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
Great acceler ations 151
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
152 Ch a pter 5
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
Great acceler ations 153
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
154 Ch a pter 5
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.
Great acceler ations 155
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
156 Ch a pter 5
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,
Great acceler ations 157
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.
158 Ch a pter 5
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
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