Earth Interventions
When Pollack and Sagan turned from icy Mars and Titan to red-hot Venus, they considered how we might shut down the massive greenhouse, or mute it, leaving just enough warming to make a planet friendly for life and possibly even humans.
The solutions fall into one of two categories: find a way to block enough sunlight or get rid of a lot of CO2. The first could be done either through injecting some kind of absorbing dust in the atmosphere (perhaps by grinding up a captured asteroid) or putting giant mirrors or sunshades in space. The second could be done by engineering some kind of microbes that spread and multiply in the clouds, metabolizing CO2 into organic carbon. Or you could remove CO2 by liquefying and sequestering large amounts at or under the surface, or by circulating hot water through the ground (once you’ve made it cool enough, through one of these other mechanisms, for liquid water to exist) to induce “weathering” chemical reactions that would pull CO2 out of the atmosphere and bind it into carbonate rocks, just as in Earth’s carbon cycle. Yet with Venus you have the additional problem that there is basically no water, beyond a trace amount in the clouds. If you really wanted to create a surface biosphere on Venus someday, you would have to import a lot of water, compensating for the oceans the planet lost, in its wild youth, to the runaway greenhouse catastrophe. Where would we ever find enough water to return oceans to Venus? Our solar system has plenty of water, out in its fringes, frozen in billions of comets. Someday, when we’ve learned how to move comets and asteroids around, the rewatering of Venus could be feasible.5 Pollack and Sagan pointed out that with the same technology we would need to build an asteroid defense system, modifying at will the orbits of potentially dangerous objects, we could also eventually move spare objects around the solar system if we wanted to use them for terraforming.
Finally, they turned their attention toward the home world, beginning by stating that “an important special case for planetary engineering is to reverse significant perturbations (e.g. global warming) in the environment of our own planet. This is surely easier than planetary engineering on other worlds.”
It is not a coincidence that the two major strategies discussed for terraforming Venus resemble more extreme versions of the same two methods now under discussion for possible geoengineering of Earth: solar radiation management (SRM) and carbon dioxide removal (CDR). Venus stands, once again, as a caricature of our home planet’s environmental processes and challenges. To return that planet to habitable conditions, we would need to block enough sunlight or remove enough CO2 and other greenhouse gases to turn down the thermostat by at least 360 degrees Celsius (or 650 degrees Fahrenheit!). On Earth we have to mitigate a potential rise of only a few degrees. That sounds easier. Yet this is our home, and it feels different to speak in a cavalier way about making adjustments to it. In contrast to our schemes for other planets, with Earth we don’t generally talk about using brute-force methods such as deliberately crashing comets, blocking the Sun out of the sky, or filling the stratosphere with dust from ground-up asteroids. Instead, we typically look at enhancing or simulating known processes that are already at work on our planet.
The basic physics of solar radiation management is familiar to climate modelers. The anti-greenhouse effect is a known entity, an observed phenomenon in the stratosphere of Titan, in the dust storms of Mars, and during large volcanic events on Earth. It’s also hypothesized and modeled to explain asteroid extinctions and nuclear winter. So couldn’t we just induce a little bit of a controlled anti-greenhouse to mitigate the warming we’ve been causing with our CO2 emissions? This might be done, for example, by spreading enough sulfur dioxide into the stratosphere to mimic the stratospheric injection by volcanic eruptions.
Approached from a simplified modeling perspective, it’s a straightforward problem: Create an idealized model planet. Then introduce some aerosols to induce an anti-greenhouse and cool off the lower atmosphere and surface. This is exactly what I did for my PhD dissertation when I simulated the climate influence of impact-raised dust on the young Earth. Compared to most of these other real-life and modeled anti-greenhouses, the adjustment needed to counter global warming here on Earth is tiny, requiring only a minor change in the energy budget of the planet. A difference of about 1 percent in the albedo (reflectivity) of the planet creates about a 3 degree (Celsius) change in temperature, which is roughly doubled when you include amplifying feedback involving water vapor. If we did this, the sky on Earth would look a little different, but not noticeably darker. Sounds easy, right? Yet the closer you look at an actual planet (this one), the messier the problem becomes. We really don’t know how it would play out in detail, globally. It would surely change precipitation patterns, likely causing droughts in some places and floods in others, but our models aren’t good enough to reliably predict this.
We’ve seen in the geological and historical records of large volcanic eruptions (the closest natural analog to a stratospheric geoengineering effort) that the effects are indeed geographically uneven. The largest volcanic eruption in the twentieth century was the great 1912 explosion of Katmai, on the Alaska Peninsula, in southern Alaska. Alan Robock, an atmospheric physicist who has studied (alongside nuclear winter, global warming, and geoengineering) the historical effects of volcanoes on climate, found that the Katmai eruption caused a drop in precipitation that led to a regional drought in the Nile River Delta. Records of Nile flow show that the year after the eruption had the lowest flow measured all century. A weakened Indian monsoon and a massive famine in Africa were also triggered in the two years following the eruption. Given what we know now and, more important, what we don’t know, there is no reason to doubt that “fixing” climate with SRM would cause similar catastrophic regional problems.
Even if we thought we knew enough to implement such a solution safely, or reached a point where the regional disruptions seemed the lesser of two evils—and just who is going to make that call?—there are several other reasons to be highly skeptical of SRM as a solution for global warming. For one thing, it doesn’t do anything to address the dangerous ocean acidification caused by rising levels of CO2. Even if we were able to cool Earth with a controlled anti-greenhouse, the oceans would still become more acidic, the coral reefs would die, and many species would go extinct, ultimately threatening the marine ecosystems we depend on for food. Further, if cooling Earth in this way led to any slacking off on efforts to reduce our carbon inputs—and one can imagine it might—this would make acidification even worse.
It also raises thorny problems of governance and continuity. A stratospheric cloud will stay aloft for only a couple of years. Such a system would have to be continually replenished and maintained. Who is going to be in charge? What happens to the climate if this effort is not continued? Sometimes I am asked by screenwriters to speculate on sci-fi scenarios that might be both plausible and make for good drama. Next time I’m asked to think of a realistic possible future where Earth has become an apocalyptic nightmare, I’ll suggest one where a stratospheric geoengineering project is started but not maintained.
Other forms of SRM (including various methods to increase cloudiness over the oceans, designs for giant structures in space, and arrays of satellites to block or reflect sunlight) suffer from the same problems of inadequate governance and woefully incomplete knowledge. And none of them does anything about ocean acidification. At best, SRM seems like an untested form of methadone for our carbon habit, transitioning us to a new dependence that might be slightly more manageable but that surely has unknown side effects and doesn’t really address our core problem.
The other major category of geoengineering being discussed, carbon dioxide removal, is more promising. If we can pull large amounts of CO2 out of the atmosphere, then we will be working closer to the root of the problem, not just compensating crudely and incompletely for warming. Earth’s carbon cycle already has large CO2 “sinks” that remove the gas from the air. Perhaps there is a way we could tweak the cycle a bit, enhancing
or mimicking these natural sinks to pull out a little more carbon and sequester it into some of its other, climate-neutral, organic or inorganic forms.
What if we could use the energy of sunlight to bind atmospheric CO2 into organic carbon, reducing the atmospheric greenhouse and simultaneously producing food or fuel? This sounds almost too good to be true, but it goes on every day. We call it photosynthesis. Photosynthesis is brilliant. No wonder the biosphere went a little bit crazy and temporarily lost its balance when it discovered this winning trick. Perhaps there is a way that we can get in touch with our inner cyanobacteria, once again transforming the world with sunlight, only this time with purpose, with a plan, with negative feedback and the concept of “enough.”
In their planetary engineering study, after considering how we could play doctor to climates around the solar system, and reviewing ideas for mitigating global warming on Earth, Pollack and Sagan concluded that most of the schemes for Earth were too risky. They did, however, strongly endorse one method of geoengineering:
We advocate instead of particle shields or sunscreens, a well-tried biological solution… We propose reforesting the world, especially in the tropics, in accordance with the ancient oriental wisdom “He who causes trees to be planted, lives long.”6
Plants and trees grow by taking carbon dioxide out of the air and fixing it into solid organic molecules, so if you increase the mass of trees, reversing the deforestation that is currently taking place, there will be a consequent drawdown of carbon dioxide. Also, tropical forests have so many other intrinsic values, such as fostering and preserving enormous biodiversity. An expanded version of this concept involves changing land use practices, for agriculture in particular, to increase uptake of CO2. There are some ideas as simple as changing when and how farmers till their fields and encouraging changes to less meat-intensive diets, which collectively can make a large difference.
You may not think of planting trees and changing farming practices as forms of geoengineering. Those who consider themselves opponents of geoengineering generally do not. Yet I would argue that, in an important sense, they are. When we do these on a large enough scale, in a conscious effort to affect the climate of the planet, we are deliberately interfering in Earth’s climate, committing acts of planetary engineering. Nobody could reasonably object to these verdant interventions. Yet, given all the carbon we are dumping into the air, and will be for decades to come, they will not, on their own, be sufficient.
There are other, more intrusive, proposals to artificially enhance the rate of photosynthesis on Earth. One is to dump massive amounts of iron flakes into large areas of the ocean. The growth of photosynthetic marine life in many ocean environments is “iron limited,” meaning that what stops much more plankton from growing is a lack of that one essential nutrient. This is why we sometimes see large algae blooms when storms blow desert sands out into the deep ocean and the iron-starved creatures instantly start to multiply like crazy. Simulating this natural process with an input of iron would cause a greening of the surface waters. Doesn’t that sound nice?
As with the ideas for changing Earth’s reflectivity with stratospheric hazes, this one sounds great from the point of view of very basic physics or chemistry, where you picture a simplified cartoon planet with boxes representing the ocean or the sky and arrows of different sizes representing inputs and outflows of radiation or carbon. However, again, as we go beyond the cartoon and look more closely at the details and mysteries of the real planet, our confidence in our ability to do this declines rapidly. Given our current level of ignorance, seeding the deep ocean with iron on a large enough scale to make a significant dent in atmospheric carbon is an obvious invitation for unintended consequences.
Like a panicked bull charging into a shop of living china, we’d be altering complex ecological systems, with their inherent biological feedbacks, in unknown and unpredictable ways. There’s a long list of research questions that would need to be answered before we could confidently try this on a large scale. It is possible that many of the side effects would be beneficial for ocean life. Increasing the fertility and biomass of the deep, remote oceans might help restore stocks depleted by overfishing. So, it could be a win, win, win for us, the algae, and the fish.
Yet we don’t know what else such a large intervention might also do. For example, it could release massive amounts of poisonous biotoxins from some of the algae species that would multiply in these altered waters. Then there are also fundamental questions about how effective such a strategy would really be. It would remove carbon from the atmospheric system in a lasting way only if a significant portion of the newly sprouted plankton died, fell to the bottom of the ocean, and was buried beneath sediments, taking its carbon with it to the grave. If, instead, this strategy mostly stimulated the growth of other species in deeper waters that then breathed the CO2 back into the air, there would be no net benefit for the climate. We should certainly continue to study this problem with better models and carefully controlled and monitored small-scale trials. Yet, like most brute-force geoengineering schemes, seeding the oceans with iron is a very long way from being ready for prime time. Ocean ecosystems are not something we want to play around unless we really know what we’re doing.
Other ideas for enhanced photosynthesis involve using algae in offshore “bioreactors” to convert sunlight and atmospheric carbon into food and fuel. This is somewhat similar in concept to the iron fertilization schemes for the deep ocean, but it would be done intensively, in controlled and confined near-shore locations. In some of these designs the bioreactors are even combined with wastewater treatment plants, using the carbon in our sewage as food for the algae, which removes CO2 from the atmosphere and creates clean water, renewable fuel, and fertilizer. Sounds too good to be true, but the basic science is sound. The question is whether it could be scaled up sufficiently to make a meaningful contribution to climate mitigation. Genetic modifications could be used to enhance the photosynthetic capacity of algae in these settings. Care and cleverness would be needed to ensure that such beasties were not inadvertently released into the ocean at large.
The solutions that make the most sense are variations on those that Gaia found long ago: use solar energy to capture carbon from the air and put it to work in the service of organic life. It seems that enhanced or artificial photosynthesis ought to be a big part of the long-term plan. Can we find ways to greatly intensify the carbon-capturing role of sunlight without dangerously interfering in other natural processes?
I don’t intend for this to be an exhaustive list of ideas for removing CO2 from the air, and I urge you to read up on it elsewhere.7 Some of the ideas for “direct air capture” (that is, machines that can simply pull CO2 right out of the air) are physically feasible but seem prohibitively expensive. However, this technology (shockingly, considering the stakes) has not yet been extensively researched. This is an area ripe for game-changing innovation. When I speak with groups of eager schoolkids hungry for knowledge of science, it always fills me with hope, knowing that soon some of them will know a lot more than I do about how planets work. When I think about discoveries that may come along this century and completely change the trajectory of the future, I suspect that one of these kids will invent a reasonably priced or (better yet) profitable way to remove CO2 from the air and lock it away or convert it into something useful.
Whatever mechanism we choose, we must make sure we know how to turn it off, lest we meet with an ironic end. A carbon-removal process with no end point or Off switch could create a complete planetary disaster. As Robert Frost wrote, “for destruction ice is also great and would suffice.” If we removed too much CO2, the world would freeze.
No Quick Fix
It is possible to love planetary engineering but remain skeptical and wary of intrusive, heavy-handed geoengineering schemes. Carl Sagan cherished the idea of terraforming. He wrote about it in The Cosmic Connection, his first popular book, and in Pale Blue Dot, his last on a space theme. He w
as always looking for ways to update terraforming schemes to take advantage of new discoveries and ideas in planetary science. In 1996, I was studying the stability of climate on Venus with my grad student Mark Bullock at the University of Colorado. We were exploring the climate feedbacks that arise because of chemical reactions between greenhouse gases and surface minerals, using techniques we learned from Jim Pollack, who worked closely with us until his death in 1994. We discovered a stable climate state that Venus might enter into that would be much cooler, though still a lot hotter than Earth. We published a paper about this in the Journal of Geophysical Research,8 and a letter soon arrived from Carl, who was at the time being treated for what would prove to be fatal myelodysplasia:
Dear David
I read your most interesting paper in JGR Planets. When you get a moment, why don’t you think about terraforming Venus?… How would you make a massive decrease in surface temperatures?
It seemed as though Carl fired this off the day after the paper came out. He didn’t miss much. This reveals something that many people don’t know about him. Even while he was engaging more and more visibly in public life, he was always still voraciously reading the literature, doing research, and carrying on vigorous scientific discussions with seemingly everyone in the planetary science community. He remained a card-carrying working scientist until the end. After I received that note, in early May, we had a fun exchange of letters about climate engineering. It was one of the last conversations we ever had. In December of that year, his weakened immune system could not fight off a sudden bout of pneumonia, and he died at the age of sixty-two.
Earth in Human Hands Page 19
