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 observations2. 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

191018901870

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

Dura

tion

of Ic

e (d

ays)

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: RussiaLocation: 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-18There 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-16Usually 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