Volcanism, Mountain Building, and Tectonics
Volcanism - major source of CO2 to the atmosphere
Mountain building - more rock weathering removes CO2 from the atmosphere, binds up in limestone
Tectonics: negative feedback maintains climate equilibrium
Climate History of Earth: Sedimentary Rocks
Warm climates:
- fossil reefs, limestones
- Al ores in tropical soils
- Salt beds from inland sea evaporation
Cold climates:
- Glacial erosion leaves distinctive marks and debris deposition
18O and certain fossil organisms indicated paleo-temp history of Earth’s climate
Oxygen Isotopes and Temperature
- Use ratio of stable isotopes of oxygen in CaCO3 sea life fossils
- Evaporation removes more light 16O, and leaves behind more heavy 18O
- Atmospheric water becomes depleted in 18O
- 18O depleted water is trapped on land as snow or ice during Ice Age -> marine organisms use 18O rich water to make their shells
Measurement of 18O/16O in CaCO3 fossils is an indicator of climate when organism lived - High 18O/16O -> colder climate
- Low 18O/16O -> warmer climate
The Last 3 Million Years
- Old, stable ice sheet on Antarctica
- North American and Eurasian ice sheets expand and shrink, affecting global climate
- Forming of Isthmus of Panama 3mil ya blocked westward-flowing ocean water, forcing it north
- Warm water in north Atlantic Ocean increased evaporation and snowfall to build glaciers
- Continental ice sheets in Northern Hemi undergo complex cycles of advance and retreat
Orbital forcing of climate in Milankovitch cycles
Orbit, tilt, and wobble
Orbital eccentricity:
- 100k year period
- More circular to more elliptical
Tilt of Earth axis:
- 41k year period
- 21.5 to 24.5 degrees
Precession of tilt:
- Bimodal period 23,000 and 19,000
- Wobble
The Last Glacial Maximum
- 20k years ago, covered 27% of today’s land
- Sea level was lowered 130m to build glaciers
- Current Ice Age continues (current glacial retreat)
- 10% of continents still buried under ice and sea lvl would rise 65m if all the ice melts
Temperature since 20k ya based on Greenland ice-core oxygen isotopes:
- Warming began, then interrupted by a cold stage
- Cold interval replaced by warming period
- Temps fell through a different interval and bottomed from 12,900 to 11,700ya
- Then current interglacial period
- Temp changes of 3-5C occurred in several years
Cause of sudden drops or jumps in temperature:
- Melting of continental ice sheets left behind huge cold lakes with ice dams
- Failure of ice dams released lots of fresh, cold water into surface layers of ocean = disrupted oceanic circulation
- Megafloods of cold, freshwater into arctic prevented sinking of arctic water -> shuts off influx of warm surface waters from equator
- Constant rise in sea level from melting of glaciers
At 7k ya:
- Warmer global temps, higher rainfall totals -> climatic optimum
- Since then, average global temps have fallen 2C
- Smaller cycles of glacial advance and retreat within cooling trend
El Nino
Warm ocean water arrives at Peru and Ecuador near Christmas time
La Nina
Occurs when cool waters move into the equatorial Pacific
- Brings cold air and high rainfall to NW US and western Canada, below average rainfall to rest of North America
- Encourages hurricanes in Atlantic Ocean, low rainfall and wildfires in SW US
Pacific Decadal Oscillation
Last 20-30 years
Mid-latitude Pacific Ocean conditions, 2º tropical effects
- El Nino lasts 6-18months, condition of tropics, 2º effect on mid-latitudes
Warm phase with more storms and rainfall
Cold phase with less storms
Unknown mechanistic cause
Volcanism and Climate
Large Plinian eruptions blast fine ash and gas to the stratosphere
- Above the troposphere where weather occurs
- Not washed out quickly by rain
- Floats for years
Ash and sulfuric acid (from SO2 gas) remain in the stratosphere as a haze, blocking some incoming solar radiation = climate cools
Worst-case scenario: flood basalt eruptions (66mil ya)
- 2.6mil km^3 of basaltic lava erupted in only 700k years
Possible effects: - Increase in atmospheric CO2 -> temp increase of 10C
- More acidic ocean waters
- Depleted ozone layers
- Punishing to life on land and in surface ocean
Mayan Civilization and Climate Change
- Successful in agriculture, irrigation, social organizations, mathematics, and astronomy over 1k years
- Centuries-long pattern of decreased rainfall -> drought -> abandonment of urban areas, stop in monument construction, breakdown of social and political order -> wars, return to a life of rural subsistence
- Significant decline in civilization due to string of events triggered by long-term climate change
Past Thousand Years
Combined effects of eccentricity, tilt, and wobble caused a cooling trend with numerous variations
Other variations within cooling trend being studied with:
- O isotopes in glacial ice layers
- Annual growth rings of corals
- Tree rings
- Tax records of grain and grape crops
- Advances and retreats of mountain glaciers
- Paintings of frock lakes, rivers, ports
- Weeks/year of sea ice around Iceland
Medieval Maximum:
warm period from 1000-1300CE
Little Ice Age
The cold period from ~1350-1850CE
- Smaller scale cooling and warming within this
- NE Canada had permanent snowfields
- Shortened growing seasons in Europe with localized famine
- Potential causes: volcanic eruptions, changes in ocean circulation, reduced solar energy
Current hypothesis for Little Ice Age
- Volcanic eruptions in 1200s added aerosols to the atmosphere = lower solar radiation at surface
- Climate cooled and glaciers advanced
- Sea ice-cooled and warm water circulation to North Atlantic was shut off
Maunder Minimum:
cooler period from 1645-1715CE
- Minimal sunspot activity -> sun possibly 0.25% weaker
Processes altering climate in past 1000yrs, changes to:
- Earth’s orbit
- Volcanic eruptions
- Solar energy output
- Sea-surface temp in tropics
- Aerosol [ ]s
- GHG [ ]s
- Interactions among above processes
20th Century
- Began as warm as any time in past 1000yrs
- Warming unprecedented in recent past
- Solar output responsible for less than 10% of warming
- Major causes of warming: GHGs and aerosols
- Avg global surface temps rose 0.6ºC in 20th century from 1910-1944, and since 1977
- 1910-1944 warming: hotter Sun, lack of volcanism
Warming since 1977 twice that of 1910-1944: due to GHG buildup
Natural causes 0.2ºC:
- Changes in Earth’s orbital patterns -> -0.02ºC
- Hotter Sun -> +0.22º
Human activities 0.4ºC increase
Start of 21st century:
- Rate of annual temp increase declined, then all-time highs in 2014, 2015, 2016
- Annual temps since 1977 have all exceeded 20th century avg annual temp
