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Climate change over the last 1,000 years Continued ……. The Orbital Monsoon Hypothesis

Climate change over the last 1,000 years Continued ……. The Orbital Monsoon Hypothesis Is the 20th century warming unprecedented, or part of a natural trend (or cycle)? What can geologists tell us about Earth ’ s climate over the last 1,000 years?

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Climate change over the last 1,000 years Continued ……. The Orbital Monsoon Hypothesis

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  1. Climate change over the last 1,000 years • Continued ……. The Orbital Monsoon Hypothesis • Is the 20th century warming unprecedented, or part of a natural trend (or cycle)? • What can geologists tell us about Earth’s climate over the last 1,000 years? • Need high-resolution “tools” ……. * The atmosphere: Ice cores, trees with rings * The ocean: Corals

  2. Climate change over the last 1,000 years Reading: • Ruddiman, W. F. (2008). Earth’s Climate: Past and Future. Chapter 16: 292 - 308.

  3. Practical Assessment • Value: 10% of course mark • When: Thursday, 31 October 2013 (start 2:30 PM) • Friday, 1 November 2013 (start 2:00 PM) • Topics: 3 practical exercises (4 for HPOs) • Notes: Designed to take ~1.5 hours. • No need to memorise equations. • Needed: Calculator, ruler, lined paper.

  4. Summary of potential climate forcing mechanisms over last 1,000 years Insolation Decreasing summer insolation (in northern hemisphere) may have cooled the high northern latitudes by ~0.2-0.3oC (~0.1oC for NH). Thousand-year-scale changes in thermohaline circulation Little Ice Age cooling from ~1400-1850 AD may have been related to slowing of thermohaline circulation. El Niños An increase in El Niño activity would tend to warm Earth. Solar variability Increase in sunspot activity (and amount of solar radiation emitted in solar flares) since 1880 AD could have warmed Earth by ~0.1oC. Volcanic eruptions Sulphate aerosols in stratosphere block incoming solar radiation (cool Earth by ~0.5oC for 1-3 years).

  5. The Orbital Monsoon Hypothesis Why study the impact of past changes in summer insolation on monsoon rainfall? Climate models indicate that land may warm faster than sea in the future due to anthropogenic greenhouse warming

  6. Assessment of the Orbital Monsoon Hypothesis: Speleothem 18O records

  7. Decreasing NH summer insolation

  8. Natural warming, greenhouse effect, or both? • Need to reconstruct trend in natural climate before the 20th century • Natural climate forcing • Insolation • Millennial bipolar seesaw • Solar variability • El Niños • Volcanic eruptions

  9. Summary of potential climate forcing mechanisms over last 1,000 years Insolation Decreasing summer insolation (in northern hemisphere) may have cooled the high northern latitudes by ~0.2-0.3oC (~0.1oC for NH). Thousand-year-scale changes in thermohaline circulation Little Ice Age cooling from ~1400-1850 AD may have been related to slowing of thermohaline circulation. El Niños An increase in El Niño activity would tend to warm Earth. Solar variability Increase in sunspot activity (and amount of solar radiation emitted in solar flares) since 1880 AD could have warmed Earth by ~0.1oC. Volcanic eruptions Sulphate aerosols in stratosphere block incoming solar radiation (cool Earth by ~0.5oC for 1-3 years).

  10. Sunspot history from telescopes

  11. History of volcanic explosions and solar radiation LIA

  12. Resolution of palaeoclimate records * * *

  13. How to extend the historical record of climate in space and time? Ice cores: Reconstruct air temperature (and snowfall?) at high latitudes and high elevations. Tree rings: Reconstruct soil temperature (and/or rainfall) where seasonal changes are large. Corals: Reconstruct sea-surface temperature (SST) and salinity (SSS) to study coupled ocean-atmosphere climate systems in the tropics (e.g. monsoon, ENSO).

  14. Global distribution of high-resolution palaeoclimate archives

  15. The “Hockey Stick” curve

  16. Global distribution of high-resolution palaeoclimate archives

  17. Coring annually layered high-elevation glaciers

  18. Quelccaya ice cap record (Peru)

  19. Recent melting of Quelccaya ice cap (Peru)

  20. Comparison of polar and tropical ice core records

  21. Analysis of tree-ring width and density

  22. Arctic and Asian tree-ring records

  23. ~2,000-year Huon Pine record from Tasmania

  24. ~300 year coral record of climate change in Kenya

  25. Urvina Bay (Galapagos) earthquake in 1954 AD exposed 11 m diameter coral

  26. UV fluorescence and annual density bands

  27. Pacific coral 18O records

  28. Stacked coral 18O record for Pacific and Indian Oceans

  29. Composite Sr/Ca and 18O record from the Great Barrier Reef

  30. Climatic impacts of the El Niño-Southern Oscillation (ENSO)

  31. Central equatorial Pacific coral 18O records of ENSO

  32. Cobb et al. (2003)

  33. Next time …… Humans and climate

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