Chapter 8: Paleoclimate
This chapter discusses:
1. Proxy data
2. Climate change at different time scales
Time Scales of Climate Change
Earth’s climate changes all the time, e.g., last 300
Myr, last 3 Myr, last 50,000 yr, and last 1000 yr.
Climate Through Time
Late 1990’s
1976
Variations of the Earth’s surface
temperature for the past 140 years
Temperatures over last 400,000 years
Quelcaya Ice Cap, Andes Mountains, Peru
Temperatures
over last 570
million years
Climate is not constant
Change occurs over a variety of
time scales
Most Earth history is warmer
and wetter than at present
1. The last time atmospheric CO2 concentrations and temperatures were much
higher than today was in the age of dinosaurs.
2. Agriculture revolution began 10,000 years ago.
3. Human population explosion in the past 100 years. Today: 6 billion
6 5 million
years
10 million
PRESENT
55 million years
years
3.5 Million years
18,000 years
1 Million years
10,000 years
230,000 years
1,000 years
100,000 years
The only mammals living at this time were small rodents.
Sea levels were much higher than today, and Texas
was mostly under water
Climate Data
Tools for studying climate and climate change
Data
Instrumental measurements (direct)
Historical documents
Natural recorders of climate or proxy data
Climate models
Understand climatic cause and effect
External factors  climate system
Feedbacks
Test hypothesis
Quantitative (put numbers on ideas) and
Predict the future
Instrumental Measurements (Direct)
Weather Stations
•
•
~40 years old
Stevenson Screen
Temperature
~140 years old
Automatic
Weather
Station
Satellite
Historical documents
The Hunters in the Snow
by Pieter Brueghel the Elder
(Kunshistorisches Museum,
Vienna)
Proxy Records of Climate
• Uses of proxy records of
climate depend on both
- time span of record
- resolution of record
Proxy Records of Climate
• Proxies that record
annual growth patterns can
indicate year to year
variations in climate
-tree rings
-ice cores
-deep lake sediments
-coral reefs
• Limited to last 500-1000
years except ice cores
Tree Rings
• Lighter, thicker wood tissue formed by rapid growth in spring and
much thinner, darker layers marking cessation of growth in autumn and
winter
• Limited to land areas outside of tropics
• Variations of tree ring width and density act as recorders of year to
year changes in temperature and rainfall
Varved Lake Sediments
• Complement tree ring records; most
common in cold-temperature environments
• Occur in deeper parts of lakes that
do not support bottom-dwelling
organisms that would obliterate
annual layers with their activity
• Layers usually result from seasonal
alternation between light, mineral-rich
debris and dark, organic rich material
brought in by runoff – act as proxy of
precipitation amount
Varves: sediments deposited annually on the bottoms of lakes that freeze in winter
and thaw in summer. Winter varve: fine sediments; summer varve: coarse sediments.
Varve thickness – length of freeze-free period – summer temperature.
Corals
multi-celled organisms that build reefs in tropical oceans
• Texture of calcite (CaCO3) incorporated in skeletons varies; lighter
parts during periods of rapid growth in summer and darker layers during
winter
• Measurements of oxygen-18 isotope concentration records sea surface
temperature and salinity (precipitation and runoff) variations
• Limited to tropical oceans
Ice Cores
• Limited to polar latitudes and mountain glaciers
• Darker and lighter layers are more or less dust blown in seasonally
• Measurements provide information on temperature, snowfall,
atmospheric composition (gases, dust, volcanic aerosols), sunspots, …
Speleothems (cave deposits)
Mineral formations occurring in limestone caves (most commonly
stalagmites & stalactites, or slab-like deposits known as flowstones)
Primarily calcium carbonate, precipitated from groundwater
Uranium can be used to determine the age
Fossils of Past Vegetation
• Climate can be inferred from distinctive
vegetation types
• Palm-tree like fossil in Wyoming 45 Myrs
ago indicating the Cretaceous warm climate
• Climate can be
inferred from leaf size
and shape.
• Climate can be
inferred from pollen in
sediments: All flowering
plants produce pollen grains with
distinctive shapes.
Marine Sediments
• Long cores drilled by specially
equipped ships
• Dating only accurate to about 40,000 years ago and can resolve
climate changes that occur on century scale or longer
Marine Sediments
• Isotopes in shells of
foraminifera can reveal
temperature, salinity,
and ice volume
• Granular debris from land
can indicate icebergs breaking
off of continental ice sheets,
suggesting cold climates
Proxy Records of Climate
• Recent times:
instrumental
• More recent times:
historical, tree rings, ice
cores
• Proxies for more ancient
climates are found in
sediments or inferred from
fossils and land forms
• Can generally only
resolve changes that occur
over 100 years or greater
Why bother studying ancient climate?
Who cares what happened a long time ago?
1. Past variability can show climatic extremes that have not been
experienced during recorded history
2. In order to understand the effects of human activity on
climate, we must establish what the planet, the atmosphere,
and climate change was like before human perturbations
3. Constructing and interpreting long-term records of climate
are the only means to determine how periodic climate
change is
(All in all, we are just a blip)
4. Past is prologue
“The farther backward you can look, the farther forward you
are likely to see.”
- Winston Churchill
Proxy Records of Climate
• Proxies that record
annual growth patterns can
indicate year to year
variations in climate
-tree rings
-ice cores
-deep lake sediments
-coral reefs
• Limited to last 500-1000
years except ice cores
The Tropical Cooling Debate
(21 k yrs ago)
• How cold were the glacial tropics?
• Evidence for a small tropical cooling
• Evidence for a large tropical cooling
• Was the actual tropical cooling medium-small?
• Relevance of global tropical temperatures to future climate
CLIMAP (Climate Mapping and Prediction) Project
Began in 1970s, published its first map in 1976 and then 1981
LGM August SST
Reconstructing the Last Glacial Maximum
Mainly based on ocean sediments
Difference between LGM and Today
Overall: 4°C cooler
than today
N Atlantic: 8°C cooler
N Pacific: 2-4°C cooler
Tropical oceans:
1-2°C cooler
What Caused The Tropical Cooling?
Insolation was close
to today
Ice sheets were too
distant.
Greenhouse gases
must have been
a major factor.
Ocean-Based Evidence for a Small Tropical Cooling
CLIMAP
Distribution of plankton species depends
on ocean water temperature.
During LGM, high-latitude coldadapted species moved to the
tropics  a large cooling in the
tropics. But …
Tropical cooling: 1.5°C
Biochemical Composition
Relative abundance of alkenone
molecules is sensitive to ocean
water temperature.
Tropical cooling: less than 2°C
Oxygen isotope measurements
Difference in δ18O values (LGM and
today) = difference by ice sheets +
difference by ocean temperatures
Tropical cooling: 2-3°C
Land-Based Evidence for a Large Tropical Cooling
Drop of the ice line
Descent of the lower limit of mountain glaciers by 600-1000 meters in the tropics.
Lapse-rate cooling: 6.5°C/1000 meters
Tropical cooling: 4-6°C
Descent of the upper limit of forests
Tropical cooling: 5°C
Temperature-sensitive noble gases (xenon, krypton, argon, neon) in groundwater
SW USA and SE Brazil cooling: 5°C
Was the Actual Tropical Cooling Medium-Small?
Ocean-based evidence: small cooling; Land-based evidence: large cooling
Critics of small cooling
Plankton relatively insensitive to temperatures at
low latitudes
Food more important than temperature for survival
The Pacific is a difficult region to apply CLIMAP
Seafloor sediments poorly preserved
(altered by dissolving)
Critics of large cooling
Drier glacial tropical climate increases lapse
cooling rate from present-day 6.5°C/km
toward 9.8°C/km of dry
air
Mountain glaciers poorly dated
Descent of vegetation due to lower CO2
Where is the truth?
Somewhere between 1.5°C (CLIMAP) and 5°C (land evidence)
Relevance of Glacial Tropical Temperatures to Future Climate
Lower values of greenhouse gases caused glacial tropical cooling (1.5 to 5ºC); how large
the future warming will be in response to large increases in greenhouse gases?
CO2 = 190 ppmv (LGM) 280 ppmv (preindustrial, 47% higher)  381 ppmv (in 2006) ? (by 2100)
CH4 = 350 ppbv (LGM) 700 ppbv (preindustrial 100% higher)  1751 ppbv (in 2006) ? (by 2100)
Tropical cooling between
1.5°C (CLIMAP) and
5°C (land evidence)
Greenhouse gases by 2100:
doubling of the
preindustrial values?
This range matches the
range of uncertainty
about Earth’s CO2
sensitivity simulated
by GCMs
Orbital-Scale Changes in CO2
Vostok Ice in Antarctica
Four 100,000-year cycles
23,000-year cycle not prominent
Maxima: 280-300 ppm
Minima: 180-190 ppm
Major CO2 cycles match marine δ18O (ice volume)
cycles in an overall sense
Which is driving which?
Difficulties: Low accuracy in dating in Antarctica
Dust reacts with CO2 bubbles in
Greenland
Climate Change in the Last 7,000 Years
• The strength of tropical monsoons
• The warmth of northern high-latitude summers
• Vegetation responses
• Bedrock rebounding and sea level fall
Causes of Climate Change Since Deglaciation
Climate controls:
21k yrs ago
Large ice sheets
Low CO2
21-6k yrs ago
Increasing summer insolation
Increasing CO2
6-0k yrs ago
Decreasing summer insolation
High CO2
Stronger, Then Weaker Monsoons
High lake levels in the north tropics
9000 years ago
Warmer, Then Cooler North Polar Summers
Pollen in lake sediments indicates
northward large-scale shifts in
spruce and oak.
Yearly Temperature Change for the Last 2000 Years
Red: recent
estimates;
Global
Warming
Blue:
earlier
estimates
Data from tree rings, corals, ice cores, and historical records are
shown in various colors. Thermometers data in black.
About 1000 y.a., Medieval Warm Period. Certain regions were
warmer than others. Warm and dry summers in England (1000-1300):
vineyards flourished and wine was produced. Vikings colonized
Iceland and Greenland.
http://upload.wikimedia.org/wikipedia/commons/b/bb/1000_Year_Temperature_Comparison.png
Yearly Temperature Change Since 1850
1998
Data from thermometers
http://commons.wikimedia.org/wiki/Image:Instrumental_Temperature_Record.png
The Earth’s Climate History
1.
Over the last century, the earth’s surface temperature has increased by
about 0.75°C (about 1.35°F).
2.
Little Ice Age = Cooling during 1,400 A.D. – 1,900 A.D. (N.H. temperature was
lower by 0.5°C, alpine glaciers increased; few sunspots, low solar output)
3.
Medieval Climate Optimum (Warm Period) = Warming during 1,000 A.D. – 1,300
A.D. in Europe and the high-latitudes of North Atlantic (N.H. warm and dry,
Nordic people or Vikings colonized Iceland & Greenland)
4.
Holocene Maximum = 5,000-6,000 ybp (1°C warmer than now, warmest of the
current interglacial period)
5.
Younger-Dryas Event = 12,000 ybp (sudden drop in temperature and portions of
N.H. reverted back to glacial conditions)
6.
Last Glacial Maximum = 21,000 ybp (maximum North American continental
glaciers, lower sea level exposed Bering land bridge allowing human migration
from Asia to North America)
7.
We are presently living in a long-term Icehouse climate period, which is
comprised of shorter-term glacial (e.g., 21,000 ybp) and interglacial (e.g., today)
periods. There were four periods of Icehouse prior to the current one.
8.
For most of the earth’s history, the climate was much warmer than today.