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Humans and climate change Continued … Has El Niño caused 20th century warming?

Humans and climate change Continued … Has El Niño caused 20th century warming? The “ Early Anthropogenic Hypothesis ” : When did humans begin to play a role in climate change? Earth ’ s sensitivity to greenhouse gases Why has the warming since 1850 AD been so small?

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Humans and climate change Continued … Has El Niño caused 20th century warming?

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  1. Humans and climate change • Continued … Has El Niño caused 20th century warming? • The “Early Anthropogenic Hypothesis”: When did humans begin to play a role in climate change? • Earth’s sensitivity to greenhouse gases • Why has the warming since 1850 AD been so small? • Delayed warming: the ocean • Cooling from anthropogenic aerosols • Brief summary of potential final exam topics

  2. Humans and climate change Reading: Ruddiman, W. F. (2008). Earth’s Climate: Past and Future. Chapter 15: 285 - 288 Chapter 16: 306 - 308 Chapter 18: 326 - 342

  3. Practical Assessment • Value: 10% of course mark • When: Today (start 2:30 PM) • Tomorrow (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. Climatic impacts of the El Niño-Southern Oscillation (ENSO)

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

  7. Cobb et al. (2003)

  8. Is the present interglacial a bit strange?

  9. “Wrong way” CO2 and CH4 trends during the Holocene? Deforestation began ~8 kyr ago Rice cultivation expanded rapidly ~5 kyr ago

  10. Spread of agriculture over the last 10,000 years

  11. Impact of pandemics on atmospheric CO2? Drop in atmospheric CO2 related to forest regrowth during two great pandemics? (1) The “Black Death”: Outbreak of bubonic plague killed 25 million Europeans (33% of population) between 1347 and 1352 AD. (2) The “American pandemic”: A host of diseases carried by Europeans killed ~50 million native Americans (85-90% of population) between 1492 and 1700 AD.

  12. Human production of CO2

  13. Where does the CO2 produced by humans go?

  14. d13C and CO2 for last 500 years from ice core bubbles at Siple Station, Antarctica d13C Ice cores show progressive increase in 12C in atmospheric CO2 from burning of isotopically light fossil fuels ~1.5‰ decrease over last century in atmosphere ~0.8‰ decrease in surface ocean CO2

  15. Estimates of 2 x CO2 sensitivity from Earth history

  16. Components of 2 x CO2 warming

  17. Why has the warming since 1850 been so small?

  18. Response times in Earth’s climate system

  19. Delayed warming in the climate system

  20. Impact of aerosol cooling on greenhouse gas warming

  21. Sulfate aerosol plumes emitted from industrial smokestacks ~15% of greenhouse warming offset?

  22. History of preindustrial and anthropogenic sulfates: The net effect is cooling.

  23. Distribution of carbon-rich brown-cloud hazes Cause: Biomass burning, cooking / heating Impact: Absorb incoming radiation ~2-3 km above Earth’s surface ~50% of greenhouse warming offset?

  24. Climate projection for a 2 x CO2 world

  25. The Indian Ocean Dipole 30oN SST anomaly +2oC +1 0 0o -1 -2 25oS 30oN Rainfall anomaly +200 mm month-1 +100 0 0o -100 -200 25oS 30oE 130oE

  26. Instrumental DMI (Saji et al. 1999) Instrumental DMI (Reynolds & Smith v2) Coral DMI Coral record of the Indian Ocean Dipole Eastern corals Coral DMI reconstruction Western corals warmer / wetter -0.5 d18O anomaly (‰) 0.0 0.5 cooler / drier 4 IOD events Dipole Mode Index (normalised) 2 0 -2 2000 1860 1880 1900 1920 1940 1960 1980 Year AD

  27. 25 y moving averages (Jul-Nov) 0.5 Rainfall anomaly (normalised) 0.0 window -0.5 -1.0 -1.5 2000 1860 1880 1900 1920 1940 1960 1980 Year AD Opposing trends in IOD-season rainfall Malindi, Kenya (wetter in western IOD sector) Padang, Sumatra (drier in eastern IOD sector) Pre-1940 mean

  28. EMSC1008 Final Exam Exam format for Earth's climate module * Short answers for questions related to palaeoclimatology and climate change lectures for weeks 10 - 13. * Each question will require about 10 minutes (1/3rd of exam, 15% of course mark) * Need to understand fundamental concepts (no calculations, but you will be asked to support some answers with sketches).

  29. Possible exam topics for climate module • (To be reviewed tomorrow) 1. Controls on atmospheric CO2 and long-term climate 2. Plate tectonics and multi-million-year climate change • From greenhouse to icehouse: the last 55 Myr • Milankovitch theory and the origin of ice ages • Glacial-interglacial changes in atmospheric CO2 and CH4 • Abrupt climate change and the "bipolar seesaw” 7. The orbital monsoon hypothesis • 8. Climate change over the last 1000 years • 9. Humans and climate change

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