What have we covered so far – the Basic Questions

1. What have we covered so far – the Basic Questions 1. Climate has not always been similar to the present; in fact has rarely been like the present Holocene climate. 2. Climate depends on a large number of variables - with abundant feedbacks, and climate change is not necessarily intuitive. 3. The scientific community does not understand some basics about climate, even for recent periods where the data are good. This includes the causes of the glacial/interglacial cycles. • Climate change can be both dramatic, and fast: the return to glacial climate during the Younger Dryas may have happened in just a few decades. 5. Computer models are just (digital) plausibility arguments, and are limited by our understanding of what are the important variables, in resolution, and in ability to predict the future.

2. Summary: Climate Conditions during Last Glacial Maximum • Insolation rates about the same as today.

3. Solar Insolation Changes • Summer insolation starts to increase at ~20K years and reaches a maximum at ~10K years. Not much ice can remain over the summer at 10,000 years BP. Raymo’s thermal gradient argument? She didn’t do her calculations forward to the present time….

4. What do present solar insulation models predict for the future?

5. Summary: Climate Conditions during LGM • Insolation rates about the same as today. • Colder (~ -4 º C globally and ~ -10 ºC (or – 20) near the poles and ~-2 to -3 ºC in tropics). • Ice Sheet volume was ~ twice today. • Sea Level lower by ~ 125m. Exposed large continental shelf areas. • Drier and dustier (globally). • Reduced atmospheric CO2 and CH4 levels • Vegetation more arctic like (tundra, steppe). • Deep Ocean circulation more sluggish.

6. colder warmer Millenial Scale Oscillations in Temperature DURING the Last Glacial Period.It wasn’t just cold back then…. During last glacial period (20-100 K yrs BP) there were many large and fast temperature swings recorded in the Greenland ice cores and deep sea sediments from N. Atlantic.

7. Summary: Rapid Climate Change • During the last 100K yrs there have been repeated oscillations between warm (Dansgaard-Oeschger: D-O) and cold (Heinrich) conditions, with very fast temperature changes (7ºC in 50 yrs in Greenland). • The most recent (strong) cold event occurred about 12K yrs ago (Younger-Dryas) during the transition from LGM to current interglacial period. • Current hypothesis is that variations in deep water formation rates in N. Atlantic driven by salinity, and thus poleward heat transport, is a likely mechanism for YD. • These rapid climate change events are strongest in the N. Atlantic, but some evidence that they occurred globally. • Generally, there is an anti-phasing of temperature fluctuations between N. and S. Hemispheres.

8. Rise in Temperature and Atmospheric Gases • Temperature, CO2 and CH4 start increasing ~18K yrs. • Implies changes in radiation budget (temperature), ocean circulation /biology/chemistry (CO2) and precipitation (CH4).

9. Ice Sheet Retreat Retreat begins ~18K and BOTH LIS and CIS ice sheets are gone by ~6K. (Real calendar age = 14C age plus ~ 1700 yrs.)

10. Sea Level Rise • Use 14C and 230Th/238U to date the age of a sequence of submerged corals that lived close to the sea surface. • The rate of sea level rise has pulses. (14C ages are too young by up to ~3K yrs.)

11. Short discussion on why 14C years differ from calendar years If you are dating corals (say from Barbados), you can use both 14C and Th/U age dating techniques. BUT they don’t agree. 14C dates are systematically younger than the Th/U dates. On the same samples.

12. Tree rings (count rings and do 14C dating on wood) show the same effect, the 14C dates are TOO YOUNG.

13. Here’ s why. 14C is generated in the atmosphere by cosmic rays hitting 14N. The 14C decays at a constant rate, but the rate of PRODUCTION of 14C depends on the strength of the geomagnetic field. If the field is strong, fewer cosmic rays hit the atmosphere. If it is weak (see below), then MORE 14C is generated.

14. Formation: 14N + neutron => 14C + proton Decay: 14C => 14N + b- + a neutrino

15. So – if the magnetic field was WEAKER than present, more 14C would have been produced then, and more would still be around (in the corals) now. This would make the corals appear YOUNGER than they are.

17. This means that you have to correct 14C dates for the changes in the geomagnetic field. If you do this, then 14C is a reliable dating technique.

18. Transition from Glacial to Interglacial Conditions • Rapid Bolling-Allerod warming event at ~14.5 Kyrs. • Transition from the LGM climate to present interglacial climate was not smooth. • Younger-Dryas is a period of cooling at ~12 Kyrs that lasted for ~ 1000 yrs. Greenland Ice Core

19. Younger-Dryas Event: atmospheric, marine and continental proxies Y-D climate signal is strongest in N. Atlantic region. .

20. Rapid Temperature Change during the Y-D Sawtooth Pattern of Y-D Heating is rapid, Cooling is slower At end of Y-D, temperature in Greenland increased by 7ºC in 50 yrs.

21. Role of Proglacial Lakes in Climate Change

22. Possible Pathways of Meltwater Flow Also out the Hudson River valley

23. Appearance of Meltwater Pulses From sea level record From d18O-CaCO3 record Why do meltwater pulses show up in d18O-CaCO3 record?

24. Meltwater Pulses as a Climate Change Trigger • Pulse of freshwater discharge into the N. Atlantic would reduce the formation rate of deep water (N. Atlantic Deep Water) by reducing salinity (density) of surface water. • This would reduce the ocean’s transport rate of heat via Gulf Stream to N. Atlantic region and cause cooling in the region.

25. Deep Water Formation: Present vs LGM

26. Possible Impact of Reduced NADW Formation Rates on Air Temperatures

27. Millenial Scale Temperature Oscillations During last glacial period (20-100 K yrs BP) there were lots of large and fast temperature swings recorded in the Greenland ice cores and deep sea sediments from N. Atlantic.

29. Heinrich Events Heinrich Events: Ice Rafted Debris. Remember, these are COLD events

30. Dansgaard-Oeschger and Heinrich Events recorded in the N. Atlantic Region Fairly consistent ~1500 yr period between warm periods (D-O events). Several of the coldest periods are associated with Heinrich events (e.g., Younger-Dryas). Sawtooth pattern of fast warming and slow cooling.

31. NOTE that there are a lot more D-O warm cycles than there are H-events. That is because in order to have H-events, you need the ice sheets to be advancing – which requires both cold temperatures AND moisture back on land to force them to advance (which takes time). I.e., the H-events come at the end of a cold period that is preceded by a warm period.

32. Possible Mechanism: Salt Oscillator Hypothesis: Changes in the salinity of the N. Atlantic, resulting from ice melting or ice formation, determines the strength of NADW formation and, as a result, changes the rate of heat transport to the N. Atlantic region. The H-event

33. Salt Oscillator: Ocean circulation has two modes

34. Heinrich events are defined as ICE-RAFTED debris, found in sediments in the mid-North Atlantic sediments. How are D-O events defined? What parameter defines a WARM D-O event?

35. Model Simulations Does S. Hemisphere warm when the N. Hemisphere cools?

36. Salt Oscillator: Explaining the time sequence of D-O and Heinrich events Hypothesis: The sawtooth pattern of temperature change is caused by a slow decrease in rate of NADW formation, until it eventually stops, which is then followed by rapid return of NADW formation after a critical value of salinity is reached. small big

37. Model Simulations Does S. Hemisphere warm when the N. Hemisphere cools?

38. Antarctic warmed during Younger Dryas • Antarctica temperature increases during Y-D cooling. Ice core data. • Globally, atmospheric CO2 levels increase. • Globally, atmospheric CH4 levels decrease.

39. Inter-hemispheric Seesaw • Model simulations of Heinrich events indicate that reduced heat transport into the N. Atlantic yields less heat loss from S. Hemisphere and thus warming. • Models indicate that when NADW formation is reduced, then Antarctic Bottom Water formation rate increases, which in turn means higher ocean to atmosphere heat transfer and warmer temperatures in Antarctica. • Temperature records during Y-D from Antarctic ice cores indicate warming while Greenland cooled.

40. Were D-O events and Younger-Dryas global? Map showing locations where abrupt climate changes (i.e., D-O events) have been documented in marine sediments (red) or polar ice (blue). Yellow dots show those locations where the last of these events (i.e., Younger Dryas) is recorded by major advances of mountain glaciers. While for most of the globe, these events are in phase, in parts of the Southern Ocean and of the Antarctic ice cap, they are clearly anti-phased.

41. Rapid Climate change evidence in Santa Barbara Basin • Warming events associated with negative d13C, which author interprets is a result of methane hydrate release. • Did ocean circulation rates change between warming and cooling events? Conclusion: a LOT of methane was released during the D-O interstadials into the BOTTOM water of SB Basin.

42. Summary: Rapid Climate Change • During the last 100K yrs there have been repeated oscillation between warm (D-O) and cold (Heinrich) conditions, with very fast temperature changes (7ºC in 50 yrs in Greenland). • The most recent (strong) cold event occurred about 12K yrs ago (Younger-Dryas) during the transition from LGM to current interglacial period. • Current hypothesis is that variations in deep water formation rates in N. Atlantic driven by salinity, and thus poleward heat transport, is a likely mechanism. • These rapid climate change events are strongest in the N. Atlantic, but some evidence that they occurred globally. • Generally, there is an antiphasing of temperature fluctuations between N. and S. Hemispheres.

43. How did the last Interglacial Period (the Eemian) end? The Eemian was warm for about 10,000 years. No ice sheets in N. America or Europe. But at the end of the Eemian, ice sheets grew slowly (over 11K years), and these were punctuated by millennial-scale cold/ice rafting periods (H-events). This has long-term implications for the future: AFTER the present period of greenhouse gas warming is over.

45. Compiled record of temperatures in Northern Europe, including tree-ring data, black line is ‘instrumental’ records, growing (vineyard) seasons.

46. The “Little Ice Age” Sea Ice off southern Iceland, compiled from records of the days that the fishing fleet could actually leave port – and had to return to port.

47. Record of the advance/retreat of mountain glaciers Derived from repeat drawings of the same scene in European paintings.

48. We actually don’t know the cause of the Little Ice Age. Other proposed causes are the Plague (by Ruddiman), that caused massive depopulation of Europe and the re-growth of farmland into forests. Which pulled CO2 out of the atmosphere and cooled the climate.

50. Ice core record of Little Ice Age from Peru (mountain) ice cap. Top graph, showing dust and d18O record of low temperatures (high winds) during interval 1600 to 1900 period. Lower plots showing (left) ice core record taken in 1980, showing annual ice core temperature record. Lower plot (right) showing same coring area in 1993, with ice melting. The first melting of the Peruvian ice cap in 1000 years.