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Climate Proxies

Climate Proxies. How can you measure the climate of the past?. Learner outcomes. At the end of this lecture you should be able to Describe how proxies differ from observations Describe how tree rings, corals, fossils, lake ice and lake duration are used to estimate local climate

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Climate Proxies

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  1. Climate Proxies How can you measure the climate of the past?

  2. Learner outcomes At the end of this lecture you should be able to • Describe how proxies differ from observations • Describe how tree rings, corals, fossils, lake ice and lake duration are used to estimate local climate • Describe how ocean sediment and ice cores are used to estimate global climate • The difference between stable and radiometric isotopes and what type of information they tell use about past climates

  3. 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.

  4. CO2 Concentrations Temperature (Northern Hemisphere) 1000 Years of CO2 and Global Warming

  5. How do we develop proxies? • Assumptions • Observations of phenomenon today • Link current observations to past records

  6. Assumptions • Observation: Solar radiation varies but overall decreases • Therefore solar radiation in the past was higher

  7. 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

  8. 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, pHlevels • Local climate

  9. 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 is year tree was cut 1930 1950 1970 1890 1910 1870

  10. Tree Rings Growth conditions recorded in rings • Wide ring-warm days sufficient water • Narrow-cold days/drought

  11. What can plant and animal fossils tell us about ancient climates? These 350 Ma fossil ferns were most likely the oldest on land, and likely required high pCO2 levels. • Certain plants and animals live only in specific environments, so their presence is a clue to local climate. A trilobite, the three- lobed king of warm, shallow Cambrian seas Soft-bodied Waptia, an arthropod from the Cambrian Burgess Shale

  12. Source: Wisconsin State Climatology Office

  13. Examples of Climate Proxies • Pollen Lake Ice Duration • Tree Rings Lake Ice Thickness • Ice Cores

  14. 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

  15. Sea Level • Glaciation –low sea level

  16. Thick levels of sedimentation can indicate heavy weathering, warmer temperatures Volcanic sediments Loss of sediment layers through erosion 55 mya Ocean Sediment Cores: 3-3.5km

  17. Frozen Core Some cores go 3 km deep!

  18. Vostok, Antarctica 78°28' S, 106°48'E: Coldest Places on Earth

  19. Ice Cores: Varves • A varve is an annual layer of sediment or sedimentary rock

  20. Section of Greenland Cores Dozen Ice Ages going back 1 billion years

  21. 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. During the Last Glacial Maximum pCO2 is estimated at 180 ppm The record of atmosphere CO2 since the Industrial Revolution

  22. Ice Core Thermometer

  23. Isotopes • Stable Isotopes-temperature • Radiometric dating-rate and date

  24. 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.

  25. 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)

  26. 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!

  27. Oxygen Isotopic Ratios vs. Amounts O18/O16 ratio versus O-18 and O-16 Usually described as a ratio

  28. 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

  29. 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

  30. 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

  31. Carbon Dating: Rate of Glacial Retreat

  32. Greenland Ice Sheet and Arctic: Northern Hemisphere

  33. Antarctica: South Pole

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