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Section 11.3 Climate History

Subsection 11.3.1 Introduction

Have the climates always been the same? What mechanisms have caused the climate to change?

  • Two categories of past climate

    • Historical, past few thousand years

    • Paleoclimate, conditions before human civilization

  • Historical and natural records reflect climate; climate changes and causes can be determined from careful study

    • Dates of freezes of lakes and rivers, farmers’ logs, diaries, newspapers

    • Animals in cave paintings indicate climate and climate change

  • But historical records only go so far; we need to turn to proxy data to reconstruct past climates

Subsection 11.3.2 Reconstructing past climates

A number of different methods can be used to estimate the climates of the past, including:

  • Tree-ring data

  • Pollen records

  • Ice sheets and trapped air

  • Marine sediments

  • Glacial features and geologic evidence

  • Radiometric and stable isotope data

Dendrochronology (tree-ring data)

  • Diameter of tree trunk increases with growth

  • In regions with distinct growing seasons, tree grown appears as concentric rings

  • Trees generally produce one ring per year; width of ring indicates how fast growth was

  • Growth varies with temperature, precipitation, solar radiation

Pollen Records

  • Pollen is useful to paleoclimatologists

    • Degrades slowly

    • Distinctive shapes for each species

    • Can accumulate in sediments in lakes, providing record of past vegetation (and thereby, climate)

  • Radiocarbon dating of pollen grains extends information back tens of thousands of years

Isotopic Applications

  • radiocarbon dating of organic materials or ice core air bubbles

  • stable isotope measurements:

    • deuterium and O-18 content

    • colder : higher O-18 concentrations in seawater

    • O isotopes also trapped in fossil shells

  • radiometric dating of fossil-bearing rock layers

Stable Isotopes (D and O-18)

  • Materials have been deposited in layers on ocean floor for millions of years

  • Animal shells are made of CaCO\(_3\)

    • May contain O-18, which is sensitive to temperature

    • Colder: higher O-18 concentrations in seawater

    • Can sequester lighter O-16 in continental ice

    • This is the benthic oxygen isotope record

  • This method works back to 2–3 million years

  • Warm periods about every 100,000 years


  • produced in upper atmosphere (cosmic rays) and then incorporated (via CO\(_2\)) into plants by photosynthesis

  • photosynthetic products are consumed by animals (and by animals eating animals) so C-14 goes through food chain

  • C-14 abundance is maintained as long as the organism is alive. There are at least a few thousand C-14 atoms decaying in your body every second.

  • C-14 is unstable and decays with a half-life of 5730 years (Beta decay of C-14 into N-14)

  • effective for dating 100-70,000 yrs

  • useful for organic material

Radiometric Age Dating

  • Provide a means to track life through the ages

  • Integral part of rocks in which they are found

  • Types of plants and animals give climate clues

  • Some plants have particular requirements; quantity of a fossil can indicate whether it was thriving in the climate

Subsection 11.3.3 Past Climate Observations

Putting these proxies together to reconstruct paleoclimate

  • Several major ice ages have occurred throughout Earth’s history:

    • Precambrian Eon (2.2 Ga)

    • late Proterozoic Eon (700 Ma; Rodinia snowball Earth?)

    • Paleozoic Era (500 Ma)

    • Permian Period (250 Ma)

  • We are currently in the Holecene epoch (0.01 Ma-present) of the Quaternary Period (2.6 Ma-present)

    • Earth had been cooling into an ice age throughout the Paleogene (65-23 Ma) and Neogene (23-2.6 Ma

    • intensification of the ice age occurred around 2.6 Ma (Quaternary)

    • recent continental glacial activity occurred during Plestocene epoch (1.6-0.1 Ma)

      • widespread geologic evidence of Pleistocene glaciation

      • most recently the Illinoian (200-130 ka) and Wisconsinian (110-10 ka) glaciation events

    • The Holecene epoch (10,000 years ago to the present) represents an interglacial period of the Quaternary ice age

      • this means that there are polar caps but no continental glaciation into the mid-latitudes

      • sea level rise of nearly 40 meters over this timescale

      • rise of agricultural and civilization in the Neolithic (stone age)

Little Ice Age

  • period of cooler temperatures, 1300-1800

  • Maunder Minimum: period of low solar activity, 1610-1680