by Curt Stager
Through most of that history, atmospheric concentrations of these two greenhouse gases fluctuated in near lockstep with each other, but something odd happened during the warm Holocene epoch, which began with an abrupt end to the last major cold episode 11,700 years ago. After an early thermal peak, temperatures began to slide back down into a long-term cooling trend. However, about 8,000 years ago, the CO2 content of the air began to rise again instead of falling, as it had normally done during cool-offs of the distant past. Several millennia later, methane lifted off independently, too. Ruddiman proposes that the unusual CO2 rise reflected widespread forest burning and land clearance for agriculture, and that methane later rose in response to the spread of Asian rice production in artificial, gas-bubbling wetlands. In that case, human impacts on world climate might have begun as early as 8,000 years ago.
Still others argue that climatic effects should not be the only criteria for tracking the history of human impacts on Earth. Most biohistorians believe that Stone Age hunters exterminated mastodonts and giant ground sloths along with many other large mammals roughly 10,000 to 15,000 years ago, and their disappearance fundamentally and artificially altered ecosystems all over the planet. In North America alone, more than half of all mammal species weighing more than 70 pounds (32 kg) vanished, and those weighing more than a ton (900 kg) were completely wiped out. One could therefore make a logical case for omitting the Holocene epoch from the geologic time scale altogether and simply folding it into the Anthropocene.
But most of us are less interested in when the Anthropocene began than in what it’s going to be like from here on out. Just as fossils and ice cores give us glimpses into the world as it once was, the new science of long-term climate prediction sketches a compelling outline of things to come. In that expansive view, the basic shape of the future already exists, and we can use it to tell the full story of carbon pollution from start to finish rather than settling for the relatively short portion that now dominates our collective thinking. The pacing of most of these coming events will be sluggish on the scale of daily human experience, but their eventual cumulative effects on ecosystems and societies will be enormous and incredibly long-lasting.
And just what is it going to be like from here on out? We’ll have to wait for time itself to reveal the details of future political systems, technologies, social interactions, and lifestyles; one never really knows what Homo sapiens will do next. But many features of the physical world are far more predictable. This book offers an introduction to those aspects of long-term climatic and environmental change that stand most clearly before us on the horizon. Here is a sampler of what is to come.
We face a simple choice in the coming century or so; either we’ll switch to nonfossil fuels as soon as possible, or we’ll burn through our remaining reserves and then be forced to switch later on. In either case, greenhouse gas concentrations will probably peak some time before 2400 ad and then level off as our emissions decrease, either through purposely reduced consumption or fossil fuel shortages. The passing of the CO2 pollution peak will trigger a slow climate “whiplash” in which the global warming trend will top out and then flip to a long-term cooling recovery that eventually returns temperatures to those of the preindustrial eighteenth century. But that process will last for tens or even hundreds of thousands of years. The more fossil fuel that we end up burning, the higher the temperatures will rise and the longer the recovery will take.
There’s much more to CO2 pollution than climate change, though. Carbon dioxide will gradually acidify much or all of the oceans as they absorb tons of fossil fuel emissions from the air. That chemical disturbance threatens to weaken or even dissolve the shells of countless corals, mollusks, crustaceans, and many microorganisms, and their loss, in turn, will threaten other life-forms that interact with them. In some ways, this situation resembles the contamination of the primordial atmosphere by microbial marine oxygen, only in reverse; we are responding 2 billion years later with a corrosive gas of our own that is moving from the air back into the sea. Eventually, the neutralizing capacity of Earth’s rocks and soils will return the oceans to normal chemical conditions, but the acid-driven loss of marine biodiversity will be among the most unpredictable, potentially destructive, and irreversible effects of Anthropocene carbon pollution.
Before the end of this century, the Arctic Ocean will lose its sea ice in summer, and the open-water polar fisheries that develop in its absence will last for thousands of years, radically changing the face of the far north as well as the dynamics of international trade. But when CO2 concentrations eventually fall enough, the Arctic will freeze over again, destroying what will by then have become “normal” ice-free ecosystems, cultures, and economies.
Much or all of Greenland and Antarctica’s ice sheets will melt away over the course of many centuries, with the final extent of shrinkage dependent upon how much greenhouse gas we emit in the near future. As the edges of today’s icy coverings draw back from the coasts, newly exposed landscapes and waterways will open up for settlement, agriculture, fishery exploitation, and mining.
Sea level will continue to rise long after the CO2 and temperature peaks pass. The change will be too slow for people to observe directly, but over time it will progressively inundate thickly settled coastal regions. Then a long, gradual global cooling recovery will begin to haul the waters back from the land. But that initial retreat will be incomplete, because so much land-based ice will have melted and drained into the oceans. At some time in the deep future, the sea surface will come to rest as much as 230 feet (70 m) above today’s level, having been trapped at a new set point that reflects the intensity and duration of the melting. Only after many additional millennia of cooling and glacial reconstruction will the oceans reposition themselves close to where they lie now.
We have prevented the next ice age. The ebb and flow of natural climatic cycles suggests that we should be due for another glaciation in about 50,000 years. Or rather, we used to be. Thanks to the longevity of our greenhouse gas pollution, the next major freeze-up won’t arrive until our lingering carbon vapors thin out enough, perhaps 130,000 years from now, and possibly much later. The sustained influence of our actions today on the immensely distant future adds an important new component to the ethics of carbon pollution. If we consider only the next few centuries in isolation, then human-driven climate change may be mostly negative. But what if we look ahead to the rest of the story? On the scales of environmental justice, how do several centuries of imminent and decidedly unwelcome change stack up against many future millennia that could be rescued from ice age devastation?
These are the sorts of extraordinary things that you’ll encounter in this book, but rest assured that it’s not just a litany of gloom. I hope instead to leave you with a well-founded sense of hope and a wake-up call. You and I are living in a pivotal moment of history, what some have called a “carbon crisis”—a crucial and decisive turning point in which our thoughts and actions are of unusually great importance for the long-term future of the world. But all is not yet lost, and climate change is not on the list of deadly dangers to most humans; as I will explain later, Homo sapiens will almost certainly be here to experience the environmental effects of the Anthropocene from start to finish. And that’s only fitting, seeing as we’re the ones who launched this new epoch in the first place.
But why, then, should we care enough about the distant future even to finish reading about it on these pages? The reason is simple. Although humans will survive as a species, we are faced today with the responsibility of determining the climatic future that our descendants will live in. It may well be a struggle to hold our carbon pollution to a minimum, but failing to take the heroic path and control our collective behavior is likely to drag us and our descendants into a realm of extreme warming, sea-level rise, and ocean acidification the likes of which haven’t been seen on Earth for millions of years. And the outlook for most nonhumans is far more worrisome than it is for our own kind. Sever
e environmental changes have happened before, even without our influence in the mix, but the situation that we and our fellow species now face is unique in the history of this ancient planet.
So welcome to this glimpse of our deep future. Welcome to the Anthropocene.
1
Stopping the Ice
One can only hope that the expected extremes of the
Anthropocene will not lead to conditions that cross the
threshold to glaciation.
—Frank Sirocko, paleoclimatologist
Shockingly long-term climatic changes await us as a result of modern human activity, but examining our effects on the deep future also raises a related question that is well worth considering: what would global climates have been like if we had left our fossil fuels in the ground rather than burning them?
In that alternative reality our descendants would still fret about climate, sea levels, and ice caps but the news would read quite differently from that of today. “There’s a massive, destructive climatic change coming, but scientists say that we can stop it if we take appropriate action now. If we go about business as usual, coastal settlements will be destroyed by sea-level shifts and entire nations will be covered with water. Frozen water. But there’s still hope. If we simply burn enough fossil fuels, we’ll warm the atmosphere enough to delay that icy disaster for thousands of years.”
I’m talking about the next ice age. When a paleoecologist like myself thinks about global climate change the exercise is as likely to involve visions of ice-sheet invasions as it is to include greenhouse warming. We still don’t know exactly why continent-sized glaciations come and go as they do, but they clearly have a rhythmic quality to them. Natural cyclic pulses take the long line of temperature history and snap it like a whip, looping it into a series of steep coolings and warmings. When viewed from a long-term perspective, major warmings of the past 2 to 3 million years can seem like brief thermal respites when the world came up for air between long icy dives; that’s why we call them “interglacials” rather than something that sounds more normal or permanent. The cyclic pattern also suggests that more ice ages await us in the future, so strongly in fact that climate scientists routinely refer to our own postglacial warm phase that we live in today as “the present interglacial.” Because of this admittedly unusual perspective, many of the paleoecologists I know balance their concerns about modern climate change with “yes, but it could also be a lot worse.”
Although such views are rare outside of narrow academic circles, I believe that they belong in the mainstream. Time perspectives long enough to include ice age prevention are not just the stuff of mind games but potentially important aspects of rational planning for our climatic future. In order to appreciate why this is so, however, it helps to look more deeply than usual into the nature of ice ages.
The last one began about 117,000 years ago and ended 11,700 years ago. During that long and terrible reign of cold, roughly a fifth of the world’s land surface resembled the icy interiors of Greenland and Antarctica today, especially in the higher northern latitudes. Most of what is now Canada and northern Europe was smothered under immense sheets of slowly creeping ice up to 2 miles (3 km) thick. The sites of today’s Chicago, Boston, and New York were obliterated, and what we now call Long Island is a plowed-up bow wave of detritus that marks the southern limit of the last major ice advance. Entire landscapes sagged under that tremendous weight, pressing down hundreds or even thousands of feet into the planet’s softer innards, and the gritty underbelly of the ice gouged deep scratches and grooves into solid bedrock that still scar the formerly glaciated regions of the world.
When you see glacial deposits and ice-scoured rock formations along a northerly roadside or trail, it’s easy to let your imagination strip away the towns and trees and crush your surroundings under great, grinding slabs of ice. I envision it quite often near my home in the Adirondack Mountains of upstate New York. Recently I was reminded of the frozen past when I stepped off a woodland path near Saint Regis Mountain to take a closer look at one of the largest glacial erratic boulders I’ve ever seen.
The massive chunk of gray anorthosite was broader and taller than my house, and the prying fingers of winter frost had plucked garage-sized flakes away from its lichen-crusted flanks. They lay in low heaps around the central body of rock like cast-off clothing. The base of the giant perched just high enough above the ground to leave shadowed crawl spaces below that made me think of of crouching hermits and cave bears. Peering into one of them I scanned its dusky floor for signs of residents but saw only earth-colored gravel. Clean, well-sorted, smoothly rounded gravel, just like the stuff in the shallow streambed nearby. Gravel that was never buried under forest soils or leaf litter and that still looked as fresh as it did when melting ice dropped this gigantic sheltering rock on top of it.
That primeval scene drew my imagination back to when these mountains were still emerging from their long, lightless imprisonment. The rustling beech, maples, and birches before me faded away, along with the duff and dirt beneath them, exposing a desolate brown wasteland of wet sand and pebbles that glistened under a cold clear sky. Not a tree in sight, not a shrub or flower, not even many lichens on the virgin boulders yet. Cloudy silt-laden streams and molten blue pools sparkled in the low spots, and remnant hill-sized blocks of decomposing ice hunkered down in the deeper hollows, sloughing off layers of dusty surface debris like old dogs shedding winter fur. Far off on the northern horizon lay an unfamiliar range of white, mile-high hills, the sun-scored southern face of the melting ice sheet. The vision lasted only a few moments, but a strong feeling of connection to long-ago times when Big Ice ruled this landscape stayed with me through the rest of my hike that day.
The author examining a glacial boulder near Saint Regis Mountain in the Adirondack Park, upstate New York. Kary Johnson
Let’s continue with this imagination game. What if it happened again?
Here and now in the Adirondacks we worry, with good reason, about the effects of acid rain, invasive species, and global warming on our local ecosystems. But those problems won’t exterminate every last Adirondack fish and fowl, and even the most extreme case of Anthropocene heating would still leave the land covered with some sort of greenery, if not all the kinds we’re currently used to.
A full glacial advance, on the other hand, is a total wipeout. Every lake is bulldozed or smothered under a thick blanket of cobbles, sand, and gravel. Every sugar maple, every golden-tinted trout lily, every tuft of moss heaves up in bow waves of dirt and stones and is crushed to pulp. Every animal with legs or wings flees southward. The Adirondack peaks vanish under a heavy white tide, the iconic ski jumps at Lake Placid topple and are ground to splinters, and every settlement from Saranac Lake to Old Forge is obliterated.
Meanwhile, farther north, most of Canada disappears. That includes Quebec City, Montreal, Ottawa, Toronto, Winnipeg, Calgary, and Vancouver, not to mention every wild area from Hudson Bay to Banff. From a human perspective, there’s no place called Canada for tens of thousands of years except in the same sense that a gigantic frosty slab called Antarctica now squats on the South Pole. And out across the Atlantic, advancing walls of white demolish Dublin, Liverpool, Oslo, Stockholm, Copenhagen, Helsinki, and Saint Petersburg, and every settlement on the rocky coastal rind of Greenland is shoveled into the sea by heavy spatulas of ice.
With much of the world’s freshwater imprisoned in frozen form on the continents, sea level falls by as much as 400 vertical feet (120 m). The site of every twenty-first-century port is stranded far inland, the long, slender thumb of Florida doubles in width, and the present location of every shallow-water coral reef in the tropics sprouts weeds and trees. The associated cooling weakens monsoons, locking much of Africa and southern Asia into chronic droughts.
This is what a climate historian is likely to have in mind when discussing climatic change. Compare it to what most experts expect modern warming to bring us in the Anthropocene future and you’
ll understand why a paleoecologist’s panic button might not be so easily pressed.
But wait. Isn’t global warming supposed to trigger the next ice age? Isn’t that what we saw happen in the apocalyptic enviro-thriller movie The Day After Tomorrow, in which the greenhouse effect suddenly shuts down climatically important ocean currents in the North Atlantic and triggers a superglaciation?
The movie isn’t totally wrong, in that the warm Gulf Stream really does help to keep northwestern Europe from becoming cooler than it already is. It’s part of a huge global conveyor belt system of interconnected currents that draws solar-heated tropical water into the cold surface of the North Atlantic, where it cools off and then sinks for a deep return journey southward. Some scientists worry that future climatic changes could disrupt that conveyor and trigger a sudden regional cooling; hence the movie scene in which a fierce wind seizes Manhattan with remorseless fangs of frost. But as gripping as that storyline is, serious questions remain about the real role of the conveyor in past and future climate change.
The engine driving the conveyor goes by several dry technical names, most recently the meridional overturning circulation, or MOC. It is also sometimes called THC, an abbreviation that is in no way connected to marijuana smoking (and tetrahydrocannabinol) but rather, reflects the upgrading of a simpler concept, that of thermohaline circulation, whose basic premise is that changes in temperature and saltiness drive major circulation currents of the oceans.