Timefulness

by Marcia Bjornerud

150 Ch a pter 5

  pressurized CO2 into deep geologic formations is not without

  challenges. The rocks to be used as a storage “container” must

  be porous enough to hold large amounts of compressed gas

  but not permeable enough to allow it to leak out— which is a

  bit like valuing a friend for his big- hearted gregariousness but

  then expecting him to keep a juicy secret. And forcing high-

  pressure fluid into rocks, whether it is CO2 or wastewater from

  hydrofracturing, can have an unsettling side effect: inducing

  earthquakes, which, ironically, could compromise the integrity

  of the carbon dioxide reservoir.

  Instead of capturing carbon from power plants, could we

  mimic photosynthesizers and extract CO2 directly from the

  air? For at least two decades, a number of academics and pri-

  vate companies have worked on developing “artificial trees”

  whose “leaves” would bind ambient CO2 in a chemical medium

  such as a strong base, like lye (sodium hydroxide, NaOH) or

  a polymer resin. An optimistic advocate for this technology

  is physicist Klaus Lackner of Arizona State University, who

  believes it is possible to engineer a “tree” that could capture

  as much as 1 ton of CO2 per day, about 1000 times more than

  the average natural tree. At this optimum level of efficiency, it

  would take 30 million artificial trees to keep up with our cur-

  rent 10 Gt/yr carbon habit, and hundreds of millions more to

  reverse the effects of a century of carbon emissions— or even

  get back to the 1990 level of 350 ppm that many climatologists

  see as a tipping point.

  A study by the American Institute of Physics estimates that

  the cost of direct air capture of CO2, using even the most prom-

  ising (but still unproven) technologies, would be about $780/

  ton of CO2, almost 10 times more than carbon capture and

  sequestration at power plants.23 Also, direct- capture “forests”

  would require large land areas, and the carbon they captured

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  would still need to be disposed of either through underground

  injection or burial in some solid form.

  K N O C K I N G O N W O O D

  All these concepts make old- fashioned photosynthesis seem

  like an incredible bargain— and we have the technology! So, is

  planting as many seeds and saplings as possible the solution? As

  the geologic record shows, the trick to reducing atmospheric

  CO2 levels is to sequester more carbon through photosynthesis

  each year than is released by decomposition. (The irony, of

  course, is that undecomposed organic carbon of the geologic

  past made the fossil fuels that got us into this predicament

  today). There is no net change in CO2 levels if carbon fixed by

  plants in the spring and summer is then released in the fall and

  winter through their decay. Fast- growing trees with a long life-

  span are therefore the darlings of carbon sequestration. While

  they don’t store carbon forever, they can keep it out of circu-

  lation for decades or centuries.

  But even the simple idea of planting trees to modulate

  carbon gets complicated in implementation. First, there is

  obviously a limit to the land area that can be reforested; we

  do need to grow food (though in the last century, parts of the

  northern United States such as Wisconsin and New England,

  which had been clear- cut and farmed in the nineteenth cen-

  tury, are now returning to forest land). Also, one might think

  that young trees, with vigorous growth rates, would cap-

  ture more carbon. If this is true, it would make sense to cut

  down old forests to make space for new plantings. But recent

  studies have shown, counterintuitively, that many species of

  trees actually sequester more and more carbon as they age,

  because their leaf area, girth, and branch volume continue

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  to increase.24 Letting old trees continue to grow while also

  planting new ones thus seems the best strategy. Still, trees

  have a finite lifespan and eventually return their carbon to

  the atmosphere.

  A more active approach to harnessing the power of photo-

  synthesis is known by the functional but cumbersome name

  “bioenergy with carbon capture and storage” (BECCS). The

  idea is to use biomass from fast- growing photosynthesizers—

  plants like switchgrass or “farmed” algae— as a fuel source, and

  then sequester the carbon emitted in the combustion of this

  fuel. In theory, this could be a truly carbon- negative process,

  since at least some carbon extracted by photosynthesis would

  be withdrawn from the atmosphere for the long term. Small-

  scale pilot projects have shown promise, but converting plant

  matter to fuel is itself energy- intensive, and carbon capture at

  biomass facilities may be even more expensive than for coal

  or gas.25

  Over geologic time, much photosynthetic carbon has been

  sequestered as marine biomass, mostly bacterial, that fell to

  the seafloor and was buried in low- oxygen sediments (some

  of which became petroleum, natural gas, or gas hydrates). Per-

  haps we could emulate this process by stimulating the growth

  of plankton communities in the oceans, in the hope that some

  of the carbon they fix will find its way into sediments and be

  locked away for geologic timescales. The best fertilizer would

  be iron, which microbes have been starving for since the Great

  Oxygen Revolution of the Proterozoic.

  Intentional manipulation of ocean chemistry, however,

  raises alarms among marine biologists. Altering the base of

  the food chain is certain to have negative and unforeseen

  consequences (we are already doing this unintentionally—

  but knowingly— by failing to mitigate phosphorus and nitrate

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  runoff from agriculture, which leads to anoxic coastal dead

  zones). This is why there was scientific outcry in 2007 when

  entrepreneur Russell George started selling shares in a com-

  pany called Planktos, which intended to fertilize a Rhode

  Island– sized area of the Pacific Ocean and sell carbon offsets

  to environmentally minded consumers. Planktos failed, but

  George reappeared in 2012 as a consultant to a First Nation

  in coastal British Columbia, the Haida people, promising to

  revitalize their anemic salmon fishery with iron fertilization.

  One hundred tons of iron sulfate were dumped in the waters

  around the Haida Gwaii (Queen Charlotte) Islands, with in-

  conclusive results, before the UN’s International Maritime

  Organization condemned the act, and the Canadian environ-

  mental ministry intervened to stop it. The scientific unease

  about cavalier alteration of seawater arises partly from the fact

  that we can’t be sure that our current understanding of ocean

  biogeo chemistry will even apply in the near future. We have

  incomplete knowledge of the global marine microbiome as it

  exists today and still less of a grasp on how it might evolve as

  the seas grow war
mer and more acidic.26

  L I M E L I G H T O N L I M E S T O N E

  If accelerating microbial growth in the oceans is off the table,

  perhaps we could imitate Earth’s long- term carbon sequestra-

  tion scheme: fixing atmospheric carbon dioxide in limestone.

  Making limestone begins with weathering silicate rocks to free

  up calcium that can then combine with atmosphere CO2 to

  form calcium carbonate or calcite. This is the process respon-

  sible for the slow drawdown of CO2 that cooled the globe as

  the Himalaya grew (Ch. 3). In nature, shelled organisms do the

  work, sopping up carbon at an estimated 0.1 Gt/yr— sufficient

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  over geologic time to have locked into solid rock 99.9% of all the

  carbon dioxide emitted by volcanoes, but 100 times too slow to

  keep up with our current annual emissions. And unfortunately,

  making shells will become an even harder task as ocean acidity

  increases, causing the already slow natural rate of limestone

  formation to decrease in the coming centuries.

  It might be possible, however, to form artificial “limestone”

  by deliberately facilitating the silicate weathering reactions that

  draw CO2 out of the air. An igneous rock type called peridotite,

  rich in the mineral olivine (whose gem form is peridot) will

  react with carbon dioxide to form a magnesium- rich carbonate

  mineral (magnesite) similar to calcite, as follows:

  Mg2SiO4 + 2CO2 → 2MgCO3 + SiO2

  Olivine + Carbon dioxide → Magnesite + Quartz

  The catch is that although peridotite is very abundant in the

  Earth— it makes up most of the Earth’s upper mantle— it is quite

  rare on Earth’s surface. But there are places, including New-

  foundland, Oman, Cyprus, and Northern California, where

  subduction went wrong and slabs of mantle rock were thrust

  up onto the edges of continents. At these locations, peridotite

  could be perforated with drill holes into which captured CO2

  could be pumped. One study suggests that the Oman peridot-

  ites alone could sequester 1 Gt of carbon per year (one- tenth

  of our annual output).27 The carbonation reaction is sluggish at

  low temperatures, but it is also exothermic, so once it begins,

  it is self- accelerating. The main problem, of course, is getting

  the gas to the rocks. Carbon dioxide must either be captured

  and transported to the rare places where mantle rocks lie at

  the surface, or peridotite must be mined in large volumes and

  spread over vast areas of Earth’s surface, where it could react

  passively with the atmosphere.

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  A I R R A I D S

  Given all the difficulties with getting rid of carbon dioxide, it

  is no wonder that the idea of cooling the planet by shooting

  sulfate aerosols into the stratosphere— inspired by the 1991

  eruption of Mount Pinatubo— is so seductive. “Solar radiation

  management” is relatively cheap (billions of dollars a year) and

  could probably be started right away using rockets, airships, or

  high- altitude jets. But it would also be a Faustian bargain. Once

  begun, a sulfate injection scheme would require a decades-

  to- century- scale commitment, since in the absence of serious

  CO2 reductions, it would mask but not reverse greenhouse

  warming (ocean acidification from rising CO2 levels would

  also continue unabated— and undermine carbonate precipita-

  tion, Earth’s slow but effective long- term carbon sequestration

  system). There is also the moral hazard that suppressing the

  symptom would reduce the political will to cure the underly-

  ing disease. Stopping injections after a period of a few years

  would lead to ferocious “catch- up” warming that could dev-

  astate the biosphere and lead to extreme alteration of weather

  patterns.

  Adding a Pinatubo- equivalent mass (about 17 megatons)

  of sulfur dioxide to the stratosphere every few years for 50 or

  100 years would fundamentally change biogeochemical cy-

  cles in ways we can only partly anticipate. And, like a drug

  addict needing larger and larger doses to get the same high,

  the amount of sulfate required to attain the same level of cool-

  ing would actually increase over the years. This is because

  both the residence time and reflectivity of the sulfate droplets

  would steadily decrease as a result of their tendency to glom

  together and grow larger; bigger particles fall out of the at-

  mosphere faster, and they have smaller surface area relative to

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  their volume than small ones, which reduces their efficiency as

  solar energy reflectors.

  Atmospheric chemists do know that large volumes of

  stratospheric sulfate would damage Earth’s radiation- shielding

  ozone layer, which has been slowly recovering since 1989,

  when the Montreal Protocol first limited the use of chloro-

  fluorocarbons. Also, the environmental impact of the sulfate

  delivery system would itself be considerable: if jet fighters

  were used, millions of flights would be required each year.28

  And for each mission to launch sulfate into the stratosphere

  10 km (6 mi) up, there is a possibility that the payload could

  fail to reach the target altitude, unleashing a localized down-

  pour of acid rain.

  A sulfate shroud would alter the wavelengths and intensity of

  light that falls on photosynthesizing plankton and plants, with

  unknown effects on natural food webs, forests, and agricultural

  crops. A particularly cruel irony is that aerosols would reduce

  the efficiency of solar power generation, especially large- scale

  solar arrays that use mirrors and lenses to concentrate sunlight,

  thereby undercutting a technology that could help wean us

  from the fossil fuels that are the root of the climate problem.29

  Because sulfate aerosols have no effect in the dark, when there

  is no light to reflect, they would reduce day/night, summer/

  winter, and tropical/polar temperature differences. This would

  likely cause dramatic shifts in global weather patterns, which

  are driven by temperature contrasts and gradients. The poten-

  tial effects on the many complex temperature- driven ocean-

  atmosphere interactions like the interannual El Niño cycle and

  the monthly to bimonthly Madden- Julian oscillations, which

  govern weather around the Pacific basin, are unclear. Multi-

  ple climate models suggest that areas affected by the annual

  Asian monsoon could see sharp reductions in precipitation,

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  although there are large uncertainties in these simulations.30

  What recourse would there be for regions adversely affected

  by atmospheric manipulation? Given the state of world gover-

  nance, it is hard to imagine that this intergenerational global

  geochemical experiment could be smoothly administered and

  promote harmony among nations. And did anyone mention

  that the sky would always be white, not blue?

  It is telling that the most vocal advocates for stratospheric

  sulfate injec
tion are either economists, accustomed to view-

  ing the natural world as a system of commodities whose “real”

  value is in dollars, or physicists, who treat it as an easily under-

  stood laboratory model. Often, the argument is made that our

  unintentional atmospheric modification from greenhouse gas

  emissions has now reached the point where there is “no choice”

  but to perform intentional “management” of climate.31 Most

  geoscientists, knowing the long and complex story of the at-

  mosphere, biosphere, and climate— the hellish extinctions and

  feverish ice ages, fragile food chains, and powerful feedback

  mechanisms— think the idea humans can “manage” the planet

  is delusional and dangerous. What on Earth makes us think

  we can control nature on a global scale, when we haven’t even

  learned to control ourselves?

  B A C K T O N AT U R E

  The carbon conundrum is not the only environmental chal-

  lenge of our time, but it underscores a more general, obstinate

  fact: that there is an immense asymmetry in the time it takes to

  consume, alter, or destroy natural phenomena compared with

  the time required to replace, restore, or repair them. This is the

  hard truth I first glimpsed in the shards of a tourmaline crystal,

  and it is the central challenge of the Anthropocene.

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  This brave new epoch is not the time when we took charge

  of things; it is just the point at which our insouciant and raven-

  ous ways starting changing Earth’s Holocene habits. It is also

  not the “end of nature” but, instead, the end of the illusion

  that we are outside nature. Dazzled by our own creations, we

  have forgotten that we are wholly embedded in a much older,

  more powerful world whose constancy we take for granted.

  As a species, we are much less flexible than we would like to

  believe, vulnerable to economic loss and prone to social unrest

  when nature— in the guise of Katrina, Sandy, or Harvey, among

  others— diverges just a little from what we expect. Averse to

  the even smallest changes, we have now set the stage for en-

  vironmental deviations that will be larger and less predictable

  than any we have faced before. The great irony of the Anthro-

  pocene is that our outsized effects on the planet have in fact

  put Nature firmly back in charge, with a still- unpublished set

  of rules we will simply have to guess at. The fossil record of