Climate History

Section 11.3 Climate History

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

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

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

Carbon-14

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

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