The post-apocalyptic genre presents a wide range of possible disaster scenarios for us to consider. They span from the classic, such as worldwide nuclear war, to the fantastic, such as the recent spate of zombie movies. While everyone enjoys a good, brain-eating thriller, any scientific consideration of post-apocalyptic scenarios should confine itself to more plausible events. For this essay we'll consider the classic SF nuclear wasteland and the related nuclear winter, as well as the more contemporary apocalyptic anxieties in global warming and world wide pandemic. Not only are these scenarios more likely to occur than, say, the robopocalypse or the grey goo ecophagy, but they point to concerns held seriously enough to find voice in our political discourse. Thus, besides easing the suspension of disbelief, scientific fact used in fiction can add a layer of emotional and narrative charge to contemporary human concerns.
Nuclear war is one of the most common forms of apocalyptic devestation, and that dates back to obvious Golden Age scientific and political concerns. The world is depicted in such accounts as either a blasted desert or a wintry wasteland. One of the prime literary examples, A Canticle for Leibowitz by Walter M. Miller, Jr., involves an abbey set in the southwestern United States. While outposts of civilization remain, they're separated by vast stretches of wasteland, populated by brigands and unfriendly mutants. The Mad Max franchise presents a similar scenario, with small, barbaric desert towns and not much in between. The final film in the trilogy, Beyond the Thunderdome, can be noted for offering an exceptional contrast in the grotto paradise inhabited by still innocent children. One can easily add to this list, whether in classic SF films and novels such as A Boy and His Dog or John Wyndham's Chysalids, as well as more recent video games like the Fallout franchise.
The actual effects of nuclear war on ecosystems have been hotly debated for decades. In the early 1980s, the consensus was that nuclear war would completely devastate the biosphere. An essay by Jonathan Schell titled "A Republic of Insects and Grass" put forward that the radiation doses deposited by nuclear war would cause a mass extinction, destroying most vertebrate life; plant life would be comparatively little affected. A number of actual observations and experiments of the effect of radiation on plant life were performed during the Cold War period. (There was even a journal devoted to the topic, Radiation Botany; it later lost this specific focus, changing its name—in 1976—to the far more generic Environmental and Experimental Botany). Actual observations of the effect of nuclear weapons on plant life made after the United States nuclear testing in Nevada and experimental controlled exposures of plants to simulated fallout radiation bear this out. One study using experimental controlled exposure to radiation simulating a fallout environment found that food crops varied widely in resistance, but that even as taste became disagreeable, leafy textures became rough, and yield lessened, even the weakest were likely to survive exposure rates that might effect 100% mortality rates in humans. Studies of the desert shrub Larica (creosote) exposed to radiation from nuclear testing showed little to no effect except at extremely high doses; one study suggested that the dust and debris thrown up by an explosion caused far more stress to the plant than the radiation. In fact, plants exposed to 4,000—6,000 roentgens (a unit of exposure) but not covered in dust did not even lose leaves. A study that utilized much higher doses of radiation, from 15,000 up to 20,000 rads (the equivalent unit of absorbed dose) from beta radiation alone, caused significant damage and death, but even then some survived. It's thus unlikely that the flora would be significantly damaged, or that a 'nuclear wasteland' would result, from radiation alone; dust thrown into the atmosphere might cause more damage, but, barring the full force of the world's nuclear arsenals, it would be unlikely to be sufficiently dense and widespread to cause a mass kill off of vegetation. Thus a more realistic depiction of global nuclear war's effects on the environment would include regional and often serious damage but not total, ecosystem-wide devastation. Pat Frank's Alas, Babylon provides such a vision, where the effects are largely confined to the targets of the weapons, and the major consequences are in fact social and political.
Another mechanism of ecological collapse that might lead to a post-nuclear wasteland environment is the destruction of the soil-dwelling, nutrient-recycling portions of the ecosystem necessary for plant growth. This is suggested by Richard Fortey in his book Life: A Natural History of the First Four Billion Years:
"Novels that seek to portray post-holocaust worlds always seem to assume that the soil will magically survive, and that a bean cast into seared soil will quietly proceed to a successful crop. But the soil is not a passive medium; it is alive. I doubt whether there would be many readers for a post-holocaust novel that was concerned with the hero's desperate search for mites. But alas for the world if the mites and their diminutive allies failed to prosper!"
Fortey is referring to the interconnected web of organisms in the soil that, while rarely seen, is crucial to the processes that make nutrients available to plants and thus to the ecosystem as a whole. These organisms include those responsible for decomposing organic matter in the soil, providing soil structure and nutrients in the process, e.g. bacteria, fungi, and even earthworms. Severe damage to this biota would cripple the ecosystem, as decay would no longer return the nutrients in dead organisms into the ecosystem. (And when dead bodies are put into an environment in which decay bacteria and the like cannot survive, they remain preserved for great lengths of time—frozen mammoths in the Siberian permafrost have survived for tens of thousands of years.) In addition to recycling nutrients from dead organisms, the soil biota also contains many bacteria that fix nitrogen into a biologically usable form.
However, it seems unlikely that the soil biota would be destroyed so completely. We see no evidence of the collapse of this part of the ecosystem in the fossil record after events such as supervolcanoes, which produce a 'volcanic winter' effect similar to that of a nuclear winter, and which may have an explosive force comparable to an entire nuclear war. Many eruptions of a truly staggering scale, such as the Taupo caldera of circa 25,000 years ago, are not accompanied by extinctions in the fossil record. The enormous Lake Toba eruption 75,000 years ago has been associated with population bottlenecks in humanity and other species, but no spike in extinctions is known at that time. It remains possible that the radiation distributed by nuclear war or similar causes would harm the soil biota; it has been suggested, for example, that the radiation from the Chernobyl disaster greatly harmed the local soil microbes. However, germination of plants around Chernobyl has not suffered, and some soil fungi have adapted quite well to the increased radiation levels, and a few actually increased their growth upon exposure to radiation .
As opposed to the blasted desert, the potential problems caused by the atmospheric consequences of large-scale nuclear weapon use is often called a nuclear winter; it has natural corollaries in the "impact winter" from an asteroid impact and the "volcanic winter" phenomenon. Cormac McCarthy's The Road offers a contemporary take on this trope, with grey skies and cold air that parallel the moral and existential void faced by the protagonist. Unlike most of the other apocalyptic scenarios, serious interest in these extended winters dates only from the 1980s. A seminal paper on the subject was published in 1983 by Carl Sagan entitled "Nuclear Winter: Global Consequences of Multiple Nuclear Explosions." Interest in the possibility of a nuclear winter was paired with the emerging hypothesis that an asteroid impact caused the end-Cretaceous extinction appeared. The impact winter effect, attributed to the impact event which caused the end-Cretaceous extinction that eliminated dinosaurs and many other Mesozoic organisms, often appears in time-travel stories as well as those set in an apocalyptic future. For instance, a large portion of Will Hubbell's Cretaceous Sea concerns the survival of time-travelers stranded in the aftermath of the 'dinosaur killer' impact during impact winter phenomena. The stranded characters must survive in a world blasted by the vast disruptions caused by the impact, such as violent shifts in plate tectonics that engender a mega-tsunami. Furthermore, the ecology is wrecked by the event, and some of the more intelligent small predators, no longer able to find their normal prey, begin to hunt humans.
But the more quantifiable danger of these extended winters, and one shown in Hubbell's work, hearkens back to our above concerns that plant life would suffer more from the dust thrown up into the atmosphere than from nuclear radiation itself. Multiple nuclear detonations would likely trigger enormous fires, sending soot and the like into the atmosphere, where it would block out sunlight. A recent study has suggested that urban fires started by nuclear explosions would provide most of the smoke that would block sunlight. This means that existing nuclear tests, such as those performed in the Pacific Islands or in Nevada, are a poor model for the effects of a nuclear war on climate, as the indirect consequences have become a greater factor in determining the climatic effects than the bomb's direct effects. If realized, a nuclear winter would damage the ecosystem by destroying the plants on which it depends, both through cold and through lack of sunlight for photosynthesis. However, even this would kill only currently growing plants, and seeds would still sprout. Many seeds can remain viable in the soil for long periods. Whatever animals survived the period of hunger could then repopulate. But the effects would still be serious, as plants whose seeds cannot enter such a stasis might risk extinction, and many animals would not survive the initial period of stress.
Yet as the spectre of the Cold War has faded into history, so has the looming threat of global nuclear annihilation. In its place, an apocalyptic fear with a long history in myth and legend has reemerged in contemporary concerns about global warming. Where the ancient stories looked to angry deities to explain civilization destroying floods, authors and scientists look to melting ice caps caused by climate change. The Drowned World by J. G. Ballard was published in 1962 and presages the fears that Al Gore is famous for raising today. In this work, floods overwhelm coastal cities, transforming some of them, such as London, into tropical, swampy landscapes. The survivors must make do in a chaotically primitive world. The 1995 movie Waterworld, directed by Kevin Reynolds and starring Kevin Costner, presents a world almost entirely underwater, the few bits of land left were once tips of the highest mountains. The last remnants of humanity have taken to ships and some have begun mutating to better adapt to the permanent new earth.
A similarly extreme version of sea level rise is presented in Stephen Baxter's recent novel Flood. In this book, deep subsurface reservoirs break open, releasing quantities of water far larger than those stored in the ice caps. In the early parts of the novel, the results are similar to those that might be expected for global-warming-driven sea level rises, but later the waters submerge even the mountains, driving the last communities together up onto the planet's highest ground, triggering violence between them, and finally—as the highest ranges go underwater—forcing them onto ship and raft communities. The sea level rises push ecosystems up the mountains, so formerly high-elevation areas become filled with species previously found in the lowlands. As the world becomes almost entirely ocean, the book also discusses climatic changes driven by the increasing ocean-land proportion, altering the planet's albedo and thus its response to sunlight. While there is evidence that pockets of water are contained in earth's crust and mantle, the tectonic events necessary to release vast quantities of this water are little understood and appear highly unlikely.
Now, assuming the former scenario, of melting ice caps alone, 60 meters is the maximum rise in sea levels that we would face—Greenland and the Arctic each contain enough ice to raise sea levels around 6 meters, and the majority of the balance coming from Antarctica. The contribution of glaciers found in high mountains, subantarctic islands, and the northern tundras would be very small in comparison. (A second source of sea level rise is the thermal expansion of ocean waters, but this is again tremendously smaller than the polar glaciers' contribution.) While 60 meters might not be mountain covering deep, the effects could still be dramatic, especially if the rise was sudden. Coastal lands would be inundated, causing untold damage to cities and infrastructure. Besides the obvious effects on civilization, ecosystems would also suffer—unique communities such as coastal grasslands, salt marshes, and others found only near coasts might be destroyed. Furthermore, the warming that caused the sea-level rise would have ecological ramifications in its own right, driving animals to higher elevations putting many species at risk due to disappearing habitats. Also, warming would change the seasonal cycles, causing plants to sprout and insects to emerge earlier than before—the migration cycles of birds then become desynchronized from the population of their food sources, leading to starving birds and unsuccessful nestings. In addition to the direct effects of warming, the climate change might have other, indirect effects by shifting weather patterns and ocean cycles, such as acidifying the oceans by increased atmospheric carbon dioxide.
As devastating as the above scenarios can be, the direct effects of these apocalyptic disasters appear more salient to us, given their immediacy and obvious impact on human civilization. Yet equally relevant to a discussion of how speculative fiction wrestles with the scientific facts of a post-apocalyptic world is how the environment will respond once relieved of human populations and industries. Often the earth, in this aspect, is presented as a 'new Eden,' green and ecologically whole. With the removal of the pernicious effects of civilization, nature reclaims the world and regenerates itself, healing the scars mankind imposed upon it. A global pandemic is often used to achieve such a world, absent humans without any catastrophic event that otherwise damages the natural order. This pandemic trope appears in George R. Stewart's novel Earth Abides, where civilization collapses, but the remainder of the earth's ecosystems remain unharmed. The novel describes the changes in some detail, and is notable for being one of the earlier works of science fiction (1949) to address the environment in a post-disaster context, in an era before ecological consciousness became widespread.
While even the removal of industrial civilization's direct impacts on the environment—in such forms as pollution, urban sprawl, roadbuilding, and mining—would not return the world to its actual state before the industrial era, there would certainly be many shifts which would return the ecology to a state more similar to the pre-industrial world. The end of direct pollution from industry, vehicles, excess agricultural chemicals and other causes would, of course, remove one source of environmental damage. The end of the fishing industry would also remove huge impacts on ocean communities, allowing many species to rebuild their populations.
Indirectly, a more natural regime of fire and regrowth would return to many lands where fires are suppressed by human activity. While it is now known that fire is necessary to many environments, even forests, and the general attitude of the mid-20th-century that all fire is environmentally destructive has nearly disappeared, natural fire can nevertheless not always be permitted in areas near to human habitation and property. Suppression of fire has allowed trees—not only the classic invasives from other continents, but even native trees such as the Ashe juniper in central Texas—to invade grassland environments.
Other shifts in species abundance indirectly caused by human activity might also recover, such as those due to overgrazing, encouraging thorny or poisonous plants in place of those livestock favor. Although domesticated livestock might not actually go extinct (though some more radically altered breeds probably would), the now feral livestock, without human aid, would be reduced to a population comparable to the original native herbivores, such as feral horses and cattle. Far more significantly, the feral livestock and pets now free of agricultural containment, would enter their new environments as invasive species. Invasive species are very widespread even at present, and nature preserves have to make a constant effort to maintain the original ecosystem against their incursions. If a collapse of civilization rendered such conservation activities impossible, ecosystems would not return to their prior state. The coastal grasslands of the United States Gulf Coast would be replaced by savanna environments under the incursion of invasive trees such as the Chinese tallow, while the waterways of the area would be choked by plants such as water hyacinth and water lettuce. The desert and semiarid regions of northern Mexico and the southwestern United States would be radically altered by tamarisk (saltcedar), an invasive tree that monopolizes water reserves in this already water-scarce environment. In Australia, the blackberry is an invasive shrub that turns whole areas of forest into thorny thickets; the prickly pear cactus overran thousands of square miles of farmland. While new invasive species would obviously not be introduced, neither would the currently present ones be controlled. Kudzu would cover the American South once more, blackberry and prickly pear and rabbits would make great inroads into the ecosystem of Australia (more than they have already done, that is).
Although such invasive species don't seem as threatening or unnatural as the genetically spliced wolvogs, rakunks, pigoons, or Crakers of Margaret Atwood's Oryx and Crake, it would certainly mark a continued influence of humanity on the post-human world. And while a new, "natural" equilibrium would certainly be reached eventually, the process would probably take significant evolutionary time. It would not happen on the timescale of a human lifetime, nor even be completed in that of a human civilization. Furthermore, the new state would not necessarily be much like the pre-industrial world, and would be far less biodiverse (at least in the geological short term). Eventually, local herbivores would adapt to feed on invasive plants, and local pathogens would adapt to attack invasive species. At which point it's hard to argue that the invasive species hasn't become local itself. This process has already begun to occur—there is a fruit fly found only in eastern North America whose larvae feed on the berries of the Japanese honeysuckle, introduced to that region only in the 250 years—thus, the fly must be a recent development; it apparently originated as a hybrid of two other fruit fly species. In time, such evolution would incorporate formerly invasive species into the ecosystem. Nevertheless, species lost to the invasives' incursion would remain extinct, and it is still likely that the process would have the net effect of a more homogenized global ecosystem (as the successful invasives are likely many of the same species across continents—house sparrows and Norway rats, for example, are common on all inhabited continents).
An interesting case of a human-specific disaster which does not quite produce the classical "new Eden" appears in S. M. Stirling's Change novels (starting with Dies the Fire), in which a mysterious event causes explosives, engines, and electricity to cease to function, collapsing industrial civilization. Many areas go back to wilderness, as by a generation after the event, the Great Plains are largely prairie again and wolves have much expanded their range in the US; escaped exotic animals from zoos and private collections also become a part of the new American ecosystems. However, in the eastern United States, where urban areas were close together, the environment has become heavily devastated, since the collapse of food distribution led to refugees from the starving cities eating most of the available food sources before starving to death, and the moldering cities remain a major part of the landscape.
Of course, we're in the domain of speculation whenever we entertain post-apocalyptic scenarios, leaving a rather thin buffer between fact and fiction. Many of the metrics we use to assess the potential consequences are limited, whether by our instruments or by the severity and complexity of the scenarios outlined. Despite this, it behooves authors who seek to write more than simple allegory to consult the scientific literature as regards the post-apocalyptic worlds that they create. As many of these issues are taken seriously in contemporary scientific literature, as might be expected of politically relevant concerns, there's no dearth of studies and metrics designed to push our understanding a little bit further. It becomes the role of the speculative fiction author to put these disconnected scientific fragments into a narrative that encompasses the whole, with protagonists who can observe, prod, and be affected by the range of possibilities inherent in the genre. By taking the time to think through their worlds scientifically, authors might not just find striking images and new apocalyptic possibilities, but will be able to engage their readers interest more thoroughly in speculation that presses beyond the pages of fiction.
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