Paleoclimatology Oral Review, 2005

1. Paleoclimatology Oral Review, 2005 Li Cao

2. outline • Forcing, response, feedback • Archives for paleoclimatology • Dating method • Paleoclimate change divided in different scale: Tectonic scale (Millions of years) Orbital Scale (focusing on the last 3 Myr) Deglacial and Millennial Scale Historical climate change

3. Forcing and response

4. Interaction and feedback

5. Paleo Archives • Ice core: Antarctic, Greenland, Mountain glacial • Ocean and lake sediment • Coral • Tree ring • Historical record

6. Plankton Planktonic foraminifera (upper left) Coccoliths (lower left) Diatoms (upper right) Radiolaria (lower right) Pollens

7. Dating method • Radiometric dating • Layer counting and flow model • Paleomagnetic dating • Orbital tuning: main method of dating ocean sediment cores. The process of constructing a time scale by using the link between astronomically dated changes in solar radiation and the rhythmic climatic responses they cause on Earth.

8. Radiometric dating

9. Paleoclimate changein different scale • Tectonic scale (Millions of years): faint young sun, snowball earth, BLAG vs. uplift weathering hypothesis; • Orbital Scale (focusing on the last 3 Myr): Monsoon, ice sheet, CO2, and CH4 cycle; • Deglacial and Millennial Scale: LGM, YD, Heinrich event, and D-O cycle; • Historical climate change: little ice age, the Medieval Warm Period etc.

11. Astronomical Control of solar radiation • Obliquity: 41, 000 yr cycle • Eccentricity: 100, 000 yr cycle • Procession: 23, 000 yr cycle

16. Orbital scale insolation change

17. Orbital monsoon hypothesis • Changing seasonal insolation will change the strength of the monsoons. Stronger summer radiation will strengthen the summer monsoon. Weaker winter radiation will strengthen the winter monsoon. It turns out that the African monsoon is very sensitive to insolation variations. • The African monsoon is responsible for precipitation over northern Africa. Today, the summer solstice occurs at aphelion. So, the summer insolation is near its minimum. As a consequence, northern Africa summer monsoon is weak. • Although the strength of the winter monsoon also varies, it has less impact on the African environment because the winter monsoon has little affect on precipitation over Africa.

18. Evidence of orbitally-controlled African summer monsoon • Lake levels across North Africa • Mediterranean circulation and deposition of marine sediments • Freshwater diatoms (small plant plankton) in the tropical Atlantic • Upwelling in the equatorial Atlantic

19. Milankovitch theory: orbital-scale control of ice sheets • In the last 3 million years, the ice sheets over North America grew and melted over short intervals. • Summer insolation controls North Hemisphere ice sheet growth. Ice growth occurs during times when summer insolation is low in high northern latitude.

20. History of North Hemisphere ice sheet

21. Deglacial and millennial climate changes • Last Glacial Maxima (LGM, 21 Kyr BP) • Younger Dyras cold event (~12.9 - 11.6 Kyr BP) • Heinrich event • Dansgaard-Oeschger event

22. CLIMAP reconstruction of LGM • cold • Great continent-sized ice sheet on North America, the biggest one is laurentide ice sheet • 110 m lower sea level than present • Dry and windy

23. Debate on tropical cooling during LGM • Small tropical cooling (~2°C ) : CLIMAP reconstruction based on the changes in planktic fauna and flora in the low-latitude oceans. Other evidences: biochemical composition of plankton shells (double bonds of alkenones), δ18O measurements on the CaCO3 shells of plankton. • Large tropical cooling (~5°C ): Mountain glacial ice line change, analyses of Sr in the CaCO3 of tropical corals, noble gases dissolved in glacial-age groundwater.

24. Millenialclimate change • Heinrich event: ice-rafting event from deep-sea cores, corresponding to Greenland ice core low δ18O. • Dansgaard-Oeschger cycle: A series of warm-cold oscillation punctuated the last glaciation from 15 to 110 Kyr BP. The D-O cycles have been marked by abrupt terminations, and often by abrupt onsets.

25. Heinrich event and D-O event

26. Proposed causes of millenial climate changeduring the late Pleistocene • Natural oscillations internal to the ice sheets: ice sheet instabilities • Interactions between climate systems: the salt oscillator • Response to solar variations

27. Three NADW mode • Modern: vigorous NADW formation in high-latitude seas that warms Northern Europe and Greenland greatly; • Glacial: less vigorous but active NADW formation, but not sinking as deeply, and occurring at lower latitudes with less warming of Greenland and Northern Europe; • Heinrich: greatly reduced NADW formation

28. Younger Dyras

29. Phenomena involved in YD • Reduction or southward shift in sites of formation of NADW formation, and probably reduction in cross-equatorial flow of warm surface waters to the NA; • Reduction in monsoon strength, causing drying; • Cooling centered on the NA but extending well beyond, and possible encompassing most of the globe; • Warming centered on the SA and including much, but not all, of Antarctic.

30. Possible YD cause • YD began when something caused NADW production decreases. This caused a reduction in warm cross-equatorial surface water flow, producing high-latitude cooling but leaving more heat in the South Atlantic to produce the Antarctic warming. Atmospheric effects of the colder North Atlantic included reduced monsoon strength, and steeper temperature gradients causing stronger winds. Feedbacks associated with the monsoonal changes and changes in wind-driven oceanic mixing then transmitted the signal farther into the southern hemisphere. Warming then may have occurred after re-diversion of meltwater drainage allowed density increase of North Atlantic surface water, leading to abrupt re-initiation of sinking.

31. Little Ice Age &Medieval Warm Period • The terms “Little Ice Age” and “Medieval Warm Period” have been used to describe two past climate epochs in Europe and neighbouring regions during roughly the 17th to 19th and 11th to 14th centuries, respectively. The timing, however, of these cold and warm periods has recently been demonstrated to vary geographically over the globe in a considerable way.

32. Reference • http://earth.usc.edu/~geol150/index.html/lectures.html • http://www.ncdc.noaa.gov/paleo/education.html • Earth’s climate past and future by William F. Ruddiman • Paleoclimatology by Raymond S. Bradley