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Climate Proxies
How can you measure the climate of the past?
Learner outcomes
At the end of this lecture you should be able to
1. Describe how proxies differ from observations
2. Describe how tree rings, corals, fossils, lake ice and lake duration are used to estimate local climate
3. Describe how ocean sediment and ice cores are used to estimate global climate
4. The difference between stable and radiometric isotopes and what type of information they tell use about past climates
Proxies
Unlike instrumental records that tell us only about the most recent century, proxy records (natural archives of climate change) enable us to place recent climatic change in the context of the last several hundred to thousand years.
Temperature (Northern Hemisphere) CO2 Concentrations
1000 Years of CO2 and Global Warming
How do we develop proxies?
• Assumptions• Observations of phenomenon today• Link current observations to past records
Assumptions
• Observation: Solar radiation varies but overall decreases
• Therefore solar radiation in the past was higher
Proxies• Corals*• Tree rings*• Pollen*• Fossils*• Sea level• Lake ice duration*• Ocean sediments• Ice Cores
*indicative more of local climate change than global climate change
Corals• Shells made of Calcium Carbonate
(CaCO3)
• Shell in equilibrium with the ocean water• Band width provided evidence of
temperature the coral grew in• Growth rates change with ocean
temperatures, pH levels• Local climate
Why should we hug trees?• Dendrochronology is the study of the annual variability
of tree ring widths, which can be extended back to 8000 years ago.
• The study of trees provides climate information regarding temperature, runoff, precipitation, and soil moisture.
• Local climate Date of last ring isyear tree was cut
1930 1950 1970
19101890
1870
Tree Rings
Growth conditions recorded in rings• Wide ring-warm days sufficient water• Narrow-cold days/drought
What can plant and animal fossils tell us about ancient climates?
• Certain plants and animals live only in specific environments, so their presence is a clue to local climate.
These 350 Ma fossil ferns were most likely the oldest on land, and likely required high pCO2
levels.
A trilobite, the three- lobed king of warm, shallow Cambrian seas
Soft-bodied Waptia, an arthropod from the Cambrian Burgess
Shale
Lake Monona Ice Duration 1855-2005
0
20
40
60
80
100
120
140
160
180
1855 1875 1895 1915 1935 1955 1975 1995
Seasons
Du
rati
on
of
Ice
(day
s)
Source: Wisconsin State Climatology Office
Examples of Climate Proxies• Pollen Lake Ice Duration• Tree Rings Lake Ice Thickness• Ice Cores
Global Proxies
• Sea level• Ocean sediment• Ice cores
– Layers (varves) in ice cores– Gases in ice cores– Stable Isotopes: O-16 to O-18 ratio in ice
cores– Radiometric Isotopes:Carbon dating of
sediment in the ice cores or glacial deposits
Sea Level
• Glaciation –low sea level
Ocean Sediment Cores: 3-3.5km
• Thick levels of sedimentation can indicate heavy weathering, warmer temperatures
• Volcanic sediments• Loss of sediment
layers through erosion
• 55 mya
Frozen Core Some cores go 3 km deep!
Vostok, Antarctica 78°28' S, 106°48'E: Coldest
Places on Earth•
Vostok Station Nationality: Russia
Location: Vostok - an outpost if there ever was one - is located near the South Geomagnetic Pole, at the center of the East Antarctic ice sheet, where the flux in the earth's electromagnetic field is manifested.
The coldest recorded
temperature on Earth, -128.6°F (-
89.2°C) was measured here on
July 21, 1983.
Ice core drilling 3.4 km to go ½ million
years into past climate
Ice Cores: Varves
• A varve is an annual layer of sediment or sedimentary rock
Section of Greenland Cores
Dozen Ice Ages going back 1 billion years
How can ancient greenhouse gases be trapped?
• Atmospheric gases (CO2, CH4, SO2, etc.) can be trapped in glaciers as frozen water metamorphoses from snow to firn to recrystallized ice.
The record of atmosphere CO2 since the Industrial
Revolution
During the Last Glacial Maximum pCO2 is
estimated at 180 ppm
Ice Core Thermometer
Isotopes
• Stable Isotopes-temperature• Radiometric dating-rate and date
How can oxygen isotopes used as paleoclimate proxies?
• isotope -- atoms of the same element with the same atomic number (chemical properties) but differing atomic weight (physical properties). Differ in number of neutrons.
• Oxygen is composed mostly of 16O and 18O, which as part of water molecules are separated by physical processes.
A typical carbon atom with 6 protons and 6 neutrons and
6 electrons.
Fractionation
• To divide or separate into parts• Ocean water is made up of both O-16 and
O-18
There is a standard or average ratio of O-18 to O-16 (standard mean ocean water as the baseline, SMOW)
• Certain physical and biological processes change the ratio (this is fractionation)
Oxygen Isotopic Ratios or Amounts
O18/O16 ratio in glacial ice indicate the atmosphere temperature in which the snow that made up the ice formed
Extent of isotopic difference (fractionation) is dependent on the temperature.
So they form a temperature proxy!
Oxygen Isotopic Ratios vs. Amounts
O18/O16 ratio versus O-18 and O-16
Usually described as a ratio
Oxygen Fractionation Summary
• If ratio O-18 to O-16 is higher than expected in the ocean, colder temperatures
• If ratio O-18 to O-16 is lower than expected in the ocean, warmer temperatures
Oxygen Isotopes in Glacier Ice
• Polar ice is preferentially enriched with O-16 relative to the ocean (O-16 locked in glacier ice). So especially during glaciation ocean water is “heavy”
• Why is glacier ice “light”?– The water source is from precipitation which
is preferentially light.• So during a glaciation you would expect
remaining ocean water to be heavy
Radiometric Isotopes
• Isotopes that decay (Carbon) can tell us the approximate date of an event or the rate at which an event took place – Glacial retreat
Carbon Dating: Rate of Glacial Retreat
Greenland Ice Sheet and Arctic: Northern Hemisphere
Antarctica: South Pole
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