Part 6. Current, Past, and
Future Climates
Chapter 16
Climate Changes: Past and Future
Introduction
Climate change is the change in the statistical
properties of one or more atmospheric
variables
• Climate changes on many different time scales
• Climate change is greatest at the Earth’s poles
and least in tropical regions
• Understanding climate change requires
understand the physical cause or causes of the
climate change
The Geologic
Column
Past climates in Earth
history can be inferred from
geologic and fossil evidence
Human history starts
Pleistocene ice
ages
Dinosaurs are wiped
out
High sea level
stands during the
Cretaceous
Major extinction
of life
High sea level
stands during the
Paleozoic
First multicell animals
Age of Earth - 4.6 billion years
Warm Intervals and Ice Ages
• For most of Earth’s history, climate was 515oC warmer than present, and ice was rare
• Brief cold ice ages interspersed generally
warm climate
– Over past 2.5 billion years, ice ages
occurred only 10-20% of the time
The Earth began a gradual
cooling phase about 55
Mya. Ice accumulated on
Antarctica about 34 Mya.
By 10 Mya, Antarctica
was covered with ice. By
4 Mya, so was Greenland.
The Earth has
demonstrated regular
glacial/interglacial cycles.
The Earth currently is in
a warm phase. The last
glacial phase peaked
about 20,000 years ago.
The last interglacial may have been the
warmest time in Earth history
• Peaked ~ 125,000 years ago
• Air temperature about 2°F to 5°F warmer
than present
• Sea levels about 20 feet higher than at
present
Pollen diagrams detail past vegetation and climate information
Ice extent during the last glaciation
During the last glaciation, North America was covered with ice more than 2 miles
thick in places; the ice extended as far south as St. Louis, Missouri. Sea level was
more than 300 feet lower than today. The extent and thickness of the ice is
estimated from geologic evidence.
Ice extent about
20,000 years ago;
the glacial ice
started retreating
about 15,000
years ago; the ice
readvanced about
13,000 years ago
for about 1200
years during the
Younger Dryas
period
Ice extent
about
20,000
years ago;
the ice left
New
England
about
12,000
years ago
Earth Temperatures for the last 1000 years
Medieval warm period
Little ice age
Current
warming
July moisture Tidewater region of Virginia and North Carolina
Historic
climate; dry
periods
affected early
North
American
settlements
The shorter term
Dansgaard-Oeschger
cycles and longer-term
Bond cycles attempt to
explain the regularity of
warming and cooling
events during the past
150,000 years. These
cycles are probably
caused by changes in
ocean circulation,
atmospheric circulation,
and insolation.
Factors Involved in Climatic Change
• Variations in
– Insolation intensity
– Earth’s orbit
– Land surface changes
– Atmospheric and aerosol composition
Variations in solar output
• Solar output regularly changes
– 0.1-0.2% change due to sunspots
– 11 year cycle for sunspots
• The Maunder Minimum was a period of few sunspots
and lower solar activity around the year 1600
– The Little Ice Age occurred during the Maunder
minimum
– Links to the quasi-biennial oscillation (QBO)-changes in stratospheric tropical winds associated
with changes in sunspots
Early faint Sun paradox
• The geologic record shows warmer early
Earth temperatures, but astrophysical models
show that the sun was about 1/3 weaker than
today
– The early warmth was probably caused
by greater CO2 concentrations in the early
Earth atmosphere
Milankovitch Cycles -- Precession
Milankovitch cycles -- regular natural variations in the
Earth’s orbit around the sun
– Obliquity -- 41,000-year period
– Eccentricity -- 100,000-year period
– Precession -- 27,000-year period
Changes in land configuration and surface
characteristics
• Plate tectonics gradually changes the
configurations of the mountains and oceans
• Mountain building and land erosion affect
climate over geologic time
• Land use changes such as deforestation and
desertification change albedo, surface
temperatures, and water balance
Changes in atmospheric aerosols affect the
amount of solar energy that can reach the
Earth’s surface
• Major volcanic eruptions inject great
amounts of aerosols into the atmosphere
over days or weeks, leading to temporary
climate cooling
• Residence times of tropospheric aerosols is a
few years
• Residence times of stratospheric aerosols is a
few decades
Mt. Pinatubo aerosols
Ship tracks
over the
Pacific leave
clouds in their
exhaust trails
(excess
condensation
nuclei)
Changes in radiation-absorbing gases
• Anthropogenic contributions of CO2
– Increased exponentially since the mid
19th century due to fossil-fuel burning
– Increased CO2 concentrations leads to
increased atmospheric absorption of IR
radiation
– Increased anthropogenic greenhouse
gases in the atmosphere can lead to
increased atmospheric water vapor (the
most important greenhouse gas)
• Exchange of CO2 between the atmosphere and
ocean
– Current CO2 emission rates increasing 3.5
ppm/yr
– The oceans are a major absorber of CO2 due to
oceanic biota photosynthesis and solution of
CO2 in the water
– Only about 1/2 of the anthropogenic CO2
emission ends up in the atmosphere (where
does the other 1/2 go?)
Feedback mechanisms are systems in which
changes in one variable lead to changes in
another
• Feedback mechanisms can be
– Negative, where the feedback acts to
inhibit further change in a variable
– Positive, where the feedback acts to
magnify further change in a variable
Examples of feedbacks
Ice-albedo feedback (positive feedback)
• Ice cover affects global albedo
Evaporation of water vapor (positive feedback)
• Water vapor is a greenhouse gas
Ocean-atmospheric interaction (positive or
negative feedback)
• Ocean levels change through thermal
expansion and glacial melting
Sea ice
Computer models of
global climate change
give predictions of
what future climate
might be. They take
into account the
climate/ocean
feedback mechanisms
that are known. This
climate prediction is
for double the
atmospheric CO2 over
current values.
This climate
prediction is for
double the
atmospheric CO2 over
current values. While
global warming
would be greatest at
the poles, changes in
the precipitation
patterns would be
much more diverse
across the Earth.
End of Chapter 16
Understanding Weather and
Climate
4th Edition
Edward Aguado and James E. Burt