fact, were followed by a period of time— hundreds of thousands
to millions of years— when microbes alone thrived while the
rest of the biosphere struggled to get back on its feet (or into
124 Ch a pter 4
its shells). The great mass extinctions challenge any conceit
that we are the triumphant culmination of 3.5 billion years of
evolution. Life is endlessly inventive, always tinkering and ex-
perimenting, but not with a particular notion of progress. For
us mammals, the Cretaceous extinction was the lucky break
that cleared the way for a golden age, but if the story of the
biosphere were written from the perspective of prokaryotic
rather than macro scopic life, the extinctions would hardly reg-
ister. Even today, prokaryotes (bacteria and archea) make up at
least 50% of all biomass on Earth.23 One might say that Earth’s
biosphere is, and always has been, a “microcracy,” ruled by the
tiny. When larger, arriviste life- forms falter, infinitely adapt-
able microbes, whose evolutionary timescales are measured in
months rather than millennia, are always eager to move in and
reassert their long- held dominion over the planet.
Perhaps most importantly, none of the mass extinctions—
even the relatively “clean” Cretaceous disaster— can be fully
attributed to a single cause; all involved rapid changes in sev-
eral geologic systems at one time, which in turn triggered
knock- on effects in still others. In some respects, this is reas-
suring; it means that it takes a “perfect storm” of convergent
causes to destabilize the biosphere. Nevertheless, many of the
malefactors— greenhouse gases, carbon- cycle disturbances,
ocean acidification, and anoxia— are uncomfortably familiar to
current residents of Earth. And if a looming catastrophe has
multiple origins, there will be no precise predictions and no
silver- bullet solutions.
The story of the atmosphere reminds us that the sky over
our head is not the only, or ultimate, one to shelter the Earth.
When there is change in the air, even after long periods of sta-
bility, it can blow through with breathtaking suddenness, as
Svalbard’s withering glaciers attest. In the aftermath of these
Changes in the air 125
winds of change, upheavals in biogeochemical cycles ripple
through ecosystems at all levels. Organisms that have invested
everything in the old world order will suffer or even be extin-
guished while microbes quietly clean up the mess and decree a
new set of rules for the survivors. Tinkering with atmospheric
chemistry is a dangerous business; ungovernable forces can
come out of thin air.
C H A P T E R 5
G R E AT
AC C E L E R AT I O N S
Any fool can destroy trees; they cannot run away.
— J O H N M U I R , O U R N AT I O N A L PA R K S , 1 9 0 1
A C C I D E N TA L VA N D A L S
At most U.S. colleges and universities, earning a degree in
geology requires completion of a rite of passage called “field
camp.” Traditionally, this is a six- week course in a Western
state with rugged topography and plenty of bare rock baking
in the sun. Aspiring geologists learn to map rock units and
mineral deposits, log stratigraphic sequences, draw cross
sections, and interpret landforms. In the old days, field camp
was the course that separated “the men from the boys.” For-
tunately, my own field camp at the University of Minnesota
was taught by professors with a more enlightened philosophy.
Even though Minnesota has plenty of interesting rocks of its
own, our field camp was set in the spectacular Sawatch Range
of central Colorado.
We had a day off each week, and during one of those times
of sweet liberty, a group of us set out on a long hike to explore
an old pegmatite mine we had heard about. Pegmatites are
exotic igneous rocks famed for their oversized crystals of rare
and colorful minerals and valued, increasingly, as sources of
rare- earth elements, which are essential for high- tech batteries,
Great acceler ations 127
cellphones, and digital storage media. Pegmatites represent the
very last stage of solidification of some granitic magmas, when
a combination of undercooling and a high content of magmatic
gases allows crystals to grow many times faster than usual. A
normal quartz or feldspar crystal forming in a magma chamber
beneath a volcano like Mount Saint Helens might grow at the
leisurely pace of about 0.6 cm (0.25 in.) per century.1 Pegmatite
crystals, on the other hand, are the baby blue whales of the
mineral kingdom, bulking up at the dizzying rate of inches per
year.2 Although they can form quickly under the right circum-
stances, pegmatites are rare— not exactly renewable resources.
The pegmatite we were hunting was an old one, formed in
Meso proterozoic time at least 1.5 billion years ago, long before
the modern Rockies existed.
We found the road into the abandoned mine diggings—
hesitating briefly at the “No Trespassing” signs— and followed a
string of waste rock piles to a hollowed- out space on the side of
a half- blasted hill. There we discovered what pegmatite zealots
(a distinct subculture of mineralogists) call a gem pocket. It was
like stepping into the pastel world of an old- fashioned Easter
sugar egg: giant crystals of white feldspar were decorated with
clusters of purple mica (lepidolite) and hexagonal prisms of
pink and green tourmaline. Some of the tourmalines were per-
fect gemstone miniatures of watermelon slices, with thin green
rinds and pink interiors. In an instant, we were all seized with
a visceral greed, a need to take as many of these treasures as
we could. We had come with our rock hammers, but the pick
ends were blunt, designed for breaking rocks, not extracting
delicate crystals. I managed to tap out a few small deep- pink
tourmalines, and then spotted a prize: a perfect watermelon-
colored crystal about 8 cm (3 in.) long. It was in an awkward
corner close to the ceiling of the excavation, with little room
128 Ch a pter 5
for wielding a hammer, but I was determined to have it. I began
pounding away, thinking ahead to how I would display this
trophy at home when, in one errant blow, I smashed it.
In that moment it seemed my vision was suddenly cleared,
as if I had been released from a malevolent spell that had en-
gulfed us when we entered the gem pocket. I abruptly lost my
appetite for the whole enterprise. After several years of immer-
sion in the world of geology, I had developed some sense for
Deep Time. And I saw that in an avaricious second I had care-
lessly destroyed an exquisite thing that had witnessed a third
of Earth’s history— most of the Boring Billion, Snowball Earth,
the emergence of animals, the great extinctions, the growth of
the Rockies. I felt sickened by the scene of devastation around
me, and my complicity in it.
bard’s glaciers— and the increasingly anemic winters we have
in Wisconsin— knowing that I am culpable for them as a person
who loves international travel and long hot showers, and more
generally as a member of a fossil fuel– addicted society. In my
lifetime, we have thoughtlessly smashed ancient ecosystems
and made a wreckage of long- evolving biogeochemical cycles.
We have set in motion changes for which there are few prec-
edents in the geologic past and which will cast long shadows
far into the future.
A N A N T H R O P O C E N E A L M A N A C
Sometime in the last century we crossed a tipping point at which
rates of environmental change caused by humans outstripped
those by many natural geologic and biological processes. That
threshold marked the start of a proposed new epoch in the
geologic timescale, the Anthropocene. The term was coined
Great acceler ations 129
in 2002 by Paul Crutzen, a Nobel Prize– winning atmospheric
chemist, and it quickly entered both the geologic literature and
popular lexicon as shorthand for this unprecedented time when
the behavior of the planet bears the unmistakable imprint of
human activity.
In 2008, a short paper by a group of stratigraphers in the
Geological Society of London provided quantitative argu-
ments for how the Anthropocene could be formally defined.3
The authors pointed to five distinct systems in which human
activities have at least doubled the rates of geologic processes.
These include the following:
• erosion and sedimentation, in which humans outpace all
the world’s rivers by an order of magnitude (a factor of 10);
• sea level rise, which had been close to nil for the past
7,000 years4 but is now about 0.3 m (1 ft) per century and
expected to be twice that by 2100;
• ocean chemistry, also stable for many millennia but now
0.1 pH unit more acidic than a century ago;
• extinction rates, now a factor of 1000 to 10,000 above
background rates;5
and of course
• atmospheric carbon dioxide, which at more than 400 ppm
is higher than at any time in the last 4 million years (before
the Ice Age), while emissions by human activities surpass
all those of the world’s volcanoes by a factor of 100.6
Other authors note that phosphorus and nitrogen efflux into
lakes and coastal waters— leading to anoxic dead zones— is now
more than double the natural rates, due to runoff of agri cultural
fertilizers.7 And through agriculture, deforestation, fires, and
other land- use practices, humans dictate one- quarter of the
130 Ch a pter 5
net primary productivity— the total photosynthetic effort of
plants— on land.8
Most geologists think these stark facts more than justify
the adoption of the Anthropocene, not only as a useful con-
cept but also as formal division of the geologic timescale, on
par with the Pleistocene (the Ice Age, 2.6 million to 11,700
years ago) and the Holocene (essentially, recorded human his-
tory). The magnitudes of human- induced planetary changes,
“achieved” in less than a century, are equivalent to those in
the great mass extinctions that define other boundaries in
geologic time. With the exception of the end- Cretaceous
meteorite impact, however, those events unfolded over tens
of thousands of years.
The International Commission on Stratigraphy— that for-
midable parliament of Time— has taken the matter up, and
the main disagreements are bureaucratic: in particular, how
exactly to define the start of the Anthropocene. Should there
be a Global Boundary Stratigraphic Section and Point (GSSP,
or “golden spike”) as for other boundaries in geologic time?
The GSSP for the base of the Holocene is a particular layer
within the Greenland ice cap, with an isotopic signal that
marks the onset of the warmer climate of the Holocene.9 Ice
is more ephemeral than rock, but the layer lies more than
1400 m (4600 ft) below the surface and is safe from melting
for now. (Also, a sample of the layer is archived in freezers
at the University of Copenhagen). The Anthropocene could
be similarly defined by a distinctive signature in polar ice—
perhaps the spike in unusual isotopes that is the shameful
legacy, the Scarlet A, of atomic bomb tests in the 1950s and
’60s. But this near- surface ice could very soon be a victim of
Anthropocene climate; glacial archives are being lost at an
alarming rate around the world. On the Quelccaya Ice Cap in
Great acceler ations 131
the High Andes, for example, 1600 years of ice has vanished in
the past two decades— destroying a high- resolution weather
record going back to the time of the Nazca people.10 The use of
the word glacial to mean “imperceptibly slow” is quickly be-
coming an anachronism; today, glaciers are among the rapidly
changing entities in Nature.
Some geologists therefore suggest that an exception be
made in defining the Anthropocene and that a calendar year—
perhaps 1950— rather than a natural archive be chosen as its
formal beginning. After all, we humans are the only ones ag-
onizing over this, and as long as we’re around we can remind
each other of the date. If at some point we vanish, it is likely
no one else will fret about the definition of the Anthropocene.
In many ways, the exact start of the Anthropocene matters less
than the concept behind it.
A subtler point for geologists is that the idea of the An-
thropocene represents a fundamental break with the philo-
sophical underpinnings of the field, established by Hutton and
Lyell. Hutton’s great insight was that the past and present are
not disjunct domains governed by different rules but linked
through the continuity of geologic processes. And much of
Lyell’s magnum opus, Principles of Geology, is a polemic in-
tended to dissuade readers from the idea that geologic change
happened faster in the past than in the present. The Anthro-
pocene now inverts this idea by emphasizing how processes
are faster in the present than in the past. In attempting to
predict the geologic future without the comfort of uniformi-
tarianism, we are in a position strangely analogous to that of
pre- nineteenth century geologists who had no guidelines for
understanding the geologic past. Still, we can refer only to
the recent geologic record for possible analogs to our present
uncertain moment in time.
132 Ch a pter 5
U N D E R T H E W E AT H E R
Climate- controlled buildings and the year- round availability of
fresh fruit allow citizens of wealthy nations in the twenty- first
century to treat the weather as a backdrop to their lives, not the
main story. We may complain about the inaccuracy of a local
forecast or be irritated when rain foils weekend plans, but as a
life. Rather than measure the value of good weather (imagine
this headline: “Last Week’s Sunshine Was Worth $10 million
to Area Farmers”), we characterize bad weather events—
blizzards, hurricanes, heat waves, droughts, floods— as costly
anomalies that deprive businesses of their “rightful” earnings.
That is, we assume that the weather is normally stable and
benign, and are constantly surprised when it is not.
The long- term imprint of weather and climate on human
civilization is the focus of a new area of interdisciplinary
scholar ship that integrates history, economics, sociology, an-
thropology, statistics, and climate science. One of the salient
patterns that emerges when one looks at the past two millennia
of human civilization is that periods of social instability and
conflict coincide, at a high level of statistical significance, with
intervals when climate deviates even modestly from normal.11
In early medieval Europe, for example, average temperatures
only one degree lower than average led to crop failures and
spurred the great mass migrations and intertribal clashes of
the period from AD 400 to 700. Sustained drought related to
changes in Pacific Ocean weather patterns around the year 900
caused the collapse of both the Mayan civilization in Central
America and the Tang dynasty in China. The Angkor kingdom
of Southeast Asia, which had flourished for 500 years, crum-
bled after just two decades of drought in the early fifteenth
Great acceler ations 133
century. Another cold period in Europe coincided with the
Thirty Years War, from 1618 to 1648, which was more devas-
tating even than World War I in terms of the percentage of the
population killed. Although the war was nominally a religious
and political conflict, the animosities were deepened and the
suffering exacerbated by climate- related famine.
We may think that in modern times we are no longer so vul-
nerable to mere weather phenomena. But analysis of global
police records from the past half- century shows that for each
standard deviation increase in average temperature in major
cities, violent crime rates rose by 4%. A similar statistical study
finds that climate stresses like water shortages have caused local
and regional intergroup conflicts around the world to increase
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