Earth in Human Hands

by David Grinspoon

  Planetary defense requires the kind of thinking we will need if our global civilization is going to survive the next couple of centuries. It’s a good arena for us to practice the art of paying attention for longer than any of us is alive. By aiding our transition to this longer-term mode of awareness and action, the challenge of planetary defense, even absent an approaching asteroid, can help us to survive the next century or two with our civilization intact and thriving.

  We’ve already begun the first phase of this: the astronomical surveys to know what Earth-crossing objects are out there. The next phase, building and operating a planetary defense system, should serve as one of many long-term goals of the world’s space programs in the twenty-first century. There are many important reasons, both pragmatic and lofty, to maintain a space program, but this project surely provides a good motivation. However, it’s not enough merely to solve the technical aspects, to learn to build the hardware for an asteroid defense system. We also need to plan for long-term, multigenerational continuity of our operational space capacities. Currently we have almost the opposite approach. Every Mars rover we build must be an entirely new design, even if building five more of an earlier design would make for good and cheaper exploration. We are so focused on innovation that we don’t think in terms of stability and continuity. Yet it doesn’t make sense to build an asteroid defense system meant to operate for only ten or twenty years. The technical puzzles occupy our attention now, but they will be the easy part. As with so many other global problems, the biggest challenges of planetary defense ultimately will not be technological but organizational, political, and cultural. We’ll need to call upon them someday, for an urgent mission to save our civilization and possibly even our biosphere, so our space systems and the organizations that operate them must be built to last.

  Space defense is ethically simpler than most other proposed or actual large-scale technological interventions with the planet. Here, human interests and the interests of the rest of the biosphere have obvious and large overlap.* An asteroid of sufficient size will ruin everyone’s day. Doesn’t it seem, then, that we have a clear moral obligation to the biosphere to pursue such a project?

  Why? Well, how do you feel about mass extinction? When I’ve described past catastrophic die-offs, the disappearance of millions of species due to planetary changes of the first and second kind, do these stories seem sad to you? Are these tragedies? Or simply “nature taking its course” like a fuzzy bunny being devoured by a fleet fox in a nature documentary? Random, or “natural,” catastrophes, even huge ones, are much easier to deal with than those which are caused, and therefore could be averted, by us. Because we have at least the possibility of foresight and avoidance, we see the mass extinction that we are currently starting as particularly tragic, and feel horrible responsibility for it.

  Now, in this context, think about the morality of not just past and current extinctions but future ones. If we get through this next century with a stabilized population and a sustainable global energy system, then it may be time for payback. What could we ever do for Earth that would possibly help atone for the damage we are doing now?

  Maybe humanity can build a planetary defense system to start to make amends for the extinction we are currently causing. As long as we, or our descendants, are on the case, Earth’s biosphere never has to be decimated by another giant impact. Maybe this could be some kind of long-term compensation to the biosphere. First we need to get a handle on the killer asteroid that is our own reckless behavior. Then, in the future, we may well be able to prevent the next mass extinction.

  Climate Control

  If we’re going to have a long future on this world, there is more that we’ll have to do in the way of active interference with previously natural processes. Once we get over our current inadvertent climate vandalism, we could thoughtfully intervene against harmful climate swings. If we want our future here to be as long as our history has been, at some point we’ll need to prevent future ice ages and episodes of natural global warming.

  As city builders and history keepers, we’ve been around long enough for the climate to change, but it really hasn’t—not much. We’ve been lucky. The current Holocene interglacial period has lasted about twelve thousand years, since the end of the Pleistocene. All of human civilization has developed in this stretch of relatively warm and stable climate. A few times, as a result of volcanic eruptions or changes in the Sun, we’ve experienced some small climate fluctuations, resulting in a few years of extreme cold or regional drought. These have caused rivers to freeze over in what we thought were temperate zones, and once-fertile valleys to turn to dust, hinting at what climate change can do. Yet, going back to before the Agricultural Revolution, ten thousand or so years ago, and certainly over recorded history, no human has ever experienced the larger swings in climate that are routine on Earth over millions of years. We’ve been lulled into an illusion of stasis by unusual climate stability during our short time here.

  If you include the rest of our history as a species (most of it), before we started keeping continuous track of ourselves, you’ll find the story is different. Over longer timescales, Earth’s climate has gone through large swings and, left to its own devices, will continue to do so. Blame it on Jupiter.

  Climate cycles on Earth are, in large part, a consequence of its existence as one planet in a solar system of many. Graphed over large stretches of Earth time, the complex warming and cooling oscillation of climate reveals polyrhythmic patterns. The major beats occur at intervals of 23,000, 41,000, and 100,000 years. We call these the Milanković cycles, after Milutin Milanković, the Serbian astronomer and mathematician who is considered one of the founders of planetary climatology. In 1916, Milanković published climate calculations of the surface temperatures on Mars, Venus, Mercury, and the Moon, some of which he had worked out while a prisoner of war during World War I. Yet he is mostly remembered for figuring out the relationship between planetary motions and climate change on Earth. Over hundreds of thousands of years, as first determined by Milanković, little gravitational nudges resulting from the motions of Jupiter, Saturn, and the Moon cause subtle but important wobbles in the orbit and spin of Earth. These induce slight changes in the seasonal distribution of sunlight, causing our planet to sway rhythmically between ice age and hothouse conditions. When Milanković died in 1958, his theory was not taken seriously. It took a while for our data to become good enough to prove him right.

  Milanković’s theory of the astronomical forcing of Earth’s ice ages was resurrected and vindicated in the late 1970s, when ice cores first allowed us to reconstruct, in detail, the climate history of the last 450,000 years. A pattern jumped out, showing superimposed climate fluctuations, with pulses of 23,000, 41,000, and 100,000 years, neatly confirming Milanković’s predictions (or retrodictions, that is, predictions of the as-yet-unknown past). More recently we have learned that some other planets in the solar system experience similar cycles—not surprising, as they travel the same spaceways of mutually interacting gravitational influences. Earth’s rotational axis stays at a nearly constant twenty-three-and-a-half-degree tilt from the Sun, thanks to the steadying gravitational hand of our big moon. Compared to this, the Martian spin axis bobs up and down like a dreidel. Over a period of 120,000 years, the tilt of Mars varies from fifteen to thirty-five degrees.* Currently it sits about halfway between these extremes. The amount of solar energy at the poles is twice as high at the maximum tilt as at the minimum. As a result, the polar terrains of Mars are exquisitely marbled with interlaced layers of ice and dust recording complex climate fluctuations on the Red (and sometimes partially white) Planet. Titan undergoes a sixty-thousand-year climate cycle driven by its orbit around Saturn. The planets are all tugging on one another and rhythmically torqueing one another’s climates.

  We know that the timing of Earth’s ice ages is determined by this interplanetary contra dance, but we’re not exactly sure why. There is still plenty of mystery about how the
very slight and subtle orbital changes in Earth’s seasonal illumination translate into dramatic climate variations. Some amplifying feedback mechanisms seem to be responsible, but these are still being worked out and debated. Yet the pattern in the climate data bears an unmistakable orbital stamp.† Despite the claims of astrologers, there is no evidence that the position of the planets at your time of birth affects your personality and fate. However, life on Earth and human evolution have been profoundly influenced by the motions of the planets over the ages as climate has danced to the music of the spheres.

  Extreme climate change is like the impact danger in that it has not hit within the recorded memory of our civilization. We modern humans have had to infer its existence through the detailed scientific study of Earth and the rest of the solar system. Going back farther, however, we see that it has definitely shaped who we are. When we were hominids but not yet humans, we experienced extreme climate change. As I’ll discuss in chapter 8, larger climate changes were extreme challenges to survival that altered human evolution. Perhaps some old stories and legends passed down by indigenous peoples refer to such cataclysmic events. In our prehistory as hunter-gatherers, we were better able to cope with minor climate change than we would be today. When we were a nomadic species, if a food or water source dried up, we could migrate to happier hunting grounds. Moving from place to place in response to shifting conditions, following the food and the water, was part of our modus operandi during ordinary times, so we were able to respond more adroitly when the world changed. Once we abandoned this lifestyle for stationary settlements with planted fields, we became much more vulnerable to changes in environment. As we built villages and city-states, and tied ourselves down to specific places, a blight or long-term drought could present a much more serious problem. Eventually we became even less resilient, dependent as we now are on global agriculture and a global economy that cannot as easily tolerate a change in conditions.

  If we left Earth to its own devices for long enough, it would eventually enter another ice age. This would be much more extreme than the kinds of climate changes we are potentially facing now. During the last ice age, up until about thirteen thousand years ago, an unbroken sheet of ice two or three miles thick extended from the poles down to the latitude of Minneapolis. Sea level dropped by around three hundred feet, completely redrawing coastlines and the paths of rivers around the world. There is no way that a civilization of many billion humans, tied to cities and dependent upon a global system of agriculture and trade, could survive such a transition intact. We don’t ever want to try to live through one, and if we get our act together, we will never have to. We have at least twenty thousand years to work the problem, probably much longer. Transitions between ice ages and interglacials are times of accelerated extinction. So, if we ever prevent an ice age, we will also save a lot of other species: perhaps, someday, another opportunity for payback.

  Have we already injected enough CO2 into the air to delay or prevent the next ice age? Quite possibly. Even if we completely stopped spewing carbon next Thursday, it would take about one hundred thousand years for the natural carbon cycle to draw it back down to preindustrial levels. That’s probably long enough to delay the ending of the current interglacial and the onset of the next ice age. Yet this is not an easy thing to predict. Projecting climate on this hundred-thousand-year timescale is even harder than modeling the next hundred years (which, for obvious reasons is where most efforts have been focused).

  Since, as I’ve indicated, we don’t fully understand the mechanism by which the orbital Milanković forcing causes large climate swings, there is a lot of uncertainty here. The last warm interglacial before the one we’re in now, the Eemian, about one hundred twenty-five thousand years ago, lasted for about ten thousand years. This length is typical. Most of these warm periods persist for roughly the length that ours has already lasted. Yet the one we’re in now seems to be weird, and not just because of us. Some models predict that, without human interference, the ice would return within fifteen thousand or twenty thousand years. Others suggest that, due to Earth’s orbit currently being in a phase of low eccentricity—meaning that it is now nearly circular, compared to other epochs, where it is slightly more egg-shaped—we inhabit an interglacial of exceptionally long duration. Models incorporating this fact suggest that, even without us, the ice would not return for another fifty thousand years. In other words, if we don’t screw it up, sending climate careering beyond the safe zone, our luck might hold for quite some time. This warm, stable climate our civilization has enjoyed for ten millennia, and come to take for granted, might last for five times again as long. Yet what about looking farther into the future, beyond just the next ice age? Might we have initiated something more long term? Could we have seriously thrown Earth off its rhythm, perhaps even permanently halting the Milanković cycle of glaciations?

  My young colleague Jacob Haqq-Misra has been studying this question. Early results from his modeling suggest we may be on our way to initiating such a change in Earth’s behavior. Jacob is small and wiry, and a dynamo at pursuing his passions. He’s a research scientist modeling planetary climate, but he also plays percussion in a touring jam band, helps run an internship program for students interested in astrobiology, and has followed his interest in environmental ethics with the same intensity as his scientific studies. My kind of guy. As a grad student at Penn State, Jacob studied with Jim Kasting, a pillar in the field. Kasting, who was himself a postdoc at Ames with Jim Pollack a decade before me, is now a sort of guru of planetary climate modeling, in the way Pollack once was. He has trained an influential cadre to be experts in big-picture climate modeling. Many of his former students and postdocs are now turning their attention to new climate modeling challenges such as exoplanets, the future of Earth’s habitability, and (in Jacob’s case) the long-term climate impact of current human industry. Jacob has started looking at how the amount of CO2 we are projected to produce in both the most likely and worst-case scenarios will affect the long-term Milanković cycles of ice ages and interglacials.

  His models are somewhat simple—intentionally so. As I’ve discussed, part of the art of planetary climate modeling is choosing a model that is appropriate for the task at hand. As the French say, Pas besoin d’utiliser un marteau-pilon pour écraser une mouche, or “You don’t need a jackhammer to swat a fly.” If you are simply trying to evaluate whether a given effect is significant, then you just need a model you know is good enough to get the magnitude approximately right. With this work, Jacob was not claiming to make a detailed and correct prediction of climate changes over the next several hundred thousand years, only to determine whether our influence could be strong enough to interrupt the Milanković oscillations over the coming millennia. His first model results suggest that the answer is yes. It is quite likely that we are extending the current interglacial, delaying the onset of the next full-on ice age that would otherwise start in perhaps fifty thousand years. Beyond that, though, Jacob’s results suggest that no matter what we do from here, our climate influence will persist for much longer.

  Even if our carbon emissions are modeled as one giant burp, as an instantaneous pulse that then ceases, the increased CO2 abundance lingers in the air for hundreds of thousands of years due to the slow (to our sensibilities) operation of Earth’s climate cycle, which will eventually resorb our excess carbon into the soils, sediments, rocks, and mantle. After several hundred thousand years, the climate would largely return to “normal.” This is assuming we stop the Great Acceleration in our fossil fuel use pretty soon. That is actually a safe assumption on the long timescale of this exercise, because soon there won’t be any fossil fuel left.3 No matter what we do from here, Jacob’s results show, we are likely to have interfered with the next few Milanković glaciations, stopping the cycle of ice ages that has been operating for hundreds of millions of years.

  Would this be such a bad thing? As I’ve indicated, ice ages are not good for the biosphere. Okay, good i
s a loaded term. They cause extinctions. You might want to argue that extinctions are inevitable, like death and taxes, or even that some level of extinction is good, or at least normal, or that at the very least the biosphere has dealt with these deathly cold spells since time immemorial and come through just fine. From a certain vantage point, they are no more traumatic to the biosphere than are Earth’s annual seasons. Yet that is a point of view that accepts extinction as a part of the way things are.

  I’m not sure we will ever have that luxury again. It’s a strange thought, but there may never again be such a thing as “natural” extinction.

  In a way, we’ve been doing what all species do: altering our environment, forcing others to adapt, wiping out those who can’t. Nothing new about that. For our entire history, human beings have been causing brutal extinctions of other species, but here’s a hopeful thought: twenty-first-century humans could be the first ever to decide not to behave like this. From now on, or as long as humans (or thinking creatures descended from or created by us) are here on the planet, extinction will be a choice. Obviously the current carnage, the mass extinction that we are now starting to manifest, has to stop. Then what? What extinction rate would we prefer? Do we wish to eliminate it entirely? Then what of biological innovation? If we choose to eliminate all extinction, then this is equivalent to saying we are the gods in charge of all future species. We will either have to play god or allow species to go extinct (which, I suppose, may still be a form of playing god). Something to think about.