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Transcript
Climate over the long term (Ch. 3 - 6 highlights) • Long-term climate changes • Plate tectonics • What maintains Earth’s habitability? • Faint Young Sun paradox • CO2 : Earth’s thermostat * • Past icehouse conditions • Past greenhouse conditions * Critical idea Long-term climate changes Long-term: consider how Earth’s climate has changed over last few hundred m.y. Why study long-term changes? -- Helps us to fundamentally understand how Earth’s climate system works -- If we don’t know this, we can’t evaluate how climate might change in future From before: Natural climate variations time scale type according to book ~few years “historical” ~10-1000 years “historical / millenial” ~10,000 years “orbital” millions of years “tectonic” Long-term climate changes Plate Tectonics • Theory that the upper portion of Earth is subdivided into ~dozen large pieces (lithospheric plates) that move relative to one another • Most volcanic activity occurs at plate boundaries, either where plates are moving apart (divergent margin), or where they are moving towards each other (convergent margin) “tectonic” means any large scale Earth movement Map of Earth’s lithopheric plates Seafloor spreading here Subduction & mountain building here Plate tectonics can affect climate because: (1) Continents can change position This strongly affects ocean currents. (2) It controls the rate of volcanism (high when plates moving fast, low otherwise). (3) It controls the rate of weathering (high when more continents collide and more mountains formed). Changing continent positions: Assembly of supercontinent Pangaea Rate of volcanism Changes in amount of uplift of continental rock could regulate amount of weathering “Uplift weathering hypothesis” Get uplift mainly when continents collide Why increased rock fragmentation leads to more weathering: Weathering depends on surface area What maintains Earth’s habitability? Earth’s climate “just right” -- at present -- mostly over geologic time geologic evidence (e.g. sedimentary rocks) & biologic evidence (fossils) indicates liquid water stable at surface for most of Earth history -- not always true in past, however Climates on three planets today Venus Earth Mars avg. temp. 460 oC 15 oC -55 oC avg. distance to sun 0.7 x Earth 1.5 x Earth solar energy 2 x Earth 0.44 x Earth input (flux) Climates on three planets today avg. temp. Venus Earth Mars 460 oC 15 oC -55 oC Just right greenhouse warming 285 oC 31 oC 5 oC avg. temp. with no 175 oC -16 oC -60 oC greenhouse Too cold Phase diagram for water Venus, Earth, Mars with no greenhouse effect (& same pressure): -16 C Faint Young Sun paradox (1) Astrophysical models indicate that sun’s brightness should have increased significantly over age of solar system (2) So why wasn’t Earth frozen earlier? Solar luminosity -- what we mean by sun’s “brightness” not same as albedo! luminosity = energy / (area * time) = Watts / m2 at surface of sun; we call this flux away from sun -- flux decreases as distance from sun increases because solar energy spread over a larger area (spreads over surface area of sphere = 4 * pi * r2) -- models suggest sun’s luminosity increased by ~30% over age of solar system Earth should have been frozen before 1.8 b.y. ago CO2 : Earth’s thermostat? CO2 is a greenhouse gas, helping to make Earth habitable today The amount of CO2 in the atmosphere may have varied in the past to keep Earth comfortable GCM results: the effect of different CO2 levels CO2 as Earth’s thermostat -- where is carbon (C) stored on Earth? -- how is C exchanged between different reservoirs? Where is carbon stored on Earth? Carbon reservoirs today Limestone (carbonate) rock: CaCO3 How is C exchanged between different reservoirs? Carbon cycle C exchange between rocks & ocean + atmosphere Note: low rates Focus on C exchange between rocks and atmosphere: • volcanic eruptions add C to atmosphere (as CO2), remove it from rocks • chemical weathering of rocks either adds or removes C from atmosphere, depending on type of rock weathered; we’ll consider removal of C from atmosphere Volcanic eruptions (Regulated by plate tectonics) More volcanism earlier in Earth history? -- Yes, more plate tectonic activity -- Could get more CO2 in atmosphere, stronger greenhouse -- But unlikely that this alone exactly balanced variations in solar luminosity No reason for volcanic activity on Earth to be related to solar luminosity ! Chemical weathering (hydrolysis): -- chemical reaction of minerals with water to form different minerals CaSiO3 + H2O + CO2 CaCO3 + SiO2 + H2O mineral in rock mineral rain atm Makes carbonic acid H2CO3 mineral Chemical weathering (hydrolysis): CaSiO3 + H2O + CO2 silicate rock rain atm CaCO3 + SiO2 + H2O limestone / carbonate -- removes CO2 from atmosphere, puts it in limestone (or carbonate) rock -- proceeds faster if more precipitation, higher temperature, more vegetation (Why?) Chemical weathering (hydrolysis): CaSiO3 + H2O + CO2 silicate rock rain atm CaCO3 + SiO2 + H2O limestone / carbonate -- removes CO2 from atmosphere, puts it in limestone (or carbonate) rock -- proceeds faster if more precipitation, higher temperature, more vegetation (Why?-- carbonic acid) Temperature - weathering feedback: Temperature - weathering feedback: CO2 : Earth’s thermostat? The amount of CO2 in the atmosphere may have varied in the past to keep Earth comfortable Chemical weathering (hydrolysis) was probably important in regulating this The weathering process involved a negative feedback Can weathering explain the Faint Young Sun Paradox? If colder (lower solar luminosity), weathering rates should have been less... … more CO2 stored in atmosphere, less in rocks... … more greenhouse effect, higher temperature. So: Yes, in principle. But there were times in Earth’s history when the (presumed CO2) thermostat was not so effective... This led to icehouse & greenhouse conditions Past icehouse conditions evidence for multiple glaciations Major glaciation 550-850 m.y. ago 3 glaciations last 500 m.y. Glacial striations in Alaska Formed by movement of ice over rock Positioning of large landmasses over polar regions help cause glaciation Note: Polar positioning is not the only reason we had past icehouse climates Past greenhouse conditions fossil evidence for warm conditions 100 Ma ago (Cretaceous period) Dinosaurs Warm-climate flora O-isotope data, deep oceans: ~13 oC cooling in last 50 m.y. 100 m.y. ago (Cretaceous): • Supercontinent Pangaea breaking apart • High sea level GCM models including changes in plate position and CO2 fail to fully explain Cretaceous climate What led to greenhouse conditions in the Cretaceous? Probably 2 factors important (1) Higher CO2 in atmosphere -- faster plate movement led to more volcanic emission of CO2 -- there was less removal of CO2 from atmosphere by weathering because there were few high mountains (no plate collisions) (2) Heat was transported in oceans differently than today Today Then Model simulation of Cretaceous ocean salinity Highly saline water is dense and can sink, even if warm If heat in Cretaceous oceans transported more efficiently, would tend to equalize temperatures more… ...discrepancies between models & geologic evidence would be explained