Ice AgesTake-Away PointsSlide 3The Last 800,000 yearsBeginning of the Ice AgesMilankovich CyclesAxis TiltAxis Tilt and the Incredible Shrinking TropicsPrecession: 26,000 year cycleShape of Orbit + PrecessionThe Greenhouse EffectThe Carbonate-Silicate CycleSlide 13Slide 14The Snowball EarthSlide 16What Causes Ice Ages?Are We Headed For Another Ice Age?Slide 19Ice Ages•Pleistocene 3 M.y. •Permian 250-220 M.y. •Ordovician 450 M.y. •Precambrian –900-650 M.y. (Snowball Earth)–2300 M.y.Take-Away Points1. The recent period of ice advances since 2.5 million years ago is called the Pleistocene2. The best record of Pleistocene ice advances is on the sea floor3. There were probably 20+ advances of the ice, of which only the last few are preserved on land.4. Over most of the earth’s history, the earth has been mostly ice free5. Tiny changes in earth’s orbit and axis tilt play a role in ice advances by varying the amount of sunlight we receive6. The carbonate-silicate cycle governs the balance of carbon dioxide in the atmosphere and in rocks and creates “icehouse” and “greenhouse” periods7. We still have no final answer why ice ages occurTime (1000 Years) North America Alps Northern Europe Poland-Russia0-18 Interglacial 18-67 Wisconsin Wurm Vistula Varsovian67-128 Sangamon Uznach Eem Masovian128-180 Illinoisan Riss Warthe/Saale Cracovian180-230 Yarmouth Hoetting Holstein Sandomirian230-300 Kansan Mindel Elster Jaroslavian300-330 Aftonian Cromer Likhvin330-470 "Nebraskan " Gunz Menapian470-540 Waalian540-550 Donau II Weybourne550-585 Tiglian585-600 Donau I600-2000 About 20 Glacial Advances2000 (2 M.Y.) Beginning of Pleistocene4000 (4 M.Y.) Dwarf forests still in Antarctica15 M.Y. First Glaciation in AntarcticaThe Last 800,000 yearsBeginning of the Ice AgesMilankovich CyclesCool Summers More Important Than Cold Winters•Tilt of Axis•Shape of Orbit •PrecessionCan’t be the whole story-have operated throughout earth historyAxis Tilt•Small Axis Tilt: Mild winters but cool summers. Favors Ice Age •Large Axis Tilt: Cold winters but hot summers. Favors InterglacialAxis Tilt and the Incredible Shrinking Tropics•Earth’s Axis Tilt is Shrinking–24.2 degrees 9,500 years ago–23.4 degrees now–22.6 degrees 10,200 years from now•Tropics are shrinking–14.7 m/year = 4 cm/day = 1.7 mm/hour•Temperate zones gain 1550 sq km/year–1080 at the expense of tropics–470 at the expense of Arctic and AntarcticPrecession: 26,000 year cycleShape of Orbit + Precession•Summer at Perihelion (Eccentric Orbit) –Cold winters but hot summers. Favors Interglacial •Summer at Aphelion (Eccentric Orbit) •Near-circular Orbit –Mild winters but cool summers. Favors Ice AgeThe Greenhouse Effect•Without a na tural Greenhouse Effect, earth would be frozen•90% due to Water Vapor•Other contributors: carbon dioxide, methane, nitrous oxides•“A little Greenhouse Effect is a good thing”–Carl Sagan•Problem: we are accelerating it with unknown final consequencesThe Carbonate-Silicate Cycle•Earth has almost as much carbon dioxide as Venus•Volcanoes add carbon dioxide to the atmosphere•Carbon dioxide is removed from the air to make carbonate rocks•“Icehouse” and “Greenhouse” episodesThe Carbonate-Silicate CycleMountain-building favors cooling•Uplift exposes rocks to weathering•Calcium silicates (plagioclase, amphiboles, pyroxenes) are chemically weathered•Calcium is carried to the sea where organisms bind it into carbonate minerals•Creation of carbonates removes carbon dioxide from the atmosphere•Weathering of carbonates returns carbon dioxide to the atmosphereThe Carbonate-Silicate Cycle•Plate tectonics carries some carbonates into the earth•Heat liberates carbon dioxide•Carbon dioxide returns to the atmosphere•The cycle does not require life but does require liquid water.The Snowball Earth•Between 900 and 600 m.y. ago, Earth froze completely (or almost) about four times•Global freezing alternated with extremely rapid sea-level rise and global warming•Evidence:–Glacial deposits on all continents, even at low latitudes–Glacial deposits immediately succeeded by thick deposits of carbonate rocksThe Snowball EarthPossible reasons: •Fainter early sun •Biological changesGlobal ice coverWeathering and erosion shut downVolcanoes continue to erupt CO2At 10% CO2, abrupt warming beginsGo from –50 C to +50 C in 10,000 years?Implications for life?What Causes Ice Ages?Within Earth (Endogenic)•Carbonate-Silicate Cycle •Volcanic Eruptions - Sudden output of CO2 (warming) or particulates (cooling) •Mountain Building - Changes in atmospheric circulation •Continent-Ocean configuration Outside Earth (Exogenic)•Changes in Sun (faint early sun) •Variations in Earth Orbit (Milankovitch Cycles) Don't Really KnowAre We Headed For Another Ice Age?•Heating & Cooling in Historic Times •Smoke, Haze, CO2 May Alter Climate •Don't Really Know•Global warming due to fossil fuels may be catastrophic in many ways, but will probably not much affect these longer-term cycles. We will have run out of fossil fuels long before the duration of a typical interglacial.Take-Away Points•The recent period of ice advances since 2.5 million years ago is called the Pleistocene•The best record of Pleistocene ice advances is on the sea floor•There were probably 20+ advances of the ice, of which only the last few are preserved on land.•Over most of the earth’s history, the earth has been mostly ice free•Tiny changes in earth’s orbit and axis tilt play a role in ice advances by varying the amount of sunlight we receive•The carbonate-silicate cycle governs the balance of carbon dioxide in the atmosphere and in rocks and creates “icehouse” and “greenhouse” periods•We still have no final answer why ice ages