by Curt Stager
The list of true threats to human existence is remarkably short, and most of the items that I’ve heard people put on that list are easy to discount. Industrial toxins are too diffuse or localized to drive us all to extinction. Volcanoes are out, too; even the most violent eruption in the history of humankind, the Toba supervolcano that exploded in Sumatra nearly 75,000 years ago, didn’t kill everybody. And intense gamma ray bursts from dying stars or newly formed black holes are too rare to represent a credible threat to our species; even if such a burst did strike us, the broad hulk of Earth would shield the residents on the other side, so the damage wouldn’t be total.
What about pestilence? One of the worst microbial killers of all time, the Black Death, killed between 75 and 200 million people during the fourteenth century, including nearly half of all Chinese, at least a third of all Europeans and Middle Easterners, and perhaps an eighth of all Africans. Nonetheless, it wasn’t even close to the End of Days. Most Chinese, Europeans, Middle Easterners, and Africans survived, even in the absence of modern treatment, which today would drop the mortality rate to 5 or 10 percent.
What helps humankind to resist total extinction by disease? It’s a process that many people don’t even believe in: evolution by natural selection. Given the large numbers of humans in the world and the ubiquity and diversity of genetic mutations, there will probably always be some individuals among us who have built-in immunity to any given microbial disease. The more of us there are, the greater the genetic diversity and the greater the chances of someone being naturally immunized against infection. This is how life adapted to, and eventually became dependent upon, global oxygen pollution during the last 2 billion years, and even the worst pandemics can be seen as natural-selection events that favor the survival of resistant variants.
Will we wipe each other out with a thermonuclear war, thereby avoiding the trials of future climate change? Even more destructive than the fireballs and radiation would be the dust and smoke-induced nuclear winter that would leave a miserable remainder of humanity grubbing for sustenance. But this planet is huge, there are billions of highly resourceful humans living on it, and radioactive fallout spreads unevenly on the winds and sinks away under soils and ocean muds. Only a full-on, suicidal conflagration among multiple superpowers could kill every last one of us, and although it is an unpleasant possibility, I believe (with fingers crossed) that it is unlikely.
And finally, we do have a potential asteroid problem in this rock-filled solar system of ours. The lightning-fast chunk of space debris that blasted a 110-mile-wide (180 km) crater into Mexico’s Yucatan Peninsula 65 million years ago might have decimated the dinosaurs. Steam and dust from the impact choked the atmosphere, and the heat of vaporizing rock ignited wildfires over thousands of square miles. But it would take a much more intense collision than that one to kill us all. The impactor would probably have to be large and fast enough to melt much of Earth’s surface or break the whole planet into pieces. For those self-tormenting readers who might actually enjoy wallowing in fearful anticipation of asteroid disaster, it’s worth experimenting with the “catastrophe calculator” that is posted online by the University of Arizona’s Impact Effects Program. The website lets you enter information about the size, speed, density, and angle of the flying object as well as the nature of the target and how far away you happen to be from the impact site. With the click of a button, the calculator produces detailed and darkly fascinating descriptions of what happens next.
Unable to resist the temptation myself, I recently found that if an asteroid 1,000 feet wide (ca. 300 m) hits the ground thirty miles away from my home at 10 to 11 miles (17 km) per second, it will blast a hole 3.5 miles (5.6 km) across and more than a mile deep into the solid rock. A fireball twenty times brighter than the sun will fill the sky and ignite the forest around me, and an earthquake measuring 6.9 on the Richter scale will shake the ground. Then, a minute and a half later, jagged chunks of stone the size of my head will begin to crash down around me. Half a minute later still, a raging blast of hot wind will flatten the burning trees, my house, and me with a deafening roar. Not a total planetary wipeout, by any means, but it would certainly distract an observer from global warming for a while. To melt the whole surface of the planet, the projectile has to be at least 4,350 miles (7,000 km) across; anything much larger will shatter Earth into a spray of new asteroids. But the calculator puts a damper on the shock value of that result by noting that rogue impactors of this magnitude don’t exist in our sector of the galaxy.
This review of options for a future apocalypse raises another important point worth keeping in mind as we gauge the severity of risks we face from our two carbon emissions scenarios. The environmental changes that may accompany them are too important to ignore, but they’re not worthy of utter panic or despair, either. If asteroids or other truly serious threats aren’t likely to destroy the human race within the next 100,000 years, then the greenhouse effect certainly isn’t going to either. In order for it to wipe us all out, it would have to overwhelm more than 6 billion members of an incredibly resilient species that has thrived for tens of thousands of years in every imaginable habitat from frigid pole to torrid desert, even without modern technology and despite abrupt swings between glacial and interglacial conditions. Show me how this is likely to happen, and I’ll change my mind.
In fact, there is a way in which CO2 really can kill people, and I’ve seen some of its horrible effects firsthand. But it has nothing to do with global warming.
In 1985, I was a postdoctoral member of a sediment-coring expedition to Cameroon, West Africa, with my former graduate adviser at Duke University, Dan Livingstone. With the help of aquatic ecologist George Kling, who was then a graduate student of Dan’s, we collected long cores from the thickly jungled crater of Lake Barombi Mbo, a mile-wide bowl of remarkably clear water an hour’s drive inland from the sea, which were later used to develop pollen records of local rain forest history. After the work there was finished, George and I headed into the cooler, grassier highlands of central Cameroon to study other more remote sites. One of those was a lovely crater lake called Nyos.
Lake Nyos lay far up a grass-carpeted slope from the deeply rutted dirt road that rings the central highlands, too far for us to lug our heavy sampling gear with ease. This would be more of a scouting trip than a sampling mission. A kindly fellow named Mr. Joseph met us in Nyos village on the lush, fertile valley floor below the lake, and he assigned one of his sons to guide us along the unmarked trail. It was a hot, sunny day, and the lake sparkled at our feet as we rested on the shore and looked out across the smooth blue surface to steep gray cliffs on the far side, about half a mile away. To us, it was a beautiful setting for a relaxing interlude in our sampling schedule.
But if we had been able to bring enough equipment with us to perform the usual analyses from our inflatable raft, we would have discovered several surprising things about Lake Nyos. Though we didn’t know it at the time, it is Cameroon’s deepest lake; more than 650 feet (ca. 200 m) at the center. And that ample reservoir was supercharged with a natural store of dissolved CO2 that entered in the form of geologically carbonated groundwater. A little more than a year afterward, on August 21, 1986, a billion cubic yards (a cubic kilometer) of that gas burst forth and killed 1,700 people in the valley below. Among the victims were Mr. Joseph and his family.
As I later described in the September 1987 issue of National Geographic, the timing couldn’t have been worse. It was the close of market day in Nyos village, and hundreds of people had come in from surrounding villages to buy and sell housewares and garden produce. Night had just fallen, and many had gathered indoors for dinner and sleep.
Hadari, a herdsman who had watched in stunned silence from a nearby hillside as the gas erupted from the lake that night, later described it to me. “It rose up like a white cloud, and then it sank into the valley like a flood,” he said. Carbon dioxide is heavier than air, so it poured down on the low-lying villages like wat
er, drowning them in a river of fumes 160 feet (50 m) deep and up to 10 miles (ca. 16 km) long. People in Nyos village suddenly fell away from their crowded dinner tables, struggled for breath, and died on their floors. Others died in their beds or on their doorsteps in the stifling darkness.
The carcasses of 5,000 cattle littered the grassy slopes, and many were still there when I helicoptered over them with photographer Anthony Suau about a month later. Even the vultures and flies were dead, “even small ants,” Hadari said. But the grasses and trees were untouched, still as luxuriantly green as ever. To plants, CO2 is the life-giving essence of air: they breathe it and thrive. To animals, ourselves included, it’s a waste product to be exhaled, and it’s toxic in high doses. Biomedical labs sometimes euthanize study mice by placing them in a closed container with dry ice in hopes of sending their subjects gently and humanely into permanent sleep, but it’s actually not a very nice way to go. High concentrations of CO2 induce sensory hallucinations, including painful burning sensations that were reported by Nyos survivors, then convulsions and death.
I will always regret having taken the easy route to Lake Nyos on that lovely day in 1985; if we had gone to the trouble of studying the lake properly, we could have warned people of the danger. Instead, survivors later found our names inscribed in Mr. Joseph’s guest book and assumed that George and I, being the only foreign scientists in the area before the eruption, had planted a bomb in the lake. We might as well have, for all the good we did for the residents of Nyos village.
And I will always think back on that awful event to remind myself of what a real CO2 catastrophe is like. Next to that, the greenhouse effect just doesn’t compare. If we end up emitting 5,000 Gtons of carbon by burning through our coal reserves, then we’ll likely trigger an intense and long-lived warm period that would be as severe as a realistic greenhouse future gets. And we should do our best to prevent it, not because it will exterminate us but because our descendants and other living things will actually have to endure it if we let it happen.
How might modern ecosystems and species respond to such a planetary fever? Nobody knows for certain, but we can make some reasonable guesses, and in the next two chapters we’ll look to the distant past for glimpses of relatively moderate to supergreenhouse conditions that are revealed in geological records. Hopefully, our grandchildren’s grandchildren will never have to face a super-greenhouse anyway. When I last spoke with Archer, he treated it like a thoroughly preventable problem. “We simply can’t let it happen,” he said. “My personal guess is that we’ll probably end up releasing about 1,600 gigatons. In that case, we’ll stabilize carbon dioxide levels close to 600 parts per million and, hopefully, avoid some of the very worst impacts.”
But whichever path we choose to take into the Anthropocene future, it’s now clear that we have already locked ourselves and our world into some uncomfortably large changes. If you’re a hard-core fatalist, you might use such points to argue for giving up and doing nothing. That, however, would be a mistake.
Paleobotanist Steve Jackson recently wrote in an editorial for Frontiers in Ecology and the Environment, “Climate change may sometimes be inevitable, but that is not a good reason to invite or accelerate it.” What we’ve already done to the global climate system will have surprisingly far-reaching consequences, but those effects are much less extreme and long-lasting than what could happen if we don’t reduce our carbon consumption as much and as rapidly as possible. The path to a grim 5,000-Gton scenario lies before us, but we’re not inescapably committed to following it just yet.
Our very existence at this pivotal moment in history gives us the amazing ability—some might say the honor—to set the world’s thermostat for hundreds of thousands of years, and we have already bequeathed a complex climatic legacy to a long line of future generations. Whether we like it or not, our behavior during this century will determine the magnitude and longevity of that inheritance.
3
The Last Great Thaw
Is there anything whereof it may be said, “See,
this is new?” It hath been already of old time,
which was before us.
—Ecclesiastes 1:10
When we look beyond 2100 AD to a deeper future, what we see there will depend on what happens on this side of that boundary line. If we limit our carbon emissions to 1,000 Gtons, then we’ll avoid the more extreme environmental changes that could accompany a 5,000-Gton scenario, but our descendants will still breathe air that’s richer in CO2 than any inhaled in the history of our species, and global average temperatures may still climb 3 to 5°F (2 to 3°C) on the way up to thermal maximum.
But how realistic are these scenarios? Has anything like the warming from a 1,000-Gton release ever happened before? And if so, how did landscapes and living things respond?
Computer models show us what can happen to climates in theoretical simulations, but geohistorical studies show what actually has happened in the past. Both approaches, the abstract and the concrete, have their strengths and limitations. Models can be constructed to simulate an infinite range of possibilities, but they may or may not be realistic. Paleoecological records are grounded in reality, but they can’t necessarily be manipulated to answer specific questions; whatever information they happen to offer is all you get. Both are most powerful when used in tandem, with history guiding the models. After seeing what CLIMBER and other models do to a virtual Earth when carbon emissions rise by “moderate” amounts, it’s also interesting to look for an example of a relatively moderate warming event that occurred in the past. Together they can sketch a compelling summary of how a version of our current warming trend has played out in a real-world setting and what it can and cannot tell us about the future.
Large-scale warmings have come and gone many times before this, so we have a long list of possible tales from which to choose. However, the biological backdrops of those events stray more and more widely from those of today the deeper we look into the past. Many of them happened before the continents reached their current positions and before most species looked as they do now. But as we probe the deep future, we want to know what living with 550 to 600 ppm of CO2 might be like, and we want it in the context of a physical world that closely resembles the one we know today.
That constraint leaves us with a much shortened list of possible paleo examples to choose from. If we’re going to limit our search to times when most of today’s plants and animals shared the planet with us, then we can’t look more than a few hundred thousand years back through evolutionary time. Fortunately, more information exists for younger events than for older ones because erosion and other processes have had less time in which to erase the traces they’ve left behind. Although nothing within the last million years perfectly matches today’s 387 ppm CO2 context, much less the 550 to 600 ppm condition that we’re considering in our moderate scenario, there is still much of value to learn from previous warmings. And this choice of a geologically recent time frame also allows us to take advantage of ice cores, the frozen archives of ancient air samples that lie buried in glaciers and continental ice sheets.
The longest such records come from the two largest ice realms on the planet: Greenland and Antarctica. These records are unique in the extreme length and high quality of their layered sequences, and the great thicknesses of those deposits attest to their age; many thousands of annual snowfalls are compressed into those icy stacks. The strata that lie beneath the highest gently rounded domes are our primary targets of study, not just because they are so numerous but also because they move little in comparison to marginal ice, which flows and churns like a slow-motion river under the pull of gravity. Such stability reduces the risk of errors from physical distortions.
At the time of this writing, the longest of all ice core records was produced by the European Project for Ice Coring in Antarctica (EPICA), a multinational effort involving scores of investigators. The core was drilled through 2 vertical miles (3 km) of ice on East Antarctica�
�s Dome C and it covers 800,000 years, long enough to encompass eight cold-glacial, warm-interglacial cycles. The most recent of those cycles are also registered in a 420,000-year-long record from the nearby Vostok station as well as in cores from central Greenland. This similarity of signals among disparate locales lends support to the climatic tales they tell.
Micropockets of ancient air trapped in the cores show that CO2 concentrations were lowest during cool ice ages, generally hovering near 190 ppm; methane, a rarer but more powerful greenhouse gas, averaged 0.4 ppm then. The highest of the warm spikes in the record barely scraped 300 ppm CO2, and its methane levels maxed out between 0.7 and 0.8 ppm. To put our modern times into historical perspective, remember that CO2 concentrations are now 387 ppm and rising, and methane is currently at 1.8 ppm. And even our moderate-emissions scenario is expected to raise those concentrations far beyond anything that the longest, oldest ice core records can tell us about.
Most of the warmings between ice ages also differed from that of today because their primary trigger wasn’t the greenhouse effect but changes in seasonal solar heating in the high northern latitudes. Greenhouse gas concentrations did rise during past interglacials, but it was mostly because higher temperatures sped up microbial decay rates and reduced the solubility of gases in the oceans. In our case, we’ve flipped the arrows of cause and effect by boosting the gas concentrations first. If enough ice remains in the distant future for scientists to read the layers set down during our times, they’ll see an unusual break in the long-standing relationships between global temperature and atmospheric composition. In the Anthropocene zones of their ice cores, rising greenhouse gas concentrations will coincide with or slightly precede, rather than follow, the warming trend.