ESS15 Lecture 16. Deep past, ice ages, the global carbon cycle.

Transcription

1 ESS15 Lecture 16 Deep past, ice ages, the global carbon cycle.

2 Half a billion years ago - Gondwana glaciation. Continents bunched up at South Pole about 500 million years ago Huge ice sheets left deposits and erosion across Southern Hemisphere

3 Really ancient climates. Late Paleozoic (~300 Ma) Most continents bunched up near South Pole (Gondwanaland) Evidence of ice sheets in Africa, South America, and Australia (contiguous) Middle Cretaceous (~120 Ma to ~ 90 Ma) No Atlantic Ocean, Australia attached to Antarctica Ocean bottom temperature ~ 15º to 20º C No polar ice in either hemisphere Plant and animal fossils ~ 15º latitude poleward of present ranges (dinosaurs in the Arctic!) CO 2 was 400% to 600% of present concentration

4 Cenezoic Climates (since the dinosaurs, 65 Ma) Gradual global cooling Gradual separation of Australia, South America, and Antarctica Antarctica moved into polar position South America and Australia moved north Opening of Drake Passage initiated Circumpolar Current in the Southern Ocean Ocean surface and bottom temperatures cooled by 10º C Cool temperate forest in Antarctica ~20 Ma gave way to ice, reached current volume ~ 5 Ma Northern Hemisphere ice sheets appeared about 3 Ma

5 The past 5 million years.

6 i-clicker: Compared to 3-5M years ago, the past 1M year has experienced: A: More dramatic swings in climate. B: Less dramatic swings in climate. C: Similar climate variability. D: Impossible to tell.

7 Slow descent into a glacial epoch. Antarctic circumpolar current isolated south polar region Antarctic ice sheet reduced Earth s albedo Northern ice sheets grow and collapse in a cycle of ice ages

8 Thinking about glaciers Europeans have been living with glaciers for millennia They knew what land at glacial margins looked like It wasn t much of a stretch to see those same landforms elsewhere!

9 Orbital theory of ice ages. Regular changes in shape of Earth s orbit and Earthsun geometry as the timekeeper of ice ages First suggested in mid 19 th Century by Adhemar and (later) James Croll Quantified by Serbian mathematician Milutin Milankovitch in early 20 th Century Hard to support with paleoclimate evidence of the day, fell out of favor until mid-1960 s Modern paleoclimatic data in 1970 s strongly supported Milankovitch

10 Tilt of Earth s axis. Changes in the tilt of Earth s axis of rotation determine the amplitude of the seasonal cycle of solar radiation

11 Eccentricity of Earth s orbit. Earth s orbit is an ellipse (not a circle) Currently slightly closer to the sun in January than July The amplitude of this variation is the eccentricity

12 Precession of Earth s orbit. Direction of rotational axis spins like a top Currently points NH away from sun at closest point This minimizes seasonal amplitude of radiation Precession reverses this periodically

13 Orbital cycles - are very slow.

14 Orbital cycles - impact northern hemisphere summer sunshine. Ice Modulator When summer sun is weak in northern high-latitudes, some snow persists Albedo increases Ice builds up When sun comes back, ice melts much more quickly than it came Sawtooth pattern

15 Deep past - in a nutshell. Rock weathering and volcanoes stabilize the climate on really long timescales (100 s of millions of years). Liquid oceans! Temperature and CO2 have varied wildly with deep time. Climate has been cooling over the last 100 million years, but really slowly. Ice is mostly a recent phenomenon. Most recently, dramatic 100,000-year ice age cycles likely driven by solar (orbital) variability amplified by positive feedbacks involving the ocean and carbon cycle. Modern CO 2 changes have already been as dramatic as those that occur during glacial cycles, but are happening much faster. Future changes in CO 2 are likely return us to concentrations not seen since many millions of years ago!

16 i-clicker: professor rap question. Choose your own adventure. A: Nerdy rap with an Eminem and MC Paul Barmon shout-out about where Exxon can sequester their carbon. B: I m sick of science, drop a rap about fishing! C: I m not interested in hearing nerdy raps, move on.

17 The global carbon cycle

18 Carbon, life and energy. Photosynthesis uses energy from the sun to convert inorganic air (CO2) to living biomass! Most of this energy is released through respiration (back to CO2) when plants are eaten by animals, bacteria, people

19 Fossil fuels Some of the stored solar energy in biomass can be preserved in fossilized remains

20 We now extract these remains, burn them for energy, and dump billions of tons of the resulting CO2 into the atmosphere every year. What happens next?

21 Global carbon cycle in a nutshell. We are mining fossil CO 2 and titrating into the oceans, (buffered by acid-base chemistry) Much of the fossil CO 2 will remain in the atmosphere for 10 s of thousands of years About half of fossil-fuel CO 2 is absorbed by poorlyquantified sink processes The strength and even the sign of potential carbonclimate feedback is among the most uncertain aspects of climate change in the 21 st century

22 Huge amounts of carbon naturally cycle back and forth between components of the Earth system. Atmosphere 775 Gt ~90 ~120 Gt/yr Ocean ~90 Gt/yr ~120 Land 38,000 Gt The ocean is the biggest reservoir 2000 Gt

23 About half the CO 2 released by humans is absorbed by oceans and land Atmosphere /yr ~90 ~120 Ocean ~90 8 GtC/yr ~120 Land Humans 38,

24 Where has all the carbon gone? Into the oceans Solubility pump (CO 2 very soluble in cold water, but rates are limited by slow physical mixing) Biological pump (slow rain of organic debris) Into the land CO 2 Fertilization (plants eat CO2 is more better?) Nutrient fertilization (N-deposition and fertilizers) Land-use change (forest regrowth, fire suppression, woody encroachment but what about Wal-Marts?) Response to changing climate (e.g., Boreal warming)

25 The Oceans

26 Anthropogenic DIC in the ocean. DIC: Dissolved Inorganic Carbon Most anthropogenic CO2 confined to top few 100 m Shoaling in tropics, convection at higher latitudes

27 Two mechanisms pump CO2 out of the atmosphere into the ocean.

28 The solubility pump CO 2 solubility in seawater depends sensitively on sea surface temperature CO 2 is highly soluble in cold high-lat waters Think can of soda (less fizzy when warm) Transported to deep ocean by convection and vertical mixing Dynamically-driven equatorial upwelling brings high-co 2 water to surface Atmospheric transport closes the loop

29 The Biological pump Ducklow et al (2001) Primary production generates communities of critters from DIC and nutrients in the presence of light Zooplankton graze on phytoplankton Bacterial decomposition and heterotrophic respiration recycle DIC and nutrients to the water column Detrital particles from dead phytoplankton and zooplankton waste coagulate into progressively larger particles Larger particles sink faster than turbulence can resuspend them, so fall below euphotic zone

30 Common misconception: When we reduce or stop the burning of fossil fuel, the CO2 will go away and things will go back to normal. CO2 from fossil fuel will react with oceans, but only as fast as they mix Eventually, fossil CO2 will react with rocks About 1/3 of today s emissions will stay in the air permanently! Archer et al, Ann. Rev. Earth Plan. Sci. (2009)

31 The Land.

32 i-clicker question: During photosynthesis, plants tend to water. A: Gain B: Lose C: Neither

33 Leaf anatomy. Stomate

34 Leaf anatomy. tomate

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36 Carbon and water. Plants eat CO 2 for a living They open their stomata to let CO 2 in Water gets out as an (unfortunate?) consequence For every CO 2 molecule fixed about 400 H 2 O molecules are lost

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38 Amazon Rain Forest

39 Deforested Pasture 20 km away

40 Disturbance

41 Disturbance and Recovery 6-year-old 18-year-old 57-year-old Year Planted: Height (m):

42 Ecosystem recovery and succession. Woodwell and Whittaker, 1968

43 Billions of Tons of Carbon per Year Emissions scenarios - what ifs by economists. A key ingredient for how predictions of future climate are done. Fossil Fuel Emissions CO2 in the atmosphere

44 There is huge uncertainty about what people will do Range of 500 ppm CO2 concentration uncertainty by end of century. Fossil Fuel Emissions CO2 in the atmosphere Billions of Tons of Carbon per Year 500 ppm Depending on which what-if scenario somes true.

45 There is almost as much uncertainty about what the natural carbon cycle will do in response to humans! Range of 300 ppm CO2 concentration uncertainty by end of century. LAND: Coupled simulations of climate and the carbon cycle depending on what the carbon cycle does. OCEAN: Given nearly identical human emissions, different models project dramatically different futures! 300 ppm!

46 Carbon cycle - summary. Emissions of CO 2 by global industry are part of a much bigger biogeochemical cycle of carbon About half of anthropogenic CO 2 emissions are removed from the atmosphere by pertubations to natural biogeochemistry that are not completely understood In addition to cloud feedbacks, uncertainties in future human emissions and in the response of global biogeochemistry to changing climate are among the leading sources of uncertainty in predictions of 21st century climate

47 Thanks. Monday is a holiday Wednesday is the midterm review lecture.