Presentation on theme: "Climate over the long term (Ch highlights)"— Presentation transcript:
1 Climate over the long term (Ch. 3 - 6 highlights) Long-term climate changesPlate tectonicsWhat maintains Earth’s habitability?Faint Young Sun paradoxCO2 : Earth’s thermostat *Past icehouse conditionsPast greenhouse conditions* Critical idea
2 Long-term climate changes Long-term: consider how Earth’s climate has changedover last few hundred m.y.Why study long-term changes?-- Helps us to fundamentally understand howEarth’s climate system works-- If we don’t know this, we can’t evaluatehow climate might change in future
3 From before:Natural climate variationstime scale type according to book~few years “historical”~ years “historical / millenial”~10,000 years “orbital”millions of years “tectonic”Long-term climatechanges
4 Plate Tectonics Theory that the upper portion of Earth is subdivided into ~dozen large pieces (lithospheric plates)that move relative to one anotherMost volcanic activity occurs at plate boundaries,either where plates are moving apart (divergentmargin), or where they are moving towardseach other (convergent margin)“tectonic” means any large scale Earth movement
7 Plate tectonics can affect climate because: (1) Continents can change positionThis 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 andmore mountains formed).
8 Changing continent positions: Assembly of supercontinent Pangaea
10 Changes in amount of uplift of continental rock could regulate amount of weathering“Upliftweatheringhypothesis”Get uplift mainlywhen continentscollide
11 Why increased rock fragmentation leads to more weathering: Weathering depends on surface area
12 What maintains Earth’s habitability? Earth’s climate “just right”-- at present-- mostly over geologic timegeologic evidence (e.g. sedimentaryrocks) & biologic evidence (fossils)indicates liquid water stable atsurface for most of Earth history-- not always true in past, however
13 Venus Earth Mars Climates on three planets today avg. temp oC oC oCavg. distance x Earth x Earthto sunsolar energy 2 x Earth x Earthinput (flux)
14 Venus Earth Mars Climates on three planets today avg. temp oC oC oCgreenhouse oC oC oCwarmingavg. temp oC oC oCwith nogreenhouseJust rightToo cold
16 Venus, Earth, Mars with no greenhouse effect (& same pressure):
17 Faint Young Sun paradox (1) Astrophysical models indicate that sun’sbrightness should have increasedsignificantly over age of solar system(2) So why wasn’t Earth frozen earlier?
18 Solar luminosity -- what we mean by sun’s “brightness” not same as albedo!luminosity = energy / (area * time) = Watts / m2at surface of sun; we call this flux away from sun-- flux decreases as distance from sun increasesbecause 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
25 How is C exchanged between different reservoirs? Carbon cycleC exchange betweenrocks & ocean +atmosphereNote: low rates
26 Focus on C exchange between rocks and atmosphere: volcanic eruptions add C to atmosphere(as CO2), remove it from rockschemical weathering of rocks either adds or removes C from atmosphere, depending on type of rock weathered; we’ll considerremoval of C from atmosphere
27 Volcanic eruptions(Regulated by plate tectonics)
28 More volcanism earlier in Earth history? -- Yes, more plate tectonic activity-- Could get more CO2 in atmosphere, strongergreenhouse-- But unlikely that this alone exactly balancedvariations in solar luminosityNo reason for volcanic activity on Earthto be related to solar luminosity !
29 Chemical weathering (hydrolysis): -- chemical reaction of minerals with water to form different mineralsCaSiO H2O + CO CaCO SiO H2Omineral rain atm mineral mineralin rockMakes carbonic acid H2CO3
30 Chemical weathering (hydrolysis): -- removes CO2 from atmosphere, puts it in limestone (or carbonate) rock-- proceeds faster if more precipitation, higher temperature, more vegetationCaSiO H2O + CO CaCO SiO H2Osilicate rain atm limestone /rock carbonate(Why?)
31 Chemical weathering (hydrolysis): -- removes CO2 from atmosphere, puts it in limestone (or carbonate) rock-- proceeds faster if more precipitation, higher temperature, more vegetationCaSiO H2O + CO CaCO SiO H2Osilicate rain atm limestone /rock carbonate(Why?-- carbonic acid)
34 CO2 : Earth’s thermostat? The amount of CO2 in the atmosphere mayhave varied in the past to keep EarthcomfortableChemical weathering (hydrolysis) was probablyimportant in regulating thisThe weathering process involved a negativefeedback
35 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.
43 100 m.y. ago (Cretaceous):Supercontinent Pangaea breaking apartHigh sea level
44 GCM models including changes in plate position and CO2 fail to fully explain Cretaceous climate
45 What led to greenhouse conditions in the Cretaceous? Probably 2 factors important(1) Higher CO2 in atmosphere-- faster plate movement led to morevolcanic emission of CO2-- there was less removal of CO2 fromatmosphere by weathering becausethere were few high mountains(no plate collisions)(2) Heat was transported in oceans differently thantoday
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