1. Glaciers

2. Astronomical Control of Solar Radiation Earth's present-day orbit around the Sun 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 axisTilt of Earth's axis Shape of Earth’s yearly path of revolution around the SunShape of Earth’s yearly path of revolution around the Sun

3.  18 O Record from Benthic Foraminifera Ice volume and T move  18 O in same direction Ice volume and T move  18 O in same direction Two main trends Two main trends  Cyclic oscillations  Orbital forcing Dominant cycles changed over last 2.75 myDominant cycles changed over last 2.75 my  Long-term slow drift  Change in CO 2  Constant slow cooling

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

5. Orbital-Scale Change in CH 4 & CO 2 Important climate records from last 750 kya Important climate records from last 750 kya  Direct sampling of greenhouse gases in ice Critical questions must be addressed 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 gases can be measured? How well do they represent atmospheric compositions and concentrations?How well do they represent atmospheric compositions and concentrations?

6. Carbon Dioxide Measurements of CO 2 concentration Measurements of CO 2 concentration  Core from rapidly accumulating ice  Merge well with instrumental data

7. Methane Measurements of CH 4 concentration Measurements of CH 4 concentration  Core from rapidly accumulating ice  Merge well with instrumental data

8. Orbital-Scale Changes in CH 4 CH 4 variability CH 4 variability  Interglacial maxima 550- 700 ppb  Glacial minima 350-450 ppb Five cycles apparent in record Five cycles apparent in record  23,000 precession period  Dominates low-latitude insolation  Resemble monsoon signal Magnitude of signals matchMagnitude of signals match

9. Monsoon forcing of CH 4 Match of high CH 4 with strong monsoon Match of high CH 4 with strong monsoon  Strongly suggests connection Monsoon fluctuations in SE Asia Monsoon fluctuations in SE Asia  Produce heavy rainfall, saturate ground  Builds up bogs Organic matter deposition and anaerobic respiration likelyOrganic matter deposition and anaerobic respiration likely –Bogs expand during strong summer monsoon –Shrink during weak summer monsoon

10. Orbital-Scale Changes in CO 2 CO 2 record from Vostok CO 2 record from Vostok  Interglacial maxima 280-300 ppm  Glacial minima 180-190 ppm 100,000 year cycle dominant 100,000 year cycle dominant Match ice volume record Match ice volume record  Timing  Asymmetry  Abrupt increases in CO 2 match rapid ice melting  Slow decreases in CO 2 match slow build-up of ice

11. Orbital-Scale Changes in CO 2 Vostok 150,000 record Vostok 150,000 record  23,000 and 41,000 cycles  Match similar cycles in ice volume Agreement suggests cause and effect relationship Agreement suggests cause and effect relationship  Relationship unknown  e.g., does CO 2 lead ice volume?  Correlations not sufficient to provide definite evaluation

12. Climate in the 20 th Century Did climate really change in the 20 th 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 20 th Century? According to IPCC’s Third Assessment Report, Climate Change 2001: 1.The global average surface temperature has increased 0.6±0.2ºC over the 20 th Century. 2.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. From Climate Change 2001: SFP WG I Variations of Earth’s surface temperature for the past 140 years

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

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  13 C = ~0.014‰ y -1 over the last 10 years Average annual decrease in  13 C = ~0.014‰ y -1 over the last 10 years Reflects the influence of fossil-fuel carbon combustion in the atmosphere 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 Additional influences of temporal variations in oceanic and terrestrial components of the global carbon cycle are also evident  13 C of CO 2

21. Climate in the 20 th Century Did climate really change in the 20 th 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 Human activity influenced atmospheric gas concentrations  CO 2 increased about 8,000 years ago  CH 4 increased about 5,000 years ago Halted the development of another ice age 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 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 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! 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.

28. Minimum Ice

30. (Ferretti and others 2005)

32. What happened in 1492?

33. Plows, Plagues & Petroleum

35. References Worth Reading Ruddiman W. F. (2005) How did humans first alter global climate? Scientific American 292: 46-53. Ruddiman W. F. (2005) How did humans first alter global climate? Scientific American 292: 46-53. Ruddiman W. F. (2005) Plows, Plagues & Petroleum, Princeton University Press, 202 p. Ruddiman W. F. (2005) Plows, Plagues & Petroleum, Princeton University Press, 202 p.

36. Homework Question 1 How does glacial growth or shrinkage result from the balance between ablation and accumulation? 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. 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.

37. Homework Question 2 How do glaciations affect sea level? 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. 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.

38. Homework Question 3 How does carbon dioxide (CO 2 ) in the Earth ’ s atmosphere affect climate? How does carbon dioxide (CO 2 ) 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. 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.

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