I’ve already covered how Earth at the last glacial maximum was in fact not a frozen hell but something closer to a paradise, one we still enjoy remnants of today…but there are some aspects of ice-age Earth that even the best refugia cannot preserve. Few places illustrate this better than the American South at the last glacial maximum, which, as any map of biomes 20,000 years ago will tell you, was a boreal forest, but one subtly unlike anywhere in today’s subarctic.
Oh, you can visit Dixie’s very own taiga even today — it waits in its alpine redoubt for the cold to return, as it has many times before when the world has warmed — but the vast expanse of boreal forest was no replica of today’s Canada or Alaska. The first aspect to understand is that the primary moisture sources for today’s South, and even eastern Canada (which the Southern Appalachian spruce-fir forest is often compared to), is the Gulf of Mexico and the Atlantic Ocean, both of which were much colder during the glacial period, which means much less evaporation, which means they largely shut down as sources of warm moist air.
One upside is that the characteristic muggy air of the eastern United States is gone, replaced by much drier air masses, the net result being air that feels much more similar to the American West’s today. And along with the American West’s air comes a precipitation pattern that westerners would find very familiar: with the Gulf of Mexico and the Atlantic Ocean largely shut down, the only remaining moisture source is deeper in the tropics, the famous “Pineapple Express”: atmospheric rivers originating in the equatorial eastern Pacific. The southeastern United States gets them even today, and they’re responsible for some of the all-time great snowstorms kids of Dixie remember — they’re just much less common than the usual Gulf lows or Atlantic nor’easters, let alone the summer thunderstorms.
But with all other moisture sources shut down, precipitation in the American South becomes dominated by episodic atmospheric rivers that form mainly in the winter. Sound familiar? It’s the characteristic California precipitation pattern, which gives it its “Mediterranean” climate. At the last glacial maximum, the entire country enjoyed dry sunny summers and wet winters, only back then the southeast was cold enough to have a subarctic climate. Summers? Warm days and chilly nights, with endless sunshine. Winters? Cold, ranging to very cold in places like what’s now Tennessee; temperatures were likely more comparable to today’s Alaskan Interior, with an average day not even reaching 0 degrees Fahrenheit during the afternoon.
Crucially, with Pacific storms being the primary moisture source, like today’s American West, and very much unlike today’s American South, precipitation will come in along with colder weather, not warmer. So with an average day already failing to reach 0 degrees, a place like Nashville would be subject to weeks where it plunges to -20 Fahrenheit or lower and snows, interspersed by more numerous weeks where it perhaps rises above 0 during the day and is bright and sunny…with ice fog galore in the river valleys and hollows.
The American South in fact has nearly ideal topography for copious ice fog, it just doesn’t get cold enough…in an interglacial. In the glacial climate that’s been normal for Earth for 90% of its recent geological history, it does get cold enough. Today’s boreal forests tend to be in landscapes scoured flat by glaciers in the very recent geological past, but the glacial-era American South reveals what a boreal forest normally looks like: sited in rolling to outright rugged terrain, not flat lake-studded plains. Ergo, much more ice fog than the average taiga today (though probably no more than Fairbanks or Yakutsk, with coincidentally also have good topography for ice fog formation).
Copious ice fog along with this “California but far colder” precipitation pattern is already odd enough, but it gets even weirder when you realize that the ice fog, which in the likes of our world’s Fairbanks and Yakutsk is fairly stable throughout much of the winter, would be subjected to strong winds scouring it away. Where from? Why, from the huge ice sheet just to the north, that’s where.
Enter “katabatic winds”, which are ubiquitous in today’s Antarctica and Greenland but just blow on coastal tundra and then out to sea. Our interglacial offers no analogue of an ice sheet whose barrier looks out to a vast rolling continent rather than to the sea, but we can fairly surmise that these flows, which tend to be strongest in winter, would cause hurricane-force winds throughout the tundra of what’s now the Ohio Valley, and cause notable gusts deep into the Tennessee Valley. In other words, precisely the colder reaches of Dixie’s taiga! In our world there is no boreal forest that’s this close to a massive ice sheet, but we can expect katabatic winds to be a major feature of the climate; it helps that the hilly terrain of the American South means that there would be any number of places that would act like wind tunnels. Where the Cumberland River cuts through the Highland Rim just northwest of Nashville, for instance, was probably a very windy area in the winter during the last glacial period when the winds blew out of the northwest.
And if you’ll recall, the glacial-era climate engenders snowfall at extremely cold temperatures by our standards, which means snowflakes that are even lighter and fluffier than the famous “champagne powder” of the Wasatch and the like in Utah. Indeed, at temperatures that may well be -40 Fahrenheit, the powder of the prehistoric South would make the fruit of the “Great Salt Lake effect” of our time seem like concrete by comparison; the water content would be about half of “normal” champagne powder. Instead of skiing through it and the powder reaching your hips you’d experience immersion in ethereal powder clear to your head as you navigated. And needless to say if you skied down those hills the temperatures would make even today’s Tetons look mild by comparison.
Katabatic winds are gravity-driven, so they’re agnostic as to the activity of atmospheric rivers and passing storms, and unlike chinook winds they don’t warm temperatures meaningfully as they blow. So while the ice fog would clear as they blew through, gale-force gusts with the usual -20 Fahrenheit or colder temperatures would be routine in much of the Tennessee Valley. If passing atmospheric rivers happened to incorporate a strong storm and coincided with katabatic winds, you could even see “katabatic-enhanced’ wind gusts exceeding 100 mph on a routine basis in some areas.
Needless to say this very windy air would loft up the snow like it weighed nothing, so presumably blowing snow and ground blizzards were much more common in the glacial-era South than anywhere on Earth today. Subtly alien…and it gets even better.
Consider that the last glacial maximum could boast of twenty times as much dust content in the atmosphere globally as today (yes, twenty times as much; there’s more ice, more desert, less forest, and a lot more wind, so it’s only natural). The air would generally be dry and very clean, with (aside from the dust) little in the way of irritants or allergens, with insect activity being minimal, aside from the deep tropics…and even the deep tropics were notably less muggy than today. Even in the hottest parts of Amazon it dipped below 70 degrees Fahrenheit overnight.
The upshot of all this dust? As in after Mount Tambora erupted, much more vivid sunsets on a global scale. A classic colorful California sunset? That’s just the baseline, everywhere. Even in biomes like thick boreal forests where you normally would not expect to see anything other than a muted sunset; somewhere like the American South could see deep golds, oranges, reds, and purples every night and every morning even more so than the Mojave does today. That’s just what the physics tells you would happen.
So in the winter, you could be perched midway up the slope to the Smoky Mountains, and all around you would be a yards-thick snowpack of champagne powder in the spruce-fir forest, with ice fog below making it feel like you were on an island in the sky, every sunrise and sunset being gorgeously vivid and colorful. It’s always very cold, but most of the time it’s sunny…until a week comes in when ethereal powder dumps in whiteout fashion, accompanied by lightning and thunder in some of the stronger bands as they hit the mountains…but forget about there being any summer thunderstorms (the air is too dry)…or indeed rain, period. By the time storm season starts, the air has already cooled to well below the freezing mark. The Tennessee, Mississippi, and Cumberland Rivers all flow mightily (during the few months they’re not frozen), but they’re fed entirely by snowmelt, not rainfall.
And in those dry summers, in nights that, upslope, could easily be outright frosty, you’d see them: noctilucent clouds. Aurora borealis still appears only at high latitudes, which in the glacial era is deep in the polar climate zone, but noctilucent cloud formation follows mesospheric temperatures, which cool along with the surface; as the northern woodlands’ characteristic climate migrates southward, the noctilucent clouds migrate with them. Somewhere like Tennessee would see more noctilucent clouds than anywhere on Earth today. Remember the dust content? At twenty times today’s dust content, noctilucent clouds would have a much easier time forming and growing (more nuclei are available even up in the mesosphere).
As with today, during the last glacial maximum they’d be most visible in summer, so in those nights in the South they’d be spectacular visitors after the warmth of the afternoon sun and the vivid colors of the sunset have faded. You might feel like you were in some high-latitude alpine woodland…until you realize that while the climate is different, the days at 37 degrees north are still the same; compared to our taiga the summer days are short, the winter days are long. Again, like the aurora, the midnight sun and the polar night are far above the latitude where ice dwells year-round.
In the summer from places like North Carolina or Tennessee you could easily see displays of noctilucent cloud more spectacular than anything seen anywhere today, much less a latitude that low. Which implies they could be prominent deeper in the fall and spring than is the case today, but by winter it seems likely they’d have faded from view.
Nevertheless, this all adds up to a picture that’s boreal forest, yes, but alien in so many subtle ways…and intriguingly intense in a way that humans would find absent from our world. But everything I describe above is not a fantasy, it was reality. The American South was actually like this…and someday will be again.
After all, interglacials like ours are but brief heat waves against the Earth’s baseline in our geological age: in the not too distant future, the ice will reclaim what is rightfully his, the trees of Mount Mitchell will sense their time has come, and the humans left behind on the mother planet will witness the return of paradise…
This post’s featured image is Southern Appalachian spruce-fir forest at Clingman’s Dome by Brian Stansberry (2011) (CC-BY 3.0)