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Timefulness

Page 16

by Marcia Bjornerud


  by at least 14% in recent decades.12 And in many ways, our

  advanced technologies make us less flexible than previous so-

  cieties in the face of change. We have made huge infrastructural

  investments in coastal cities based on a bet that sea level will

  remain constant. We have built sprawling cities in the desert

  on the assumption that snow and rain will keep refilling reser-

  voirs. We have a food production system that is predicated on

  the belief that old, familiar weather patterns will always return.

  But the weather is getting weird. Ten of the hottest years on

  record have occurred since the start of this millennium. “‘One

  hundred- year”’ and “500- year” flood events are happening once

  a decade. The new rules of the Anthropocene are even making it

  difficult for Earth scientists to use the quantitative models they

  have developed to study geologic systems. Such models are

  based on the concept of stationarity— the idea that natural sys-

  tems vary within a well- defined range with unchanging upper

  and lower bounds, an assumption that has yielded reasonable

  predictions in the past. A sobering report by an international

  group of leading hydrologists recently stated that “stationarity

  134 Ch a pter 5

  is dead and should no longer serve as a central, default assump-

  tion in water- resource risk assessment and planning.”13 In other

  words, the main prediction about weather and the water cycle

  is that they will become increasingly unpredictable.

  Yet the public clings to an optimistic belief in uniformitari-

  anism. This is partly understandable, because it is rooted in the

  geologic fact that climate in the Holocene Epoch, which saw

  the rise of everything we associate with human civilization—

  agriculture, written language, science, technology, govern-

  ment, fine arts— has been exceptionally stable. In fact, this sta-

  bility is arguably the very thing that allowed humans to build

  civilizations at all. The large- amplitude climate oscillations

  of the Pleistocene, in contrast, probably kept nascent human

  societies in check. The “Ice Age” wasn’t, in fact, entirely icy;

  instead, for 2.5 million years, the climate fluctuated manically

  over many timescales— as glaciologist Richard Alley memo-

  rably puts it, like someone “playing with a yoyo while bungee

  jumping off a roller coaster.”14 Understanding what exactly was

  going on in the Pleistocene is essential for putting current rates

  of climate change in perspective. The story of deciphering the

  Ice Age takes us back once again to Lyell, but also involves

  Swiss farmers, a Scottish janitor, and a Serbian mathematician.

  WA R M I N G U P T O I C E

  Here in Wisconsin, large boulders of granite and gneiss are com-

  mon centerpieces of upscale landscaping around medical com-

  plexes and office buildings. In the early nineteenth century, such

  stones— often completely different in composition from the

  local bedrock— were among the most vexing mysteries facing

  geologists in the Great Lakes states and northern Europe. These

  erratics, scattered far from their sources, seemed to support the

  Great acceler ations 135

  biblical idea of a Great Flood. Consequently, the rocks, and the

  clayey deposits they were often lodged in, became known as

  diluvium (sediment left by the deluge) or drift (a rather gentle

  term, considering the force of water that would have been nec-

  essary to transport such material). The latter term persists,

  anachronistically, in the name used for the distinctive region of

  deep bedrock valleys in southwestern Wisconsin— the “Driftless

  Area,” where no erratics or other types of diluvium are found.

  A Swiss geologist, Louis Agassiz (1807– 1873), is usually

  credited as the first to propose, in 1838, that great ice sheets, not

  floodwaters, might have carried erratic boulders long distances.

  Agassiz is championed in geology textbooks as a revolutionary

  thinker, but it seems a German naturalist, Karl Schimper— who

  in fact coined the term Eiszeit (“Ice Age”)— had earlier reached

  the same conclusion and shared it with Agassiz on a joint outing

  in the Alps.15 Schimper’s insights, in turn, may have come from

  Swiss farmers, who understood glaciers and to whom it was ob-

  vious that large boulders strewn far down alpine valleys marked

  the former positions of ice masses. More unforgivably, Agassiz

  later used his scientific credentials and his position as a Harvard

  professor to advance completely unscientific and abhorrently

  racist theories of human evolution; in my view, he should have

  an asterisk next to his name in the annals of science, like an

  athlete whose medal was rescinded for doping. Unfortunately,

  he remains the eponym of a giant late- Pleistocene lake, glacial

  Lake Agassiz, that covered much of North Dakota, Minnesota,

  and Manitoba (and left them so famously flat).

  While Charles Lyell disavowed the Flood, he also disliked the

  idea of an Ice Age during which large areas of now- temperate

  Europe and North America had been covered by ice. If not

  exactly catastrophic, it was certainly non- uniformitarian. But

  as geologists began to map the patterns of “drift,” the idea of

  136 Ch a pter 5

  a great Ice Age was shown to have explanatory power. In the

  upper Great Lakes region, it became clear that there had in

  fact been not one but several ice advances and retreats, each

  leaving distinct deposits (though each, strangely, avoiding the

  Driftless Area). What could be causing such cycles of warming

  and cooling?

  As early as the mid- nineteenth century, some scientists

  began to explore the hypothesis that variations in Earth’s or-

  bital habits could affect the way sunlight falls on the Earth

  and potentially trigger episodic ice ages. The gravitational in-

  fluences of the Moon and neighboring planets cause cyclical

  changes in three aspects of Earth’s motion in space: (1) the

  elliptical shape, or eccentricity, of the Earth’s orbit around the

  Sun, which stretches and shrinks on a 100,000- year timescale;

  (2) the tilt, or obliquity of Earth’s rotation axis, which varies

  between about 21.5° and 24.5° every 41,000 years; and (3) the

  slow wobble, or precession of the planet, like a toy top, which

  changes the hemisphere that is pointed toward the Sun at the

  solstices over a cycle that averages 23,000 years. Today, these

  three variables are called Milankovitch cycles for the allitera-

  tively named mathematician Milutin Milankovitch (1879–

  1958) who, in spite of his status as a displaced person for most

  of two world wars, managed to work out the combined effects

  of these cycles on solar irradiance of Earth.

  But it was actually a self- educated Scotsman, James Croll

  (1821– 1890), who had performed the first arduous calculations

  of the orbital cycles, more than 50 years earlier (a fact Milan-

  kovitch fully acknowledged). Croll had a keen mathematical

  mi
nd and great interest in science but was too poor to attend

  even secondary school. After some years as an innkeeper, he

  took a job as a janitor at Anderson College in Glasgow, where he

  would study scientific volumes in the library late at night (in a

  Great acceler ations 137

  nineteenth- century real- life version of the plot of the 1997 film

  Good Will Hunting). In the 1860s, he began a correspondence

  with Charles Lyell about his calculations of orbital variations

  and their effects on climate. Lyell, who by this time had reluc-

  tantly accepted glacial theory, was impressed by Croll’s clear

  brilliance and helped him gain a position at the Geological Sur-

  vey of Scotland. (Croll also exchanged letters with Darwin on

  the question of erosion rates). Croll’s work seemed to suggest

  that ice ages would be out of synchrony in the Northern versus

  Southern Hemispheres owing to the opposite effects of preces-

  sion in the two regions. This reasoning appealed to Lyell, since

  it meant that on average, the Earth maintained a steady state,

  an idea Lyell could not relinquish. A half- century later, Milan-

  kovitch would recognize that because of the disproportionate

  concentration of landmasses in the Northern Hemisphere, the

  influence of precessional cycles on northern latitudes actually

  dominated global climate.

  Neither Croll nor Milankovitch had any high- resolution geo-

  logic data against which to test their calculations, however. By

  the 1880s, the eminent Wisconsin geologist T. C. Chamberlin

  (for whom my once- glaciated Svalbard valley was named) had

  documented four distinct glacial periods, which he named for

  the states in which their deposits were best preserved— starting

  from the most recent, the Wisconsinan, Illinoisan, Kansan, and

  Nebraskan. But there was no way to know the absolute ages of

  these episodes nor whether there were still- earlier cycles of ice

  growth and recession. The problem with land- based records

  is that each ice advance, like a Zamboni resurfacing the ice

  between periods in a hockey game, will tend to erode and over-

  print the record of the previous events. Wisconsin (outside the

  Driftless Area) was glaciated in all four ice advances, but it is

  often difficult to recognize the deposits of the earlier three.

  138 Ch a pter 5

  In the last years of the nineteenth century, Chamberlin and

  many others speculated about the causes of the Ice Ages, invok-

  ing not only orbital cycles but also volcanism, mountain build-

  ing, and ocean circulation. In 1896, Swedish chemist Svante

  Arrhenius made the case that certain trace atmospheric gases,

  particularly carbonic acid (H2CO3, carbon dioxide combined

  with water vapor), could be important in governing climate be-

  cause they are transparent to incoming short- wavelength radi-

  ation (light) from the sun but block outgoing long- wavelength

  energy (heat) reradiated from the Earth’s surface.16 (He even

  surmised that emissions from coal burning might one day “im-

  prove” Sweden’s climate). All these ideas would eventually

  prove to be at least partly correct, but at the time, none could

  be rigorously tested without higher- resolution information

  about how climate had changed over time. There were many

  climate suspects, but it was premature to bring them to trial;

  the evidence was still too circumstantial.

  E S P R I T D E C O R E S

  Finally, in the 1970s, two new, rich archives of climate data that

  revolutionized climate science were opened— as if someone

  doing scholarly work with random volumes in a used bookstore

  suddenly had access to the Library of Congress. These were

  (1) deep- sea sediment cores obtained from a new generation

  of oceanographic research vessels and (2) polar ice, sampled

  through heroic international drilling operations in Antarctica

  and Greenland. The deep seafloor and polar ice caps are simi-

  lar in being sites where slow, continuous accumulation occurs

  without interruption or disturbance, like dust gradually blan-

  keting furniture in a closed- up room. Today, deep- sea cores

  from all the world’s oceans provide a 160 million- year record

  Great acceler ations 139

  of global climate change (extending back long before the Ice

  Age), encoded as variations in geochemistry and microscopic

  fossils, at a resolution of thousands of years. Ice cores, in turn,

  document 700,000 years of atmospheric variations that can be

  read to the year, at least in young ice. Teasing climate informa-

  tion from seafloor ooze and old snow, however, requires code-

  breaking— translating the cryptic record of stable isotopes in

  shell and ice.

  Oxygen, like carbon, has two main stable isotopes, and in the

  same way that light carbon (12C) is “preferred” by photosynthe-

  sizing organisms over the heavier form (13C), light oxygen (16O)

  is more likely to be taken up as water vapor during evaporation

  than heavier 18O. This means that at any given time, precipita-

  tion, including polar snows, will have less 18O and more 16O

  than ocean water, and this sorting effect is further enhanced

  during cold periods. During ice ages, when a significant fraction

  of Earth’s water is locked up in glaciers and ice caps, the oceans,

  and the organisms that form their shells from ocean water, will

  have particularly high ratios of 18O to 16O. Conversely, glacial ice

  will have particularly low values of this ratio. Ratios of ordinary

  hydrogen (1H) to deuterium (2H) vary in a similar way, and so

  in glacial ice (which is, after all, H2O) there is a second proxy

  record of climate. Isotope ratios in sea sediments and ice thus

  provide high- fidelity documentation of both global ice volume

  and temperature over time.

  The ice cores, and the much longer sea- sediment records,

  reveal that Chamberlin’s four ice advances were just the most

  recent of 30 that occurred over the 2.6 million- year span of the

  Pleistocene. And the throbbing signal of the Croll- Milankovitch

  cycles— a strong regular beat, with superimposed flutters— is

  unmistakable.17 For the first 1.5 million years of the Pleistocene,

  the 41,000- year obliquity cycle is especially evident. Then,

  140 Ch a pter 5

  around 1.2 million years ago, the pulse slows to the calmer

  100,000- year rhythm of the eccentricity cycle, like an electro-

  cardiogram readout for a patient who is falling off to sleep.

  This is called the mid- Pleistocene transition, and its cause is

  not completely understood. For one thing, of the three orbital

  variables, eccentricity has the smallest effect on solar radia-

  tion received by Earth, but for some reason the 100,000- year

  cycle became amplified by geologic processes. There are also

  higher- frequency “harmonics” in the climate records that do

  not correlate with orbital variations. Recurrent temperature

  oscillations of about 1500 years— the so- called Dansgaard-

  Oeschger cycles— seem to be a characteristic internal rhythm />
  corresponding to the timescales of global ocean currents. This

  means that the planet is not simply a puppet dancing to the

  imposed rhythms of astronomical cycles but that it takes those

  rhythms and riffs on them in its own way.

  H E AT O F T H E M O M E N T

  There is an even more important difference between the pre-

  dicted effects of the combined orbital cycles and the observed

  records of climate, and it further illustrates Earth’s capacity

  to improvise on themes by Milankovitch. The Milankovitch

  cycles are all essentially sine waves— symmetrical, palindromic

  hills and valleys. When superimposed, they create more com-

  plex patterns, but overall there is no systematic directional-

  ity to them— at a glance, it wouldn’t be clear which way time

  is flowing. The actual climate records from sea sediment and

  ice, in contrast, have an asymmetric, sawtooth geometry: long

  periods of cooling when Earth slid slowly into ice ages are

  punctuated by short and abrupt episodes of warming. That is,

  in each cycle, a tiny orbital nudge toward warmer conditions

  Great acceler ations 141

  was magnified by something in the Earth system into a heat

  wave, like a thermostat gone haywire. The cause of this amplifi-

  cation is preserved in the ice itself: greenhouse gases, especially

  carbon dioxide and methane (CH4), or swamp gas.

  When snow accumulates, air pockets remain between the

  crystals (making snow shelters surprisingly warm, because they

  are well insulated). At the poles, where snow doesn’t melt from

  one season to the next, it compacts as it is buried, and at a depth

  of about 60 m (200 ft) recrystallizes into ice. In the process, the

  air pockets shrink, but vestiges remain as bubbles suspended in

  the ice, like insects caught in amber. While there may be some

  migration of air between layers in this process, the gas bubbles

  trapped in polar ice are a direct record of past atmospheric

  compositions at the resolution of at least decades. These tiny

  bubbles tell us that over the last 700,000 years, global tempera-

  tures have been correlated at the very highest level of statistical

  significance with the concentrations of the greenhouse gases

  carbon dioxide and methane.

  So how could greenhouse gases take a small increment of

  Milankovitch warming and magnify it into a meltdown? The

 

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