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Chapter 13

Chapter 13. Climate During and Since the Last Deglaciation. David Apostalon, Ines Cobeljic, Zak Mohamoud, Jeremy Shaw, Merhawi Zerai. Chapter Overview. History & Causes of Deglaciation Milankovitch Theory Climate records Deglacial two-step The Younger Dryas Deglacial feedbacks

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Chapter 13

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  1. Chapter 13 Climate During and Since the Last Deglaciation David Apostalon, Ines Cobeljic, Zak Mohamoud, Jeremy Shaw, Merhawi Zerai

  2. Chapter Overview • History & Causes of Deglaciation • Milankovitch Theory • Climate records • Deglacial two-step • The Younger Dryas • Deglacial feedbacks • Effects of Deglaciation • Glacial lakes & floods • Sea level rise • Tropical monsoons • Changes in vegetation • Seasonal temperature variations • Current & Future Climate • Insolation predictions • Another glaciation? • Agricultural humans

  3. History of Deglaciation • What is deglaciation? • 2.75 million years of Ice Age • Shifts between glacial and interglacial periods • ABLATION: rate of ice loss

  4. History of Deglaciation • Causes of ablation • Solar radiation • Conduction of heat by air or rain • Calving

  5. History of Deglaciation • Milankovitch Theory • Proposed by Milutin Milanković • Summer insolation controls glaciation • Snow & ice accumulate during winter • But...warm summers will melt ice • So...cool summers required to maintain ice each year • Glaciation occurs during low summer insolation at northern latitudes • Deglaciation occurs during high summer insolation at northern latitudes

  6. History of Deglaciation • What is insolation? • INSOLATION: incident incoming solar radiation(W/m²) ...or... Arriving solar radiation per unit area

  7. History of Deglaciation • Insolation affects temperature

  8. History of Deglaciation • Insolation levels change over time • Determined by cycles of Earth’s precession, tilt, and eccentricity

  9. History of Deglaciation • Summer insolation maximum • Changes in Earth’s tilt and precession caused a summer insolatin maximum at northern latitudes ~10,000 years ago • Insolation rises... deglaciation begins

  10. History of Deglaciation • Shifting climate forces • Glaciers melt • Insolation increases • CO₂ levels increase from 190 to 280 ppm • CH₄ levels double • Balance of power shifts • Melting of glaciers accelerates

  11. History of Deglaciation • Coral reefs: climate records of deglaciation • We can measure the rate of deglaciation by indirect means using coral reef data • Coral reefs grow in shallow waters • As sea levels rise, corals die • We can date ancient corals to determine ancient sea levels • Sea level changes can be converted to ice volume • 1 meter of sea level = 400,000 km³ of ice Sea level rises, old coral dies

  12. History of Deglaciation • Coral reefs: climate records of deglaciation • This data gives us a rate of deglaciation by measuring changes in ancient sea levels using radiocarbon and thorium/uranium dating. 2,000-3,500 year discrepancy...?

  13. History of Deglaciation • Why the discrepancy? • ¹⁴C ages are younger than Th/U ages • Th/U ages are more accurate when compared to tree ring data • ¹⁴C ages are too young!

  14. History of Deglaciation • ¹⁴Carbon Dating • ¹⁴C production in atmosphere varies • Cosmic rays convert ¹⁴N into radioactive ¹⁴C • Weaker magnetic fields = more ¹⁴C • Based on half-life (decay) of ¹⁴C • more ¹⁴C = less time has elapsed • But....extra ¹⁴C causes appearance of less elapsed time

  15. History of Deglaciation • THE DEGLACIAL TIME PERIOD • North American ice sheet began to retreat 15,000 14C years ago • Reached a midpoint ~10,00014C years ago • Ended ~6,00014C years ago. • Timeline is supported by Radiocarbon dating of material found in, under, or atop moraines deposited by the ice • Smaller Scandinavian ice sheet began retreating at the same time, but disappeared a few thousand years earlier

  16. TIMING OF ICE SHEET MELTING History of Deglaciation • N. American ice sheets began retreating 14,000 14C years ago and was gone by 6,000 14C years ago. • To convert area covered by retreating ice to ice volume (thickness x area = volume). The thickness of the ice is debatable because it can be effected by the conditions in its basal layer.

  17. THE DEGLACIAL RISE IN SEA LEVEL History of Deglaciation • Larger glaciers = more melt water • Generally consistent with Milankovitch theory • Rates of sea level rise changed dramatically • Based on coral reef data • Deglacial Two-Step • Rapid rise from 20K to 14K years ago • Slow from 14-12K years • More rapid rise after 12K years

  18. History of Deglaciation • The Deglacial Two-Step pattern points to more complex accelerations and decelerations in melting rates. • Two major influxes of freshwater into oceans due to melting glaciers. • The rates of ice melting were at least four to five times faster during the earlier and later intervals.

  19. History of Deglaciation AN EARLY RAPID MELTING OF ICE SHEET! • A pulse of unusually negative d18O values early in deglaciation. • measured in planktic formanifera • The flux in d18O is result of early melting of nearby Barents ice sheet, north of Scandinavia. • A low- d18O pulse found in cores from the Gulf of Mexico indicates a short-term increase in meltwater down the Mississippi River from N. American ice sheet.

  20. History of Deglaciation • Climate records of Younger Dryas event • All three graphs have correlating time Younger Dryas

  21. The Younger Dryas History of Deglaciation • Northern Hemisphere unexpectedly returned to near-glacial conditions. • Interruption in general deglacial warming • An Artic plant called Dryas reached Europe • Reversal toward Artic vegetation • Evidence comes from pollen records

  22. History of Deglaciation • Last Glacial Maximum • Polar water reached southward across the North Atlantic (45 N°) • southern margin defined by polar front, area of fast transition to more temperate waters to south • 15,000 years ago (early deglaciation) • polar front shifted northwest to point close to eastern Canada • warm water began to flow northward along European coast to moderate climate • moderate climate allowed trees to advance northward from prior full-glacial spot in far-southern Europe

  23. History of Deglaciation • 13,000 years ago - 11,700 years ago (Younger Dryas period) • polar front suddenly shifts back to the south almost reaches glacial position • North Atlantic Ocean cools, and Artic vegetation (Dryas) return to northern Europe • 11,700 years ago polar front quickly retreats north, and forest begin final push into north-central Europe

  24. Why the Younger Dryas oscillation? History of Deglaciation • Wally Broecker(geochemist) • lower N. Atlantic surface density prevented formation of deep water • criticized because the factor of global melting rate was slowing during Younger Dryas • The cause of the Younger Dryas remains a mystery

  25. Feedbacks History of Deglaciation • Positive feedbacks accelerated loss of ice • Between 17,000 and 14,000 years ago spikes in sea level rise caused by ice melting • Increases in concentrations of greenhouse gases • Ice sheets melted and CO₂ and methane levels increased at nearly the same time • Increases in these greenhouse gases caused warming and more melting of ice

  26. Effects of Deglaciation Proglacial Lakes proglacial lakes develop in bedrock depressions left by melting ice sheets. over time lakes move north behind the ice sheets,while the land farther south rebounds toward its undepressed elevation.

  27. Lake Agassiz Largest pro-glacial lake in north America. covered more than 200,000 km2, at depths of 100 m or more. Effects of Deglaciation

  28. Effects of Deglaciation A) Missoula flood deposits. B) ripples in the land, too large to be seen on the ground.

  29. flooding of coastlines Large-scale change on the earths surface by deglacial rise of sea level. Effects of Deglaciation

  30. Increased insolation produced Monsoons Effects of Deglaciation Stronger summer monsoons near 10,000 years ago due to earth's configuration. summer insolation were 8% higher than today in N. Hemisphere Kutzbach Orbital Monsoon model simulations supported by geological observations. Lake levels higher in: - Arabia -North Africa - Southeastern Asia

  31. Timing Effects of Deglaciation 14C dates for lake deposits in N. Africa Match the 10K insolation maximum When corrected for greater 14C production

  32. Effects of Deglaciation • Upwelling in the Arabian Sea • Strong Monsoon winds blowing across Somalia and eastern Arabia • Enhanced Coastal upwelling • altering the planktic foraminifera Species Upwelling in Arabian Sea happened 9,000 years ago

  33. Climate evidence Effects of Deglaciation • Evidence for wet climate range from: • Large dry river valleys in deserts • fossil evidence includes: • Grass pollen in lake deposits • Variety of water loving animals

  34. Shifts in Vegetation Effects of Deglaciation Strong summer insolation led to a northward shift from glaciers. Oaks (warm vegetation) transition from southeast US to Mid Atlantic State Spruce (Cold Vegetation) transition from Central US to Northeastern Canada Models versus Observed data, Spruce and Oak

  35. Effects of Deglaciation • No Analog Vegetation • No record of such vegetation in modern times • Caused by difference change in temperature and environmental variables • Specific spruce tree can’t be found at Midwest US • From Individual pollen record

  36. Vegetation-moisture feedback Effects of Deglaciation High summer insolation peaked at about 9,000 years ago. Wetter soils and increased vegetation provided positive feedback, bringing more moisture farther into the continent of Africa.

  37. insolation reduced monsoons Effects of Deglaciation decreased summer insolation expected to weaken summer monsoons Lake levels in N. Africa match well expected patterns.

  38. Peak Warmth Effects of Deglaciation • with atmospheric CO2 levels steady and high, glacial ice largely melted. • summer insolation and vegetation changed • changes affected temperatures. • insolation 5 percent higher warmed higher latitudes. • displacement of high-albedo tundra by low-albedo spruce caused positive feedback.

  39. Cooling Down Effects of Deglaciation 6,000 Years earth tilt and precession motion move Northern Hemisphere toward Aphelion position 5% decrease overall Tundra move southward replacing forest

  40. Evidence for cooling summer Insolation Effects of Deglaciation Ice cap record from ice cores shows summer melting before 5,000 years ago Greenland diatoms from sea-ice were less before 5,000 years ago Glacier margin reappear 3,500 despite glacial margin significantly melted before 3,500 years Greatly affect diatom species that’s sensitive to temperature (great change in Norway’s southwest coast)

  41. Current and Future Climate • Current and Future Climate • Where will we be 10,000 years from now, climatically speaking? Let’s begin by looking exclusively at astronomical indications (via Milankovic Theory): • The future according to axial tilt • Dominant effect on higher latitudes • Diminishing tilt with time => reduced seasonality => reduced insolation at the higher latitudes • The future according to general precession • Dominant effect on low latitudes (and the global average) • Northern summer solstice will take place at perihelion 10,000 yrs from now; implies greater insolation at the northern tropics and stronger monsoons

  42. Current and Future Climate • The current interglacial period: is global cooling long overdue? • All records of the past indicate that glaciation should be well underway by now • Future models indicate only further heating over the next 50,000 yrs • Agricultural humans: the turning point? • Our video presentation of this last leg – • Placing these projections in the context of changing axial tilt, precession, eccentricity, and so on! Please see: • http://www.youtube.com/watch?v=oRdyNn1tB-E&list=PLmen0eQI4-Lof_GS8Amc9sysdRMrRYzps

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