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METEOROLOGY
GEL-1370
Chapter Fourteen
Climate Change
Goal for this Chapter
We are going to learn answers to the following
questions:
• How the global climate has changed in the past
• Theories on why the global climate has changed?
If all the ice locked up in glaciers and ice sheets were
to melt, what would happen to this town in coastal
south Florida? (65m or 213 ft rise)
If all the ice melts what happens??
Existence of Land Bridge in the Bering Strait
Changing Climate
• Glaciers cover ~10% of earth’s land surface (mainly
Greenland and Antarctic ice sheets)
• If the all glaciers and ice sheets melt, global sea level
rise will be ~ 65 m (213 feet)
• Fossil pollen of a tundra plant in sediment layer in New
England dated to be 12,000 yrs --- climate was much
colder; ocean sediment records and ice core records--• ~18 kyr ago, alpine glaciers extended to river valleys
and huge ice sheets covered vast areas of North America
and Europe; New and Ohio River Valley were also
covered with ice
• Glaciers advanced and retreated 10 times during the last
2 million years – Are we still in the ice age (warmer part
of it??)
Past climate changes
• CLIMAP (Climate: Long-range Investigation Mapping
and Prediction) brought wealth of information
• Studies on Oxygen isotope ratios -- d18O = [(18O/16O)sam - (18O/16O)sam]/ (18O/16O)samx 1000
• Reconstruction of earth’s surface ocean temperature for
various times during the past
• Vertical ice cores in Antarctica and Greenland --- info on
past temperature patterns
• Glacier formation: Snow accumulates --- compaction --recrystallize into ice --- ice begins to move by gravity –
glacier is born
• In CLIMAP, the earth’s surface ocean temp for various
times in the past was reconstructed;
Sea surface isotherms during August 18 kyr ago
and today (Left: 18 kyr ago)
Earth’s changing climate
• Global climate was much warmer (8-15° C) than today
in the geological time frame; polar region was ice-free
• 700 and 300 myr ago glacial periods existed
• 65 million years ago, the earth was warmer than now –
polar ice did not exist; beginning 55 million yrs, the
earth entered a long cooling trend and polar ice caps
appeared; ~10 myr ago, deep blanket of ice covered
Antarctic; snow and ice began to accumulate in high
mountain valleys of the Northern Hemisphere, and
alpine or valley glaciers soon appeared
• The Pleistocene epoch or the Ice Age began about 2 myr
ago; glaciers appeared in the Northern Hemisphere,
marking the beginning of the Pleistocene Epoch
Earth Changing Climate – contd.
• Pleistocene: Glaciers advanced and retreated over large
portions of North America and Europe
• Between the glacial advances , warmer periods that
lasted for 10 kyr or more (interglacial periods)
• During the last glacial maximum (LGM), 18-22 kyr, sea
level was 126 m before present --- vast areas exposed;
Siberia & Alaska were connected (migration of animals
and humans from Asia to North America)
Past climate – contd.
• Bubbles of air trapped in the ice --- past composition of
the atmosphere, wind direction and intensity
• Dendrochronology: Changes in the thickness of the
rings indicate climatic changes from one year to the next
(useful in trees that are stressed by temp or moisture
during their growing season)
• Reconstruction of past climate is based on a) lake and
ocean sediments; b) loess and other soil deposits c)
pollen in the ocean sediments, deep ice caves, soil
deposits; d) fossils in sediments; e) oxygen isotope
ratios of corals and other carbonate species (foram, etc);
f) cave deposits (stalactites & stalagmites)
Pollen concentration vs depth
Coiling pattern vs climate
Oxygen isotope composition
Climate change during the last ~ 2 myr
Climate through Ages – 18 kyr
• Ice began to retreat ~14 kyr ago as surface temp slowly
rose; ~11 kyr ago, average temp suddenly dropped and
northeastern North America and northern Europe
reverted back to glacial conditions (Younger Dryas)
• ~10 kyr ago, the cold spell ended and ~8 kyr,
continental ice sheets over North America disappeared;
• ~5-6 kyr BP, climate was ~1°C warmer than at present
(Mid-Holocene maximum
• Holocene Epoch: The second epoch of the Quaternary
Period, beginning ~10 kyr ago and continuing to the
present time
• ~5 kyr BP, cooling trend set in, alpine glaciers returned,
but not continental ice sheets
Average air temp variations for the past 18 kyr
Average temp variations of eastern Europe for the
last 1200 yrs
Climate during the last 1 kyr
• Ice core date from Greenland reveal rapid shifts in
climate (from ice stage conditions to a warm state)
• ~1200 AD, mild climate of western Europe changed to
extreme variations; climate grew stormy; great floods
and great droughts occurred; cold winters followed by
relatively warm ones; several famines during the 1300s
• Little Ice Age: Period from ~1550 to 1850 when average
global temperatures were lower (0.5°C), and alpine
glaciers increased in size and advanced down mountain
canyons
• 1816: Unusually cold summer followed by a bitterly cold
winter
Climate during the last 1-kyr – contd.
• 1970s – early 1980s: Average yearly temp showed
considerable fluctuation from year to year & from region
to region, with overall trend pointing to warming
• Warning trend continued into the 1990s
• 20th century warming is ~0.7°C; warming is not uniform;
greatest warming over the mid-latitude continents in
winter and spring while a few areas (North Atlantic
Ocean) have actually cooled in recent decades; US has
experienced little warming
• Warming of 0.7°C is quite significant, as the global temp
have varied no more than 1.5°C during the past 10,000
yrs
• Part of the warming is due to an enhanced Greenhouse
effect caused by increasing levels of greenhouse gases
Causes of Climate Change
• Intricate relationship between atmosphere-oceancontinent coupling --- not fully known
• Feedback mechanisms is not fully characterized
• Positive Feedback: Initial increase in temperature is
reinforced by the other process
• Example: Earth has entered a slow warming trend --temperature rises --- ocean water evaporates rapidly --water vapor content in the atmosphere increases --- more
IR energy is absorbed --- leads to warming --- more
water vapor evaporation --- and so on – Water vaportemperature rise feedback – Such a chain reaction is
called ‘Runaway greenhouse effect’ – no evidence that a
runaway greenhouse effect ever occurred on earth!
Causes of climatic change – contd.
• There are checks and balances; for example, when air
temp increases, outgoing IR energy increases and this
helps to slow the temp change and stabilize the climate
• Example for negative feedback: Air warms --- more
evaporation --- more moisture (and precipitation) --global low cloudiness increases --- large percentage of
incoming sunlight is reflected --- less energy to heat the
surface --- cooling
• All feedback mechanisms work simultaneously in both
directions; example: increase in global surface air temp -- to melt snow and ice more --- decrease in albedo --more solar radiation to reach the surface
Climate change and feedback mechanisms
• Lower temp --- greater snow cover --- increase in albedo
--- more light reflected back --- further drop in temp --runaway ice age --- not found on the earth
• CLIMATE CHANGE, PLATE TECTONICS, &
MOUNTAIN-BUILDING:
– Glacial features found near the sea level in Africa today – area
underwent a period of glaciation hundreds of millions of years
ago
– Fossil remains of tropical vegetation can be found under layers
of ice in polar regions today
If land masses are redistributed (concentrated in middle
& high latitudes), ice sheets are likely to form (more
light reflected --- cooling) –influence the path of
ocean currents --- affect the transport of heat & wind
Relationship between global change and plate tectonics
• Rate at which plates move (currently 1-10 cm/yr) might
be related to the climatic changes took place in millions
of years; rapid spreading, a relatively wide ridge forms,
causing sea level to rise relative to the continents; at the
same time, an increase in volcanic activity vents large
quantities of CO2 into the atmosphere --- increased
greenhouse effect --- increase in global temperature
• Increased temperature leads to high precipitation --increased chemical weathering and hence faster removal
of CO2
• Mountain building processes lead to collision of two
continental plates --- can have a marked influence of
global circulation patterns (e.g., Tibetan Plateau)
Geographical distribution of landmasses 180 myr ago
(a) & today (direction of continental movement)
During times of rapid spreading, increased volcanic activity
may promote global warming by increased CO2 emission
Milankovitch Theory
• Three separate cyclic movements combine to produce
variations in the amount of solar energy that falls on the
earth
– Changes in the eccentricity of the earth’s orbit (100,000 yrs);
the difference in distance is only 3% while the increase in the
solar energy is ~7% received at the top of the atmosphere
from July to January; when the difference in distance is 9%,
the difference in solar energy will be ~20%
– Precession (23,000 yrs) of the equinoxes; as the earth rotates
on its axis, it wobbles like a spinning top; Presently, the earth
is closer to the sun in January and farther away in July. Due to
precession, the reverse will be true in ~11,000 yrs
– Changes in the tilt of earth (41,000 yrs); present tilt is 23½°,
but during the 41,000 yr cycle it varied from about 22° to
24½° ; smaller the tilt, less variations between seasons
Earth’s orbital changes (eccentricity) – 100 kyr
cycle
Milankovitch theory
(a): Changes in the tilt (41 kyr); (b): presently earth is closer
to the sun when N.Hemisphere is in winter; (c) 11 kyr from
now, earth will be closer to sun in July when N.
Hemisphere is in summer
Past climate changes
• CLIMAP project results: during the past 800 kyr, ice
sheets have peaked about every 100 kyr; superimposed
on this are smaller advances that show up at intervals of
about 41 kyr and 23 kyr
• CO2 levels were about 30% lower during glacial
periods than during interglacial periods; ice core studies
also reveal CH4 follows CO2 pattern
• Why CO2 levels have varied as glaciers expanded and
contracted – Due to changes in the biological
productivity (phytoplankton productivity)
• Changing levels of CO2 indicate a shift in ocean
circulation patterns---changes in the precipitation &
evaporation rates & wind intensity and direction
Climate Change - contd
• Other factors that can affect temperature variations are:
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Dust and other aerosol contents in the atmosphere
Changes in the albedo of the ice sheets and clouds
Isostatic changes due to depression by ice (during interglacial period)
Presence of other trace gases
Aerosol sources: Factories, auto emissions, wild fires, dust
storms, etc; some particles (soil dust & sulfate particles)
reflect and scatter incoming sunlight while others (smoky
soot) absorb sunlight – warming
Overall net effect of human-induced aerosols on climate is to
cool the climate
Sulfate aerosols: serve as cloud condensation nuclei, and scatter
and reflect incoming sunlight
Volcanic eruption and climate
• Sulfur-rich gases have the greatest impact
• Two largest volcanic eruptions in the last 100 yrs are: El
Chichòn in Mexico (April 1982) & Mount Pinatubo in
Philippines (June 1991)
• Mathmatical models predict global mean temp can drop
by 0.2 to 0.5°C
• Acidic ice was uncovered from ~AD 1350 to ~ 1700 –
corresponding to Little Ice Age – sulfur-rich volcanic
eruptions may have played an important role in
triggering this cool period? Other cool periods were also
triggered by such events??
Changes in average global air temp from 1990-1992 after
eruption of Mt. Pinatubo, June 1991; temp decreased by
~0.5°C
Variations in Solar Output
• 11-year sunspot cycle
• Sun’s energy output appears to change slightly with
sunspot activity
• Maunder minimum: Between 1645 and 1715, there were
few sunspots; this period is called so
• Changes in sun’s energy output – cause for climate
change??
CO2, Greenhouse effect and global warming
• Annual average in 1999 368 ppm; 1.5 ppm/yr increase;
500 ppm by end of this century
• Increase in CH4, N2) CFCs effect ~ CO2 in their ability
to increase greenhouse effect
• Temperature increase of 1-3.5°C
Human activity contributes to global warming
Ocean conveyor belt in the N. Atlantic
chapter –14- Summary
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Variations in the oxygen-18/16 ratios-climate info
CLIMAP Project; global climate change in the past
Glaciers – glacial and interglacial periods
Effect of volcanoes on global change
Positive and negative feedbacks
Milankovitch theory – 3 periodicities; factors that cause
Sunspot cycle and climatic changes
Maunder minimum
CO2 levels during glacial and interglacial periods
Relationship between increased temp and amount of pptn
Fertilization effect, ocean as a source/sink of CO2