CHAPTER 11 CLIMATIC CHANGE AND VARIABILITY
Introduction to Climatic Change and Variability
 Random variability
 Periodic variability
 Variability changes
 Temporal trends
 Step changes
Climatic Changes in Geologic History
 Temperature
o Faint Young Sun Paradox
o Global demise of methanogens may have caused first global ice age, nearly 2.5
billion years ago, in the beginning of the Proterozoic Eon
o Snowball Earth hypothesis from evidence dating to 800-600 million years ago
o Gondwanan Ice Age – named for the large southern hemisphere continent that
resulted from the break-up of Pangaea 320-250 million years ago
o K-T extinction at 65 million years ago
o Because “permanent” ice exists on earth today (ice sheets of Greenland and
Antarctica, along with numerous mountain glaciers), we are currently in the final,
major, known ice age in geologic history
o Ice ages are subdivided into intervals when “permanent” ice is displaced
equatorward (glacial phases) and intervals when ice retreats poleward
(interglacial phases)
o Currently, in interglacial phase
o Wisconsin Glacial Phase in the Pleistocene Epoch (approximately 18,000 years
ago), “permanent” ice covered most of North America north of the Ohio River and
extended across the pole to cover most of Europe and Asia
 Ice Ages and Sea Level
o During the Wisconsin Glacial Phase, eustatic sea level may have been as much
as 170 m (550 ft) below today’s level, exposing large sections of the continental
shelves
o A common misconception is that the extra sea ice during cold periods would
displace the same amount of water as its liquid equivalent, so its formation does
not contribute to sea level decreases
o It is a misconception that melting sea ice during warm periods does not contribute
to increased sea levels
o If all of the world’s current cryosphere were to melt, then sea level would rise by
about 67 m (220 ft), and substantial areas of the coastal plains would be flooded
worldwide
o Some places in Scandinavia and Alaska that were covered by substantial depths
of ice during the Wisconsin Glacial Phase are now rising (Isostatic rebound)
because of the release of the tremendous pressure/weight of the ice
 Recent Trends
o Younger Dryas, (in Europe known as the Loch Lomond Stadial) between
approximately 8,000 and 5,000 years before present, a time known as the
Hypsithermal, in which global average temperatures were perhaps 2 C° (4 F°)
above today’s levels
o Approximately 900 A.D., a warm interval known as the Little Climatic Optimum
(LCO – also known as the Medieval Warm Period) began
o During the Little Ice Age (LIA) from about 1450 to 1850, streams in Europe that
do not freeze today were observed to freeze regularly
o The ten warmest years of the instrumental record all occurred since 1990
o An estimated global sea level rise of approximately 15 cm (6 in) has accompanied
the warming of the 20th century
How Do We Know What We Know about Past Climatic Changes?
 Paleoclimatologists can make educated guesses using proxy evidence to ascertain
climatic conditions of the distant past and assess the timing and magnitude of climatic
change or variability
 Basic principles
o Preponderance of evidence is used to reach conclusions about the climate of a
particular place at a particular time in the past
o Principle of Uniformitarianism - evidence that would accompany a certain
climatic type today would be assumed to suggest the same kind of climate at the
time in the past when similar evidence was produced
o Catastrophism - Catastrophic changes do not occur uniformly, but from one
equilibrium to a drastically different equilibrium in only a short time geologically
 Radiometric dating
o Based on the principle of radioactive decay – rate at which spontaneous nuclear
disintegration of certain isotopes of certain elements occurs
o Compare the amount of parent isotope remaining to the amount of resulting
daughter isotope using mass spectrometry techniques
o Radioactive isotopes that have relatively short half-lives are more useful for
relatively young samples
o Advantages - the only absolute dating techniques that go back farther in geologic
history than a few hundred years; scientifically solid method
 Lithospheric and Cryospheric Evidence
o Principle of Superposition – Older evidence will be found successively lower in
sedimentary rock layers
o Cross-cutting relationships – Many regions exhibit igneous rock formations that
cross-cut pre-existing sedimentary rock layers
o Relic landforms can also suggest features of past climates
o Glaciers leave behind erosional and depositional landforms or features that
provide geological hints of their presence in the past including glacial moraines,
rock striations, deposits of loess, and specific erosional and depositional
landforms related to past glacier formation, motion, and ablation
o Glacial erratics suggest that a glacier must have moved rock from a location near
other rocks similar to its composition to present location
o The discrepancy between local soils and vegetation characteristic of colder
climates and a warmer regional climate present in their locations today can give
hints of a past climate to which the local environment has not adjusted
o Ice cores from deep holed drilled into ice sheets can be sampled in a laboratory,
and clues about the paleochemistry of the atmosphere can be found from air
preserved within bubbles trapped in ice when the ice formed
o Varve thickness can be related to climatic fluctuations on an annual or even
seasonal scale
 Biological Evidence
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o Fossilized rock allows paleoscientists to infer the climatic conditions that must
have existed at the time and place that the fossil was formed
o Foraminifera can provide clues about the salinity of the oceans during their
existence, and salinity is related to the energy and water budgets of the
atmosphere
o Composition of the exoskeletons of other fossilized microscopic marine organisms
also provides clues about ancient atmospheric temperature
o Palynology – the scientific study of pollen and spores – coring and radiocarbon
dating can identify a temporal sequence of pollen/spore types; a useful tool that
can provide important information about the paleoclimatic conditions, including
circulation, especially when used in conjunction with other types of paleoclimate
evidence
o Dendroclimatology can provide climatic evidence dating back hundreds or even
thousands of years before present and offers the advantage of an annual and
even a sub-annual record; especially useful in precipitation and streamflow
reconstructions
o Other sources of biological evidence include: analysis of the proportion of calcium
carbonate (CaCO3) generated from exoskeletons in marine sediments, coral reefs,
and treeline fluctuations
Historical Data refers to written information about climate but does not include formal
meteorological records
o Advantage: abundance of evidence for recent times
o Disadvantage: results are not usually quantified, so it can be difficult to ascertain
meaning
Converging evidence from several proxy data sources must be used before reaching a firm
conclusion
Natural Causes of Climatic Change and Variability
 Continental Drift and Landforms
o Positions of continents can affect ocean circulation, and therefore temperature, to
a significant extent
o The positions of mountainous regions can be important producers of climatic
change
o Mountain uplift can affect the synoptic-scale atmospheric circulation
 Milankovitch Cycles
o Eccentricity of earth's orbit around the sun – gradually changes from a more
circular to a more elliptical orbit, with a periodicity of approximately 95,000 years.
o Tilt – the angle of tilt of earth’s axis does not change through the course of the
year on human time scales, but on geologic time scales the tilt does slowly change
from 21.8° from the vertical to 24.4° from the vertical
o Precession of the equinoxes - Earth's axis wobbles like a spinning top because
the earth is not a perfect sphere
o Climatic changes can result from changes in eccentricity, tilt, and/or precession
 Volcanic Activity
o The uneven frequency of volcanic activity may be explained by differences in the
number and strength of subduction zones through geologic time
o Periods of active volcanism may enhance the greenhouse effect by contributing
CO2 to the atmosphere, leading the earth into an ice-free period, and the slow
removal of that CO2 may allow for the onset of an ice age
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o On shorter time scales, high concentrations of dust ejected during periods of
intense and frequent volcanism may diminish insolation at the surface, cooling the
climate
o The addition of matter to the atmosphere from volcanism can disrupt the global
atmospheric circulation, resulting in unusual and/or extreme weather events
Variations in Solar Output
o Although t is assumed that increased solar output would be associated with a
warmer climate, an inverse relationship between solar output and temperature
could possibly exist
o Sunspots – huge magnetic storms that appear as darker regions on the sun’s
surface, varying in number and intensity over time
ENSO Events
o ENSO affects the mid-latitude atmospheric circulation systems, because the
shearing motion associated with circulation at the poleward edges of the Hadley
cells affects mid-latitude circulation
o Monsoon circulations tend to weaken during El Niño, and Atlantic tropical
cyclone activity tends to be suppressed during El Niño because of the abnormally
strong westerly winds, which shear off the tops of developing tropical cyclones.
o El Niño or La Niña events produce significant climatic variability
o Quasi-biennial Oscillation (QBO) is a periodic reversal of the equatorial
stratospheric winds between westerly and easterly, with a cycle of approximately
26 months