Lecture 8_ climate change through earth history (1).pdf

Lecture 8: climate change through earth history Key terms: Weather — state of the atmosphere over a short time Climate — weather averaged over a long period of time (>30 years) Global warming — the rise in global temperatures due mainly to the increasing concentrations of greenhouse gases in the atm...

Lecture 8: climate change through earth history Key terms: Weather — state of the atmosphere over a short time Climate — weather averaged over a long period of time (>30 years) Global warming — the rise in global temperatures due mainly to the increasing concentrations of greenhouse gases in the atmosphere Climate change — encompasses all types of changes in the atmospheric climate that ccan be and have been observed from regional to global scales Receding glaciers: ● Both continental glaciers (Greenland, Antarctica) and alpine glaciers ○ E.g. Franz Josef glacier, South Island of New Zealand ● Last glacial maximum: ○ Cold period in earth’s climate from ~26,000-20,000 years ago ○ Maximum extent of recent glaciations ○ Global temperature about 6 degrees C lower than today ● Laurentide ice sheet ○ Covers most of Canad at LGM ○ Primary feature of Pleistocene epoch ○ Up to 3km thick at its maximum; several km’s covers the GTA ● Ice cores ○ By measuring oxygen isotopes in glacial ice cores, scientists can reconstruct global temperature trends – paleo climate ■ E.g. ice corres from Antartica and Greenland can have ice as old as 100,000’s years ○ Interpretation from Paleo climate data: ■ Warming and cooling cycles on earth ■ Ranges between about 10 degrees ■ Even the CO2 level in atmosphere is changing a lot ■ The climate excursions seem to be repeating (periodic) ■ The period of the main signal seems to be about every 100,000 years ■ Time scale: humans were around but not doing much ● Main causes ○ Most energy into the earth’s climate system is from the sun ○ In early 20th century, geophysicist Milutin Milankovitch suggested that glaciers advance and retreat through changes in earth’s orbital motions – since termed the “Milankovitch cycles” ○ Eccentricity ■ Measure the departure of the earth’s orbital ellipse around the sun from circulartiy ■ A more eccentric orbit = more seasonal variation ○ Obliquity ■ The angle of the earth’s axial tilt with respect to the orbital plane ■ Current tilt of 23.44 degrees is halfway between maximum and minimum tilt ■ Increased tilt generally = more extreme seasonal variations ○ Precession ■ The trend in the direction of the earth’s axis of rotation relative to a fixed distant point ● E.g. during northern hemisphere winter, we are currently at the closet point to the sun in the earth’s elliptical orbit ■ When earth precesses away, this will make for a colder winter (glacial) ■ Cyclical variations in earth’s orbit ○ Cyclical variations in Earth’s orbit ■ These variations change how much energy Earth receives from the sun ■ Put all these together, and we get the periodic variations in global temperature that seem to correspond roughly to the ice core data ■ So the earth itself is spinning itself in and out of these glacial/interglacial cooling/warming periods The disappearance of the Mediterranean sea: ● From about 5.9-5.3 myr ago, the mediterranean sea entirely dried up ○ Called the Messinian salinity crisis ○ The sea closed off from the Atlantic and evaporated over ~1000 years ○ Probably due to tectonics and a warm/dry climate ○ Strait of Gibraltar opened up again at 5.3 Ma ○ Zanclean flood Paleocene-Ecoene thermal maximum: ● Approx. 55 Ma, there was a hothouse Earth ● Global temperature rose by 5-8 degrees ● hot/wet climate dominated the entire planet, evven the arctic regions ● Probably caused by enhanced CO2 degassing of planetary interior by volcanism ● Eventually planet recovered with enhanced biological activity: moving carbon from atmosphere to ocean floor Snowball Earth: ● Its been proposed that Earth has gone through periods where entire planetary surface is frozen or galciated ● Most notably in the Neoproterozoic (about 650 Ma) ● Evidence is glacial deposits at this age occurring globally, and even at equatorial paleolatitudes ● Most of the earth’s surface energy comes from the sun ○ As more heat is immediately reflected, earth’s surface and atmosphere become cooler ○ A measure of how radiation is reflected from the surface of a body is called albedo ○ When snow falls on land or ice forms at sea, increase in albedo causes increased cooling which stabilizes snow and ice — ice-albedo feedback ○ As ice forms at lower and lower latitudes on earth, planetary albedo rises at a faster and faster rate (since surface area increases towards equator) ■ Ice-albedo feedback causes runaway freezing → a snowball earth ● How did earth get out of the snowball? ○ No carbon sink from atmosphere to surface ocean (no precipitation and weathering) ○ However, plate tectonics continues beneath the ice, and volcanism spews CO2 to the atmosphere: warming the planet