Proxy Climate Data

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Chapter 8 Paleoclimate

• This chapter discusses
• Proxy data
• Climate change at different time scales

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Time Scales of Climate Change
Earths climate changes all the time, e.g., last
300 Myr, last 3 Myr, last 50,000 yr, and last
1000 yr.
3
Climate Variability and Climate Change
Climate variability Diurnal cycle Seasonal
cycle Interannual variability
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Climate Variability and Climate Change
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Climate Through Time
Late 1990s
1976
Quelcaya Ice Cap, Andes Mountains, Peru
Variations of the Earths surface temperature for
the past 140 years
Temperatures over last 570 million years
Temperatures over last 400,000 years
Climate is not constant Change occurs over a
variety of time scales Most Earth history is
warmer and wetter than at present
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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
PRESENT
100,000 years
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The only mammals living at this time were small
rodents.
8
Sea levels were much higher than today, and Texas
was mostly under water
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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Historical documents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
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Instrumental Measurements (Direct)

• Weather Stations
• Stevenson Screen
• Temperature

Satellite
40 years old
140 years old
Automatic Weather Station
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The Little Ice Age(1270-1850)
The Hunters in the Snow by Pieter Brueghel the
Elder (Kunshistorisches Museum, Vienna)
Historical documents
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Proxy Records of Climate

• Uses of proxy records of climate depend on both
• - time span of record
• - resolution of record

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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

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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

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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.
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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

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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,

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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 ground
water Uranium can be used to determine the age
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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.

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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

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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

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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

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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
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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

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(21 k yrs ago)
The Tropical Cooling Debate

• 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

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CLIMAP (Climate Mapping and Prediction) Project
Began in 1970s, published its first map in 1976
and then 1981
Reconstructing the Last Glacial Maximum
LGM August SST
Mainly based on ocean sediments
Difference between LGM and Today
Overall 4C cooler than today
N Atlantic 8C cooler
N Pacific 2-4C cooler
Tropical oceans 1-2C cooler
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What Caused The Tropical Cooling?
Insolation was close to today
Ice sheets were too distant.
Greenhouse gases must have been a major factor.
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Ocean-Based Evidence for a Small Tropical Cooling
CLIMAP
Distribution of plankton species depends on ocean
water temperature.
During LGM, high-latitude cold-adapted species
moved to the tropics ? a large cooling in the
tropics. But
Tropical cooling 1.5C
Biochemical Composition
Relative abundance of alkenone molecules is
sensitive to ocean water temperature.
Tropical cooling less than 2C
Oxygen isotope measurements
Difference in d18O values (LGM and today)
difference by ice sheets difference by ocean
temperatures
Tropical cooling 2-3C
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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 6.5C/1000 meters
Tropical cooling 4-6C
Descent of the upper limit of forests
Tropical cooling 5C
Temperature-sensitive noble gases (xenon,
krypton, argon, neon) in groundwater
SW USA and SE Brazil cooling 5C
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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 6.5C/km toward 9.8C/km of dry
air
Mountain glaciers poorly dated
Descent of vegetation due to lower CO2
Where is the truth?
Somewhere between 1.5C (CLIMAP) and 5C (land
evidence)
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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 1.5C (CLIMAP) and 5C
(land evidence)
Greenhouse gases by 2100 doubling of the
preindustrial values?
This range matches the range of uncertainty about
Earths CO2 sensitivity simulated by GCMs
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Orbital-Scale Changes in CO2
Vostok Ice in Antarctica
Major CO2 cycles match marine d18O (ice volume)
cycles in an overall sense
Four 100,000-year cycles
23,000-year cycle not prominent
Which is driving which?
Maxima 280-300 ppm
Difficulties
Low accuracy in dating in Antarctica
Minima 180-190 ppm
Dust reacts with CO2 bubbles in Greenland
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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

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Causes of Climate Change Since Deglaciation
Climate controls
21k yrs ago
Low CO2
Large ice sheets
21-6k yrs ago
Increasing CO2
Increasing summer insolation
6-0k yrs ago
Decreasing summer insolation
High CO2
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Stronger, Then Weaker Monsoons
High lake levels in the north tropics 9000 years
ago
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Warmer, Then Cooler North Polar Summers
Pollen in lake sediments indicates northward
large-scale shifts in spruce and oak.
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Yearly Temperature Change for the Last 2000 Years
Red recent estimates Blue earlier estimates
Global Warming
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
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Yearly Temperature Change Since 1850
1998
Data from thermometers
http//commons.wikimedia.org/wiki/ImageInstrument
al_Temperature_Record.png
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The Earths Climate History

• Over the last century, the earths surface
  temperature has increased by about 0.75C (about
  1.35F).
• Little Ice Age Cooling during 1,400 A.D.
  1,900 A.D. (N.H. temperature was lower by 0.5C,
  alpine glaciers increased few sunspots, low
  solar output)
• 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)
• Holocene Maximum 5,000-6,000 ybp (1C warmer
  than now, warmest of the current interglacial
  period)
• Younger-Dryas Event 12,000 ybp (sudden drop in
  temperature and portions of N.H. reverted back to
  glacial conditions)
• 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)
• 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.
• For most of the earths history, the climate was
  much warmer than today.