Geology and Climate Change

• What is Paleoclimatology?
• Study of past climate
• What is the difference between climate and weather?
• "Climate is what you expect, weather is what you get"

• Why should we care?
• Being able to predict weather can save billions of dollars and thousands of lives
• Need to understand climate variability to recognize anthropogic climate change (e.g., global warming)

• How do scientists study paleoclimates?
• Proxy data - fossils, sediments, geochemistry

• Fauna and flora
• For example, reef corals and ferns indicate warm climate; variations in tree pollen and tree rings give clues to climate variation
• Coal - humid climate (could be temperate or tropical)
• Bauxite, kaolinite - warm, moist (highly weathered soils)
• Evaporites, calcrete - dry climate
• Glacial sediments, tillites - cold, glaciation
• Also glacial striations

• Sea level change - rapid drop (over 100s - 1000s of years) can indicate glaciation

• Especially important for studying Tertiary and Quaternary climate
• Oxygen isotopes - ¹⁸O/¹⁶O
• As shells are precipitated, the ¹⁸O will be enriched in CaCO₃ relative to water
• the higher the temperature, the less the enrichment

• Relation between ¹⁸O/¹⁶O ratio (reported as d¹⁸O) and temperature
• measurements mostly of mollusk (clam) shells

Ocean Isotopes in Eocene (40 Ma) Gastropods

Proxy data - corals

Measures of Paleoclimate Oxygen Isotopes and Ice Volume

• Additional factor - d¹⁸O of seawater increases as glaciers grow
• Removal of low d¹⁸O water as glacial ice increases d¹⁸O of sea water

• High d¹⁸O of shell: cold temperatures, glaciation, or both

• Quaternary glaciation cyclical
• ¹⁸O/¹⁶O ratios of foraminifera (shells) from marine sediment cores
• 100-k.y. cycles over last million years; 42 k.y. cycles from 1 - 2 m.y.

Measures of Paleoclimate Oxygen Isotopes and Ice Cores

• d¹⁸O in meteoric water (rain, snow) increases with increasing temperature
• Ice from cores in glaciers provide a record of this temperature change

• Based on d¹⁸O of glacial ice
• Present warm temperatures; previous cold events (Younger and Older Dryas, Last Ice Age)

• Based on ice cores from Antarctica

Earth’s Climate Modes
"Icehouse" vs. "Greenhouse"

• Paleoclimate animation - An animation of paleoclimatic indicators collected by A.J. Boucot & Chen Xu.
• Key: green =coal, yellow=evaporite, orange=calcrete, blue=bauxite, magenta=kaolinite, black=tillites
• Icehouse periods - those with continental glaciers
• 2 b.y. - Precambrian
• Late Precambrian to Cambrian
• Late Ordovician to Early Silurian
• Carboniferous to Permian
• Neogene - present

• Solar intensity
• Atmospheric CO2 levels
• Albedo
• Paleogeography
• Earth’s orbit
• Solar intensity
• Increasing with time
• For example, was roughly 95% at the beginning of the Paleozoic (538 Ma)

• CO₂ is a greenhouse gas
• Greenhouse gas - absorbs infrared radiation
• Keeps energy from being lost to space
• Atmospheric carbon dioxide
• Dry air: 78% N₂, 21% O₂, 1% other gases (Ar, CO₂, etc.)
• CO₂ level increased >25% in last 150 years (from 280 to 360 ppm)

• Why wasn’t the Earth frozen over?
• Early Earth had higher atmospheric CO₂ levels
• Burial of carbon as coal and carbonate minerals decreases atmospheric CO₂ levels; keeps Earth from warming up too much

• Albedo - amount of solar radiation reflected back to space (Fig. 20-8)
• Average Earth: ~30% albedo
• Tropical forest: 5 - 10%
• Snow fields and glaciers: 80 - 90%
• Ice sheet area increases albedo
• Causes cooling;  promotes more glaciation - positive feedback

• Paleogeography
• Location of continents near poles
• Cambrian versus Carboniferous
• Effect on ocean circulation - Circum-polar current
• Paleogene versus Neogene

• Quaternary glacial cycles
• ¹⁸O/¹⁶O ratios of foraminifera (shells) from marine sediment cores
• 100-ky cycles over last million years; 42 ky cycles from 1 - 2 m.y.

• Variations in Earth’s orbit (Fig. 12.36) - Milankovitch Hypothesis
• Eccentricity - shape of earth’s orbit; 100,000 year cycle
• Obliquity - change in angle of earth’s axis to plane of earth’s orbit; 41,000 year cycle
• Precession - cycle of wobble in earth’s axis = 23,000 years
• Glaciation enhanced by mild winters, cool summers
• Decreased tilt will reduce seasonality in high latitudes

• Modern temperature change from temperature records - 0.6 to 0.8°C increase suggested

Atmospheric CO₂

• Seasonal variations - CO₂ during summer in northern hemisphere
• Long-term increase due mostly to burning of fossil fuels

• Is global warming occurring?
• If so, what is the cause?
• Greenhouse gases
• Variations in solar irradiance

• Ground thermometer measurements
• Choosing a baseline - extrapolating 20% to 40% coverage to entire globe

Considerations in Measuring Global Temperature

• Ground thermometer measurements
• Choosing a baseline - extrapolating 20% to 40% coverage to entire globe
• Correction for urban warming
• Satellite and weather balloon measurements versus ground measurements

Considerations in Measuring Global Temperature (cont.)

• Tree ring data

What is the Answer?

• I don’t know!
• Clearly, a longer record of global climate is needed
• monitoring by satellite
• better coupling of modern, historical, and geologic records
• We will know a lot more in 10 years