet would likely produce you’d probably see 13 inches of snow fall in an hour for much of the month. Earth’s ice sheets didn’t build quite this rapidly, in all likelihood, but people seriously underestimate how fast glaciation can happen if conditions become favorable. The math suggests that within a few years you’d see lobes of glaciers whole fractions of a mile high descend down the mountains and into the valleys, gradually covering the continental plains with ice.
Glaciers, when stimulated, can flow faster than you think too: Greenland’s fastest-moving ice tongues flow across the landscape at a rate that converts to 3 miles per hour. That’s about as fast as a man walks! Only it’s a wall of ice creeping across the landscape, crushing anything in its path.
It gets even fancier: as ice covers more and more of the new polar continent, more sunlight is reflected back into space, cooling temperatures on land, so even as the ocean works out its heat, the land nearby gets even colder, so the temperature gradient would remain steep for an extended period after the climate changes, keeping storms fully fueled for decades to possibly centuries, by which time you would have already built an ice sheet several miles high that covers virtually the entire continent.
Sea levels would drop accordingly…and keep in mind, on the now-tropical continent that’s been deglaciated, the land is rising! Which is a double whammy. Post-glacial rebound in most places is quite slow on Earth today, but immediately after deglaciation the most affected regions are known to have uplifted at a rate of up to one foot every three years, which is quite rapid geologically. That’s 30 feet per century of elevation change! On a planet with a more elastic crust, which could be envisaged, the rate could be even faster, perhaps a foot per year or so, or even more rapid. This would cause a lot of earthquakes and volcanism, but again, nothing catastrophic to the point of apocalypse.
The net effect is that immediately after deglaciation, huge swaths of the now-tropical continent would be drowned — this would be especially true if the planet rotates at an Earth-like rate instead of very slowly, since the equatorial bulge affects water more than the underlying bedrock, meaning that all other things being equal a continent at the equator will find much higher sea levels than at the poles (Earth’s continents don’t move fast enough for this to be a factor, since the underlying continental crust can gradually adjust like rafts floating on the oceanic bedrock, but it’s covered in speculative documentaries like “What if the Earth Stopped Spinning?”…without the equatorial bulge, the oceans rush toward the poles, drowning most of the northern continents in ocean water; with it, of course, the opposite effect prevails). But as sea levels rapidly drop due to more ocean water being effectively converted to ice via atmospheric moisture transport, and the land rises, the lands would re-emerge from the sea. Potentially quite rapidly.
Consider that from the mouth of the Mississippi up to the confluence with the Ohio is 315 feet of vertical elevation difference. In this schema, you could see the sea start out at the Ohio’s confluence, and it’d recede hundreds of miles “downstream” in a matter of a few decades. From year to year it would be plainly visible, the recession of the ocean and the exposure of new land. Very freaky, fast-motion geology.
Rivers would of course rapidly “incise” downward along their courses, immersion in the ocean would alter landscapes, and all manner of hydrological and glaciological violence would reshape landscapes as the cycles repeated in ways that Earthlings might find vaguely familiar but would have no exact terrestrial analogue for.
Fun stuff, and of course such a violently changing planet would have a variety of fascinating implications for the biosphere as well, not just the climate, but how you would worldbuild off of that is a topic that deserves its own post. Already here the exploration has been fascinating; yet another example of what can happen when you take a “crazy” topic, look up the kernel of scientific truth, and reconstruct the general idea along hard-science-fictional lines. Oftentimes the scientifically informed result is much more compelling a picture than some slap-dash fantasy. You just have to marry the science with the imagination, that’s all…
This post’s featured image is a rendering of Ice Age Earth, by Ittiz of Wikipedia (CC-BY-SA 3.0), rotated by me