So, I was skimming cover copy at the regional B&N recently, and found yet another, “humanity goes away and paradise returns,” this time with terraforming gone wild tossed in. Yawn. Now, granted, I did not buy the book and read it, so I don’t know all the details of the world building, either in-book or otherwise. However, if there were dams and irrigation systems, a rapid decline in population will not lead to a return to Nature. The opposite. Because we (humans) have seen this before.
One of the big surprises that arose when people got serious about using the tools of archaeology and hydrology for environmental history was the realization that the end of the Roman Empire and the population decrease in certain regions led to vastly increased erosion. That was completely counterintuitive. The thinking had been that when people stopped cutting down trees and overgrazing, and when the irrigation diversions silted up and went away, erosion would return to the pre-development baseline, then improve (build up instead of cut down.) That’s not what the dirt showed. Oops.
How awkward.
For one thing, in a lot of places, the “forest primeval” hadn’t existed since the Neolithic. Humans abhor dense, dank wildernesses. So they cleared out spaces for desirable species, thinned the undergrowth, encouraged “good plants” and generally did everything they could to get rid of “pristine Nature.” Unless it was the high mountains, or nasty swamps, people all over the world modified, improved, and tweaked “Nature.” And that’s before the engineers appeared . . .
When you deal with water control structures, like flood-control dams and irrigation systems, you generally have to keep in mind silt and other sediments. Silt, the very fine particles of soil and other things, will slowly settle out of slow moving or non-moving water. Give it long enough, and it will fill-up your reservoir. Moving streams have an energy balance: Slope of bed X volume of flow = amount of sediment X speed of flow. Change one and the others will adjust to balance. So if you slow the flow, silt settles out. Speed up the flow of non-silty water (say, at the outlet of a dam), and you will have increased erosion until the water collects enough solid particles to return to balance. Fast moving water can carry more stuff than slow moving or still water. It’s common sense, but the ratio wasn’t known until the late 1800s-early 1900s.
So, back to the Roman water systems. When Rome retreated, the engineers and excess population needed to justify keeping the dams and irrigation systems working also went away. So the systems failed from floods, lack of repair, the occasional sabotage, and hap-hazard maintenance. All the dirt in the ponds behind the dams started to flow downstream, silting up the place. Irrigation systems became erosion channels, eating into older fields, especially if terracing failed as well and gravity helped move soil down the slopes. The greatest amount of erosion seen in parts of Europe prior to the 1800s came between AD 460-600 CE. When the population dropped and resource use declined, in other words. Reforestation didn’t happen fast enough, and the bare soil that had been fertile fields washed away, causing more erosion until it was reclaimed in the Middle Ages.
That’s the problem with archaeology and environmental history. We keep finding things that make simple, tidy stories messy and complicated. We upset Natural apple-carts. So a failure of terraforming leading to the disappearance of the human colonists would not return the planet to “pristine Nature.”
Ya know, studying environmental history ruins so many sci-fi and fantasy concepts. SIGH.
Cat Rotator's Quarterly 
Slightly off topic, but I saw a blurb for a book that basically went as follows.
“A bunch of young people realize civilization has ended and that’s great”.
IE The idiots think that they’ll survive just fine after high technology is gone. Of course, the idiot author apparently believed that would happen. đŸ˜¡
When I was teaching high school science, at least once a year I would have a student declare that they didn’t need civilization to survive! (Rural school…always farm boys.)
My response? Start asking questions. “How are you going to…? Do you know how to…?”
My favorite question “Do you know how to make black powder?” Even though some of the kids used black powder rifles for hunting they hadn’t a clue.
Some days it feels like civilization ended a decade or two ago.
Two steps forward, one step back…
There is a lot of discussion as to what is sentience, and will we recognize it in an alien species. My opinion is that one factor in determining sentience is the ability to modify and actually modifying the environment to make it more comfortable, habitable, productive and less subject to the whims of nature. There could be an argument made that our pets have figured out how to manipulate us into modifying the environment for their well being. However, without us to operate the can opener and clean the litter box, etc. they will revert to feral animals subject to whims of weather, climate, predation and disease.
I had also read that established Roman cities and farming communities in North Africa began dying when maintenance (for whatever reason, mostly invasion and slaughter) began failing on the systems of reservoirs and aqueducts which supported those cities and farms. North Africa reverted from a breadbasket to the desert that it is today.
*wags paw* That’s a very controversial topic right now, because there’s a sub-branch of late Antiquities and Roman Imperial history that argues against the importance of water control in North Africa outside of urban areas. They point to environmental patterns (rainfall, farming techniques) in rural areas, and claim that irrigation didn’t play a role outside of gardens for feeding the Roman cities. Water in the cities went to baths and fountains and so on as a way to display power, not to feed people. I’ve read both sides, and am waiting for more data. Given how difficult it is to get hard data about how much of what was grown where in that part of the world . . . I think the desertification-by-neglect is overstated for the end of Rome. Not arguing against it, just overstated by later authors. YMMV.
And in some places like the Northeast there is NO way to return to the forests as they were when the Pilgrims hit Cape Code and moved further up the coast to Plymouth. Why you ask? Because two of the predominant treas of that old growth hardwood forest the American Elm and the American Chestnut are extinct (or effectively so in the Chestnut’s case). Those trees leaf mould tended to drive out other species and limit undergrowth under those large canopied trees. The maples and birches that are predominant now don’t, although Oaks do some degree. There is some work on to get Chestnuts back via some genetic manipulation, but I think the Elms are gone forever
Plus, you’d need to return to periodic burning if you want to return to a close-to-Native-American biota. And that also requires how to work in the results of swidden agriculture, in those places where it was practiced, and the effects of fish traps. Oh, and removing all the other dams and water control structures, which will cause a real mess for several decades until the hydrologic balance returns. And there will be a lot more wetlands. I’m not sure the urbanized folks currently living in New England and upstate New York want that.
Burning! Most of the tallgrass prairie existed because of periodic fires. Without agriculture or fire, those areas revert to woodlands.
I don’t know if fire is as critical in Northeast old growth forests as it is in Western ones (some pines depend on fire to open their cones). Certainly fire was part of the environment, I think its coming back in small degree as the enviro weenies won’t let them manage the forests so you get underbrush build up with lots of fuel. There were some beauties of fires here up towards Gloucesterr this summer with the drought conditions.
William Cronon’s book *Changes in the Land* goes into some detail about the reported ground cover and burning described by pre-Puritan and later observers. Carolyn Merchant’s *Ecological Revolutions* goes into slightly different detail about the same time period, with more emphasis on Native Americans and women’s role in eco-management.
The northeastern forests are not pyrophillic (have to have fire) like the tallgrass-prairie and northwestern forests are, but they were indeed fire-managed, as were the southeastern forests (to encourage deer and discourage ticks, among other things.)
Sigh… There you go again with those pesky facts… Stop that! đŸ™‚
Late again … but I read the claim that North America had no earthworms until Europeans brought plants in soil and worms with them. This author went on to claim that before the earthworms, the leaf mold was a foot and a half deep in places, and thanks to the invasive species that will never come back.
This claim doesn’t seem plausible to me, but I haven’t had the chance To Ask Someone Who Knows. So now I’m asking.
That’s a riff on Charles Mann, who wrote the very good summary book _1491_. [NOT Mann the “climatologist” who gave us the infamous Hockey Stick Graph based on one tree-ring set from Siberia]. If I recall correctly, there were indeed earthworms, but not the aggressive European kind (nightcrawlers) that came in soil with crops from England and the Continent. Like there were smaller native bees, but not honey bees. So there was more leaf mold, but not that much extra in most places.
Again, it’s been a while since I read _1491_, and I don’t have a copy at hand.