13 Oct Past Climates Test Review

Transcription

1 13 Oct 2009 Past Climates Test Review

2 Loose End: Wind Stress Climatology U E = V E = 0 & $% 0 & $% u E dz = " y # 0 f v E dz = $ " x # 0 f Risien and!chelton 2008, Journal of Physical Oceanography 2

3 Gondwana and South Pole: Polar Position Hypothesis to describe presence of ice sheets Polar Position Hypothesis 1. Ice sheets appear when continent close to pole 2. No ice anywhere on earth if no continents near pole 430 MY glaciation in Sahara consistent Theory Problems Glaciation brief compared to time at pole Lack from Other factors needed! =>Polar land mass needed. Pangea breaks up,ant & Aus back at pole 3

4 Simulations Super-continent Pangea Can use models to study 200My 180Lon, 70N & 70S What level of CO2 needed to explain Pangean Climate? Do GCM simulations match geological record? Model land is symmetric, seasons sea level - like today topography, used 1000m CO 2? Sun was 1% weaker but Pangea was warmer than today,no ice sheets at lat of greenland & N. America Plants suggest no freeze at 40N More CO 2 likely, 1650 ppm (280 preindustrial value) RUN MODEL 15 & use last 5 4

5 Pangea - GCM Results show no ice sheets since summers warm FILL IN Dry interior as expected, rainy ocean Evaporite evidence consistent Large T range over year (no ice) Monsoonal Circulation Wet Summers & Dry Winters in East 5

6 Geography alone not enough so need BLAG (Spreading Rate) Hypothesis for CO 2 Changes and Climate FILL IN Age of the Sea Floor, Pacific is young & Spreading faster 6

7 1983, Geochemists Carbon moves from earth s interior to atmosphere Basic idea: Changes in sea floor spreading rates controls the amount of CO 2 injected into atmosphere. Faster spreading leads to more CO 2 injected in the atmosphere. last 100 Myr know spreading rates because this is oldest age of sufficient sea floor. BLAG Hypothesis for CO 2 7

8 Negative Feedback in BLAG Hypothesis for CO 2 Need chemical weathering mechanism to complete feedback sea floor spreading, chemical weather, subduction into deep earth, brings it back to the sea floor through magma Self regulating mechanism 8

9 Chemical Weathering Removes CO 2 from Atmosphere Hydrolysis of silicates (granite) removes CO 2 from atmosphere Excellent example of interactions between parts of earth system 9

10 BLAG Spreading Hypothesis FILL IN Agrees with geological evidence 10

11 Uplift Weathering Hypothesis Chemical weathering a more active component to this theory. Weathering is a function of fresh rock available & sfc area rock. Weathering decreases exponentially with time of moraine exposure During Tectonic uplift fresh fragmented rock is more abundant Steep inclines More earthquakes Fast weathering draws more CO 2 from atmosphere Uplift - subduction (slow and steady) & collisions (fast and abrupt). 11

12 Mountain Uplift & Weathering Hypothesis Uplift explains the periods when the Tibetan plateau was being formed 55 My ago; Sahara 430Myr still not explained Compare BLAG & Weathering Hypothesis. BLAG - more weathering from warm wet regions like lower Amazon basin Weathering - more weathering from young Andes RESULTS: 80% of chemical weathering from Andes highlands, WHY? Andes are younger than amazon basin, Fresher! Uplift - what is limiting factor? Maybe just that uplift is limited on earth otherwise lots of uplift makes earth freeze. Then rock is not exposed. Chemical weathering may be source of climate change as well as a moderating factor (thermostat) - uplift areas it impacts climate 1-2% area - elsewhere it may slow with cooling/drying 12

13 Warm Cretaceous Earth s geography easier to establish for last 100Myr Pangea had broken apart from 175 Myr - 100Myr Warmer more CO 2 & Higher Sea levels (~200m), most recent warm greenhouse period (this is why it is of particular interest to climate scientists) 13

14 Evidence for a Warm Cretaceous: Seen in Polar Regions Breadfruit tree fossil in polar regions Dinosaurs, turtles, crocs north of Arctic circle 14

15 Warm Cretaceous-How to simulate correct Temperature Eric Barron & colleagues pioneered Cretaceous simulations Fill in this stuff Poles 20+C warmer than today Amplification of the polar regions Simulation 1: Use Model and Cretaceous continent placement and altitude of land and still poles are too cold Ice sheets specified Simulation 2: Include 4xCO 2 and get warmer everywhere. But not quite right. Need to warm polar (tropics) regions more (less) 15

16 Ocean Heat Transport Hypothesis: keep subtropics from getting too warm & keep poles warmer Change in deep water formation locations==> Salty dense water sunk in Subtropical Locations, helps to move heat poleward Poleward heat flux at depth Still a problem getting much warmer poles and slightly warmer tropics 16

17 Sea Level Changes Fill in this stuff 17

18 Factors Impacting Sea Level Changes Volcanic activity decreases, less hot (expanded crust), reduce ocean ridge volume Asteriod - large biological impact but small climate impact... 18

19 Summary and What Next? Key Ideas for Today Three types of data instrumental historical paleoclimate Plate movements and role in climate, Pangea simulations Three parts of theories for a ice sheets presence/absence Warm Cretaceous, hard to model but want to in order to understand climate change For Next Meeting: exam, Room 501 9:15 to 11:15 19