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Glaciers

Glaciers. Astronomical Control of Solar Radiation. Earth's present-day orbit around the Sun Not permanent Varies at cycles from 20,000-400,000 years Changes due to Tilt of Earth's axis Shape of Earth’s yearly path of revolution around the Sun. d 18 O Record from Benthic Foraminifera.

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Glaciers

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  1. Glaciers

  2. Astronomical Control of Solar Radiation • Earth's present-day orbit around the Sun • Not permanent • Varies at cycles from 20,000-400,000 years • Changes due to • Tilt of Earth's axis • Shape of Earth’s yearly path of revolution around the Sun

  3. d18O Record from Benthic Foraminifera • Ice volume and T move d18O in same direction • Two main trends • Cyclic oscillations • Orbital forcing • Dominant cycles changed over last 2.75 my • Long-term slow drift • Change in CO2 • Constant slow cooling

  4. Lowest level of submerged corals is 120 meters below present-day sea level Corals formed at 15-20 meters below present-day sea level Corals found about 6 meters above present-day sea level

  5. Orbital-Scale Change in CH4 & CO2 • Important climate records from last 750 kya • Direct sampling of greenhouse gases in ice • Critical questions must be addressed • Before scale of variability in records determined • Reliability of age dating of ice core? • Mechanisms and timing of gas trapping? • Accuracy of the record? • How well gases can be measured? • How well do they represent atmospheric compositions and concentrations?

  6. Carbon Dioxide • Measurements of CO2 concentration • Core from rapidly accumulating ice • Merge well with instrumental data

  7. Methane • Measurements of CH4 concentration • Core from rapidly accumulating ice • Merge well with instrumental data

  8. Orbital-Scale Changes in CH4 • CH4 variability • Interglacial maxima 550-700 ppb • Glacial minima 350-450 ppb • Five cycles apparent in record • 23,000 precession period • Dominates low-latitude insolation • Resemble monsoon signal • Magnitude of signals match

  9. Monsoon forcing of CH4 • Match of high CH4 with strong monsoon • Strongly suggests connection • Monsoon fluctuations in SE Asia • Produce heavy rainfall, saturate ground • Builds up bogs • Organic matter deposition and anaerobic respiration likely • Bogs expand during strong summer monsoon • Shrink during weak summer monsoon

  10. Orbital-Scale Changes in CO2 • CO2 record from Vostok • Interglacial maxima 280-300 ppm • Glacial minima 180-190 ppm • 100,000 year cycle dominant • Match ice volume record • Timing • Asymmetry • Abrupt increases in CO2 match rapid ice melting • Slow decreases in CO2 match slow build-up of ice

  11. Orbital-Scale Changes in CO2 • Vostok 150,000 record • 23,000 and 41,000 cycles • Match similar cycles in ice volume • Agreement suggests cause and effect relationship • Relationship unknown • e.g., does CO2 lead ice volume? • Correlations not sufficient to provide definite evaluation

  12. Climate in the 20th Century • Did climate really change in the 20th Century… or perhaps earlier? • How much of this change can be attributed to human impacts on the climate system? • Do we have to do anything about it?

  13. Carbon Dioxide

  14. Did climate really change in the 20th Century? • According to IPCC’s Third Assessment Report, Climate Change 2001: • The global average surface temperature has increased 0.6±0.2ºC over the 20th Century. • Globally, it is very likely that the 1990s were the warmest decade and 1998 the warmest year in the instrumental record, since 1861 (and likely the warmest in the last 1000 years). 2001 was second warmest according to NCDC.

  15. Variations of Earth’s surface temperature for the past 140 years From Climate Change 2001: SFP WG I

  16. Variations of Earth’s surface temperature for the past 1000 years From Climate Change 2001: SFP WG I

  17. How much of this change can be attributed to human impacts on the climate system? Climate Change 2001:“There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activity.” NRC 2001: “The changes observed over the last several decades are likely mostly due to human activities.”

  18. How do we know?? • Measure increases in atmospheric concentrations of known greenhouse gases. • Calculate the effects of these gases on the Earth’s heat budget. • Model the response of global climate to calculated radiative forcing. • Look at similar episodes in geologic record

  19. Indicators of human influence on the atmosphere:

  20. Average annual decrease in 13C = ~0.014‰ y-1 over the last 10 years • Reflects the influence of fossil-fuel carbon combustion in the atmosphere • Additional influences of temporal variations in oceanic and terrestrial components of the global carbon cycle are also evident d13C of CO2

  21. Climate in the 20th Century • Did climate really change in the 20th Century? YES +0.6ºC • How much of this change can be attributed to human impacts on the climate system? YES about +0.4ºC • Do we have to do anything about it? ??

  22. (Ruddiman 2005)

  23. Ruddiman’s Hypothesis • Human activity influenced atmospheric gas concentrations • CO2 increased about 8,000 years ago • CH4 increased about 5,000 years ago • Halted the development of another ice age • Without increase in greenhouse gases, northern hemisphere would have cooled by 4ºC • Cool enough to form glacial ice

  24. Ruddiman’s Hypothesis • Challenges the conventional assumption that greenhouse gases released by human activities have perturbed the earth’s climate only with the last 200 y • New evidence suggests instead that our human ancestors began contributing significant quantities of greenhouse gases to the atmosphere thousands of years earlier by clearing forests and irrigating fields to grow crops • As a result, human beings kept the planet notably warmer than it would have been otherwise – and possibly even averted the start of a new ice age!

  25. Agricultural terraces have been constructed for ~2000 years. The photo on the right are terraces in Guizhou Province, China.

  26. Minimum Ice

  27. (Ferretti and others 2005)

  28. What happened in 1492?

  29. Plows, Plagues & Petroleum

  30. References Worth Reading • Ruddiman W. F. (2005) How did humans first alter global climate? Scientific American292: 46-53. • Ruddiman W. F. (2005) Plows, Plagues & Petroleum, Princeton University Press, 202 p.

  31. Homework Question 1 • How does glacial growth or shrinkage result from the balance between ablation and accumulation? • If ablation is equal to accumulation, the glacier is gaining as much ice as it is losing and will neither grow nor shrink. If accumulation is greater, the glacier will grow. If ablation is greater, the glacier will shrink.

  32. Homework Question 2 • How do glaciations affect sea level? • The majority of the snow that forms glaciers originates as water vapor from the oceans. As ice accumulates on land, moisture is permanently removed from the oceans and sea level goes down. As glacial ice melts, sea level rises.

  33. Homework Question 3 • How does carbon dioxide (CO2) in the Earth’s atmosphere affect climate? • Carbon dioxide is a "greenhouse" gas. This means that it absorbs infrared radiation and warms the atmosphere. Increases in the atmosphere's carbon dioxide content are expected to correspond to warmer climates.

  34. Homework Question 4 • When did humans 1st begin to affect Earth’s climate? • Subject covered in today’s lecture…

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