ice ages - earth, atmospheric, and planetary physics · 2014-11-05 · ice ages •there have been...

Ice ages

What is an ice age?

• Geological period of long-term reduction in the temperature of the Earth's surface and atmosphere which results in the formation and expansion of continental ice sheets, polar ice sheets and alpine glaciers

History

• Late 18th/early 19th century Europeans observed that erratic boulders dispersed due to the retention of glaciers caused by climate chance

Glacial erratic

• A piece of rock that is different than the native rock in the same area

• Carried by glacial ice over hundreds of kilometres

• Can be small pebbles to huge boulders (Big Rock in Alberta is 15,000 metic tons)

Big Rock

Significance

• Can be traced back to its parent bedrock to determine the ice flow route

• Erratics dropped by melting icebergs in the ocean allow for the tracking of Antarctic/Arctic glacial movements before retention

• Can be correlated with ocean temperatures to help understand and calibrate global climate models

Evidence

• 3 main types

• Geological – glacial moraines, drumlins, erratics

• Chemical – isotopes, ice cores

• Paleontological – geographical distribution of fossils

Ice ages

• There have been at least 5 major ones in the Earth’s history

• Determined by permanent ice sheets

• During non-ice ages, the Earth is completely ice free at all latitudes

• We are currently in an ice age, the Pliocene-Quaternary glaciation, which started 2.58 million years ago

Glacials and interglacials

• Within the current ice age, there have been cycles of glaciation with ice sheets advancing and retreating on 40,000 and 100,000 year intervals due to more temperate and more severe periods

• The colder periods are called glacial periods, the warmer periods interglacials

Glacials

• Cooler and drier climates

• Large land and sea ice masses extending outward from the poles

• Mountain glaciers extend to lower elevations, lower snow line

• Lower sea levels due to removal of large volumes of water above sea level as icecaps

• “next ice age” actually next glacial

Last Glacial Maximum(19,000-26,500 years ago)

Interglacials

• Earth is in an interglacial period known as the Holocene

• Less ice cover, higher sea levels

Milankovitch cycles

• Milutin Milanković, a Serbian geophysicist, calculated that irregularities in Earth's orbit could cause the climatic cycles now known asMilankovitch cycles

Eccentricity

• The earth has an elliptical (oval) orbit that rotate gradually primarily due to interactions with Jupiter and Saturn

Tilt

• The inclination (tilt) of Earth's axis changes periodically between 22.1° and 24.5°

• The tilt is responsible for seasons

• The greater the tilt, the greater the difference between summer and winter temperatures

Milankovitch cycles

• The orbital eccentricity of Earth changes on 100,000 year intervals

• Changes in the tilt occur in a cycle 41,000 years long and wobbles on Earth's spin axis, complete every 21,700 years

• 100,000 year problem – eccentricity variations have a significantly smaller impact on solar forcing than precession or obliquity

Other factors?

• Atmospheric composition (GHG)

• Plate tectonics

• Ocean currents

Atmospheric composition (CO2)

• Evidence that greenhouse gas levels fell at the start of ice ages and rose during the retreat of the ice sheets

• Next glacial delayed due to the burning of fossil fuels?

• 50,000 years away if CO2 levels increase to 750 parts ppm (presently 385 ppm) 15,000 years away If CO2 drops to 210 ppm

Plate tectonics

• Geological record has shown that ice ages start when the continents are in positions which block or reduce the flow of warm water from the equator to the poles which cause ice sheets to form

• Starts positive feedback loop by increasing albedo

Ocean currents

• Modified by continent position, sea levels and salinity, and other factors

• Have the ability to warm (Antarctic Circumpolar Current keeps warm ocean waters away from Antarctica helping it maintain huge ice sheets) and cool (British Isles a temperate as opposed to a boreal climate due to warming by water from the Gulf Stream)

Plate tectonics and ocean currents

50 million years ago

THE END!