Ocean Studies
Introduction to Oceanography
American Meteorological Society
Chapter 12
The Ocean and Climate Change
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Case in Point
– People and other animals living in the Arctic face an
uncertain future because of a recent warming trend.
• Inuit people (also called Eskimos) live around the Arctic
Ocean, in Greenland, Canada, Alaska, and Siberia, one of
Earth’s coldest regions and among the most remote and
inhospitable for humans.
– Still depend largely on hunting seals, caribou, and polar bears,
herding reindeer, and gathering berries and other foods from
the land
– Should the warming trend continue, the Arctic Ocean
could be nearly ice-free during summer as soon as
the year 2013.
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The Ocean and Climate Change
• Driving Question:
– How and why does climate change and how
does the ocean participate in and respond to
climate change?
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The Ocean and Climate Change
• In this chapter, we examine:
– The instrument-derived and reconstructed
climate record
– How climate changes through time
– What is known about the climate future
particularly relating to the world ocean
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The Climate Record
• Reliable instrument-based record of past
weather and climate is limited to not much more
than a century or so.
• For information on earlier fluctuations in climate,
scientists rely on reconstructions of climate
based on historical documents and longer-term
geological and biological evidence such as
bedrock type, fossil plants and animals, pollen,
tree growth rings, glacial ice cores, and deepsea sediment cores.
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The Climate Record
• MARINE SEDIMENTS AND CLIMATE
– Much of what we know about the climate fluctuations of the
Pleistocene Ice Age is based on analysis of the shell and
skeletal remains of microscopic marine organisms that are found
in deep sea sediment cores.
• Identification of the environmental requirements of these organisms
plus oxygen isotope analysis of their remains enables scientists to
distinguish between cold and mild climatic episodes of the past.
• Scientists use a special property of water to reconstruct large-scale
climate fluctuations of the Pleistocene Ice Age.
• A water molecule is composed of one of the two stable isotopes of
oxygen, O16 or O18.
– The lighter isotope (O16) is much more abundant than the heavier
isotope (O18).
– Small but significant variations occur in the amount of light oxygen
compared to heavy oxygen circulating in the global water cycle.
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The Climate Record
Sediment cores extracted from beneath the ocean floor provide
valuable information on the geologic and climatic past
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The Climate Record
• MARINE SEDIMENTS AND CLIMATE
– Seawater has more O18 at subtropical latitudes where
evaporation exceeds precipitation and less in middle
latitudes where rainfall is greater.
– Over time the proportion of light to heavy oxygen in
ocean water decreases with increasing glacial ice
cover.
– Oxygen isotope analysis of deep-sea sediment cores
indicates that the Pleistocene was punctuated by
abrupt changes between numerous glacial and
interglacial climatic episodes.
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The Climate Record
• OTHER PROXY CLIMATIC DATA SOURCES
– Pollen is a valuable source of information on late Ice
Age vegetation and climate, especially over the past
15,000 years.
• Pollen is dispersed by the wind and accumulates on the
bottom of lakes (and in other depositional environments)
along with other organic and inorganic sediments.
– Scientists use a corer to extract a sediment column
(core) that chronicles past changes in pollen (and
therefore, vegetation).
– Changes in dominant pollen type in a core signal
changes in nearby vegetation, likely in response to
climate change.
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The Climate Record
• OTHER PROXY CLIMATIC DATA SOURCES
– Tree growth rings can yield a year-to-year record of
past climate variations stretching back many
thousands of years.
• Tree growth rings are especially sensitive to moisture stress
and have been used to reconstruct lengthy chronologies of
drought prior to the era of instrument based records.
– Ice cores extracted from glaciers yield a record of
past seasonal snowfall preserved as thin layers of ice.
• Using the oxygen isotope technique scientists can distinguish
between cold and mild episodes in the past and through
chemical analysis of tiny air bubbles trapped in the ice.
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The Climate Record
This 6-m (20-ft) long ice core was
extracted from the Greenland
ice sheet and is a source of
information on past variations in
climate and atmospheric composition.
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The concentration of
atmospheric carbon dioxide
in parts per million by volume
from about 425 million years
ago to 2007 based on glacial
ice core analysis from the
Vostok station in Antarctica.
The Climate Record
• GEOLOGIC TIME
– The geologic past and its climate record is subdivided using the
geologic time scale, a standard division of Earth history into
eons, eras, periods, and epochs based on large-scale geological
events.
– Geologic evidence points to an interval of extreme climate
fluctuations about 570 million years ago, corresponding to the
transition between Proterozoic and Phanerozoic Eons.
– The Mesozoic Era, from about 245 million to 70 million years
ago, was characterized by a generally warm Earth free of large
glacial ice sheets.
– The Cenozoic Era was a time of great climatic fluctuations.
– By 40 million years ago, however, Earth’s climate began shifting
toward colder, drier, and more variable conditions setting the
stage for the Pleistocene Ice Age.
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The Climate Record
• PAST TWO MILLION YEARS
– Over the past two million years mountain ranges, continents, and
ocean basins were essentially as they are today.
– The climate was unusual in favoring the development of huge
glacial ice sheets.
– During the Pleistocene Ice Age the climate shifted numerous
times between glacial climates and interglacial climates.
• A glacial climate favors the thickening and expansion of glaciers.
• An interglacial climate favors the thinning and retreat of existing
glaciers or no glaciers at all.
– During major glacial climatic episodes of the Pleistocene, the
Laurentide ice sheet developed over central Canada and spread
westward to the Rocky Mountains, eastward to the Atlantic
Ocean and southward over the northern tier states of the United
States.
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The Climate Record
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Extent of glacial ice cover over North
America about 18,000 to 20,000 years ago,
the time of the last glacial maximum.
The Climate Record
• PAST TWO MILLION YEARS
– An increase in the magnitude of a climatic change
with increasing latitude is known as polar
amplification, indicating that polar areas are subject
to greater changes in climate.
– A general warming trend followed the last glacial
maximum, punctuated by relatively brief returns to
glacial climatic episodes.
• Relatively cold interval from about 11,000 to 10,000 years
ago known as the Younger Dryas.
– The present interglacial is known as the Holocene.
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Reconstructed records of (A) the variation in
global glacial ice volume over the past
600,000 years based on analysis of the
oxygen isotope ratio of shells in deep-sea
sediment cores, and (B) temperature
variation over the past 160,000 years
derived from oxygen isotope analysis of an
ice core extracted from the Antarctic ice
sheet at Vostok and expressed as a
departure in Celsius degrees from the 1900
global mean temperature.
Reconstructed temperature variation
over the past 18,000 years based on
a variety of proxy climatic indicators
and expressed as a departure in
Celsius degrees from the 1900
global mean temperature.
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The Climate Record
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Reconstructed temperature variation over the past 1000
years based on analysis of historical documents and
expressed as a departure in Celsius degrees from the
1900 global mean temperature
The Climate Record
• INSTRUMENT-BASED TEMPERATURE
TRENDS
– The most reliable temperature records date from the
late 1800s with the birth of national weather services
including those of the U.S. and Canada, along with
the predecessor to today’s World Meteorological
Organization (WMO).
– The trend in global mean temperature is generally
upward from 1880 until about 1940, downward or
steady from 1940 to about 1970, and upward again
through the 1990s and early 2000s.
– In spring 2000, NOAA’s NCDC reported that global
warming accelerated during the final quarter of the
20th century.
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Instrument-derived trends in mean annual global (land plus ocean),
sea-surface, and land temperatures; expressed as departures in
degrees Celsius and degrees Fahrenheit from the 125-year period
average.
The Climate Record
• INSTRUMENT-BASED TEMPERATURE
TRENDS
– A general consensus in the scientific
community holds that a global-scale warming
trend has prevailed since the end of the Little
Ice Age.
• The simplest scientific explanation for the
observed warming trend is the steady build-up of
carbon dioxide in the atmosphere and the
consequent enhancement of the natural
greenhouse effect.
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Lessons of the Climate Past
– Climate is inherently variable over a broad spectrum
of time scales ranging from years to decades, to
centuries, to millennia.
– Variations in climate are geographically non-uniform
in both sign (warming or cooling) and magnitude.
– Climate change may consist of a long-term trend in
various elements of climate and/or a change in the
frequency of extreme weather events.
– Climate change tends to be more abrupt than
gradual.
– Only a few cyclical variations can be discerned from
the long-term climate record.
– Climate change impacts society.
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Factors Contributing to Climate
Change
• CLIMATE AND PLATE TECTONICS
– Plate movements are so slow compared to the span
of human existence that we can consider topography
and the geographical distribution of the ocean and
continents as essentially fixed controls of climate.
– Over the vast expanse of geologic time plate
tectonics was a major player in large-scale climate
change.
• Changes in the location of continents (continental drift)
altered the local and regional radiation budget and the
response of air temperature.
• Opening and closing of ocean basins changed the course of
heat-transporting ocean currents and altered the
thermohaline circulation.
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Factors Contributing to Climate
Change
• CLIMATE AND SOLAR VARIABILITY
– Fluctuations in the sun’s energy output, sunspots, or
regular variations in Earth’s orbital parameters are
external factors that can alter Earth’s climate.
– Satellite monitoring reveals that the sun’s energy
output varies directly with sunspot number.
– More sunspots may contribute to a warmer global
climate and fewer sunspots may translate into a
colder global climate.
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Factors Contributing to Climate
Change
Variation in mean annual sunspot number
since the early 17th century.
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Factors Contributing to Climate
Change
• CLIMATE AND EARTH’S
ORBIT
– Milankovitch cycles are
regular variations in the
precession and tilt of
Earth’s rotational axis and
the eccentricity of its orbit
about the sun.
• Caused by gravitational
influences exerted on
Earth by other large
planets, the moon, and the
sun
• Drive climate fluctuations
operating over tens of
thousands to hundreds of
thousands of years
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Factors Contributing to Climate
Change
• CLIMATE AND EARTH’S ORBIT
– The tilt of Earth’s spin axis changes from 22.1
degrees to 24.5 degrees and then back to 22.1
degrees over a period of about 41,000 years.
• As the axial tilt increases, winters become colder and
summers become warmer in both hemispheres.
– The shape of Earth’s orbit about the sun changes
from elliptical to nearly circular in an irregular cycle of
90,000 to 100,000 years.
• Changes the amount of solar radiation received by the planet
– Milankovitch cycles do not alter appreciably the total
amount of solar energy received by the Earthatmosphere-ocean system annually, but they do
change significantly the latitudinal and seasonal
distribution of incoming solar radiation.
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Factors Contributing to Climate
Change
• CLIMATE AND VOLCANOES
– Only explosive volcanic eruptions rich in sulfur dioxide
are likely to impact global or hemispheric climate and
then only for a few years at most.
• A violent volcanic eruption can send sulfur dioxide high into
the stratosphere.
• Sulfur dioxide then combines with water vapor to form tiny
droplets of sulfuric acid and sulfate particles, collectively
called sulfurous aerosols.
– Can remain suspended in the stratosphere for many months to
perhaps a year or longer before they cycle to Earth’s surface
– Absorb both incoming solar radiation and outgoing infrared
radiation warming the lower stratosphere
– Also reflect solar radiation to space
– A violent sulfur-rich volcanic eruption is unlikely to
lower the mean hemispheric or global temperature by
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Factors Contributing to Climate
Change
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Large-scale cooling often followed
massive volcanic eruptions that emitted
sulfur dioxide into the stratosphere.
Factors Contributing to Climate
Change
• CLIMATE AND EARTH’S SURFACE
PROPERTIES
– Any change in the physical properties of Earth’s water
or land surfaces or in the relative distribution of
ocean, land, and ice may affect Earth’s radiation
budget and climate.
• Variations in mean regional snow cover may contribute to
climate change because an extensive snow cover has a
chilling effect on the atmosphere.
• Changes in Earth’s sea ice or glacial ice coverage are likely
to have longer-lasting effects on climate.
• Changes in ocean circulation and sea-surface temperatures
(SST) contribute to large-scale climate change.
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Factors Contributing to Climate
Change
• CLIMATE AND HUMAN ACTIVITY
– In 2007, the Intergovernmental Panel on Climate
Change (IPCC) concluded that global warming since
the mid-20th century very likely (estimated probability
of greater than 90%) was caused mostly by human
activities.
– Many human activities affect climate over broad
ranges of spatial and temporal scales:
• Modification of the landscape
– Clear-cutting of forests
– Urbanization (the urban heat island effect)
• Combustion of fossil fuels
– Alters concentrations of certain key gaseous and aerosol
components of the atmosphere
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Factors Contributing to Climate
Change
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Trend in atmospheric carbon dioxide concentration
since 1957, based on measurements made at the
Mauna Loa Observatory in Hawaii.
Factors Contributing to Climate
Change
• CLIMATE AND HUMAN ACTIVITY
– Humankind’s contribution to the buildup of
atmospheric CO2 began thousands of years ago with
the clearing of land for agriculture and settlement.
– By the mid-19th century, growing dependency on coal
burning associated with the beginnings of the
Industrial Revolution triggered a more rapid rise in
CO2 concentration.
• The concentration of atmospheric CO2 is now about 35%
higher than it was in the pre-industrial era.
– The ocean takes up 56.2% of the carbon dioxide of
anthropogenic origin (via photosynthesis and cold
surface waters absorbing CO2 and sinking) while
terrestrial biomass is a sink for 13.7%.
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Factors Contributing to Climate
Change
• CLIMATE AND HUMAN ACTIVITY
– Oceanic uptake of carbon dioxide is likely to
decline in the future as the ocean becomes
more stratified in response to higher global
temperatures.
– Rising levels of other infrared-absorbing
gases will also likely enhance the greenhouse
effect.
• Methane, nitrous oxide, and halocarbons
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The Climate Future
• GLOBAL CLIMATE MODELS
– A global climate model is a simulation of Earth’s
climate system.
• Differs from numerical models used for weather forecasting
in that it predicts broad regions of expected positive and
negative temperature and precipitation anomalies and the
mean location of circulation features such as jet streams and
principal storm tracks over much longer time scales
• Used to predict the potential impacts on climate of rising
levels of atmospheric carbon dioxide and other greenhouse
gases
– Using current boundary conditions, a global climate model
simulates the present climate. Then, holding all other variables
constant, the concentration of carbon dioxide (or another
greenhouse gas) is elevated and the model is run to a new
equilibrium state.
• Scientists take the ensemble approach to climate simulation.
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The Climate Future
• ENHANCED GREENHOUSE EFFECT AND
GLOBAL WARMING
– According to the 2007 IPCC Assessment Report, the
climate models predict that over the next 20 years,
the global mean annual temperature will increase at
an average rate of about 0.2°C per decade.
– Enhancement of the greenhouse effect could cause a
climate change that would be greater in magnitude
than any previous climate change over the past
10,000 years.
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Impact of Climate Change on the
Ocean
• SEA LEVEL FLUCTUATIONS
– During glacial climatic episodes, glaciers on land
thicken and expand, and the volume of water in the
ocean basins decreases.
• Conversely, during interglacial climatic episodes, glaciers on
land thin and retreat, and the volume of water in the ocean
basins increases.
– Seawater always contracts when its temperature
drops and expands when its temperature rises.
– Persistence of the global warming trend appears
likely to cause sea level to rise in response to melting
of land-based polar ice sheets and mountain glaciers,
coupled with thermal expansion of seawater.
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Impact of Climate Change on the
Ocean
• SEA LEVEL FLUCTUATIONS
– Waxing and waning of glaciers plus ocean
temperature fluctuations are two factors that govern
eustasy, the global variation in sea level brought
about by a change in the volume of water occupying
the ocean basin
Annually averaged
sea level variation
based on
TOPEX/Poseidon
measurements for
1993-2000
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Impact of Climate Change on the
Ocean
• SEA LEVEL FLUCTUATIONS
– Climate models predict that global warming
will cause a rise in mean sea level in the
range of 30 to 40 cm (12 to 16 in.) during the
21st century.
• Thermal expansion of ocean waters would account
for more than 60% of the rise with the balance due
to melting glaciers.
• Rising sea level would disrupt coastal ecosystems
and could threaten historical, cultural, and
recreational resources.
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Map of the U.S. Geological
Survey’s Coastal
Vulnerability
Index (CVI) for Cape Cod
National Seashore, MA
showing the vulnerability of
the coast to changes in sea
level. The CVI is based on
tidal range, wave height,
coastal slope, historic
shoreline change rates,
geomorphology, and historic
rates of relative sea level
change due to eustatic sea
level rise and tectonic uplift
or subsidence.
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Impact of Climate Change on the
Ocean
• ARCTIC SEA ICE COVER
– Shrinkage of Arctic sea ice is likely to trigger
an ice-albedo feedback mechanism that
would accelerate melting of sea ice and
amplify warming.
– As sea ice cover shrinks, the greater area of
ice-free ocean waters absorbs more solar
radiation, sea surface temperatures rise, and
more ice melts.
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Impact of Climate Change on the
Ocean
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Positive ice-albedo feedback in the Arctic
is likely to accelerate warming of surface
waters and shrinkage of sea ice cover.
Impact of Climate Change on the
Ocean
• ARCTIC SEA ICE
COVER
– Less sea ice cover on the
Arctic Ocean is likely to
increase the humidity of the
overlying air leading to
more cloudiness.
– Shrinkage of the Arctic sea
ice may be the direct
consequence of higher air
temperatures or indirectly
the result of changes in
ocean circulation.
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Arctic sea ice has been shrinking at
least since the time satellite monitoring
began in 1979, reaching a new end-ofseason record low in 2007.
Impact of Climate Change on the
Ocean
• MARINE LIFE
– Climate change could alter the physical and chemical
conditions in the ocean, perhaps exceeding the
tolerance limits of organisms.
• Shrinkage of Arctic sea ice may have serious implications for
organisms living on, within, or under the ice.
• Global warming could raise the sea-surface temperature
sufficiently to threaten coral reefs.
– Organisms are particularly vulnerable to
environmental change that affects a limiting factor.
• A limiting factor is an essential resource that is in lowest
supply compared to what is required by the organism.
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Mette Nielson and Rolf Gradinger are part of an international team
studying the effects of diminishing Arctic ice cover on the marine
community living on, within, and below the ice. Here, they are
extracting ice cores for later analysis.
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Conclusions
– For information on prior climate regimes,
scientists must rely on a variety of
documentary, geological, and biological proxy
climate indicators.
– Although the climate record loses detail,
continuity, and reliability with increasing time
before present, it is evident that climate
changes over a broad spectrum of time
scales.
– The interaction of many factors is responsible
for the variability of climate.
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