Presentation on theme: "Climate During and Since the Last Deglaciation"— Presentation transcript:
1Climate During and Since the Last Deglaciation Chapter 13Climate During and Since the Last DeglaciationDavid Apostalon, Ines Cobeljic, Zak Mohamoud, Jeremy Shaw, Merhawi Zerai
2Chapter Overview History & Causes of Deglaciation Effects of DeglaciationGlacial lakes & floodsMilankovitch TheorySea level riseClimate recordsTropical monsoonsDeglacial two-stepChanges in vegetationThe Younger DryasSeasonal temperature variationsDeglacial feedbacksCurrent & Future ClimateInsolation predictionsAnother glaciation?Agricultural humans
3History of Deglaciation What is deglaciation?2.75 million years of Ice AgeShifts between glacial and interglacial periodsABLATION: rate of ice loss
4History of Deglaciation Causes of ablationSolar radiationConduction of heat by airor rainCalving
5History of Deglaciation Milankovitch TheoryProposed by Milutin MilankovićSummer insolation controls glaciationSnow & ice accumulate during winterBut...warm summers will melt iceSo...cool summers required to maintain ice each yearGlaciation occurs during low summer insolation at northern latitudesDeglaciation occurs during high summer insolation at northern latitudes
6History of Deglaciation What is insolation?INSOLATION: incident incoming solar radiation (W/m²)...or...Arriving solar radiation per unit area
7History of Deglaciation Insolation affects temperature
8History of Deglaciation Insolation levels change over timeDetermined by cycles of Earth’s precession, tilt, and eccentricity
9History of Deglaciation Summer insolation maximumChanges in Earth’s tilt and precession caused a summer insolatin maximum at northern latitudes ~10,000 years agoInsolation rises... deglaciation begins
10History of Deglaciation Shifting climate forcesGlaciers meltInsolation increasesCO₂ levels increase from 190 to 280 ppmCH₄ levels doubleBalance of power shiftsMelting of glaciers accelerates
11History of Deglaciation Coral reefs: climate records of deglaciationWe can measure the rate of deglaciation by indirect means using coral reef dataCoral reefs grow in shallow watersAs sea levels rise, corals dieWe can date ancient corals to determine ancient sea levelsSea level changes can be converted to ice volume1 meter of sea level = 400,000 km³ of iceSea level rises, old coral dies
12History of Deglaciation Coral reefs: climate records of deglaciationThis data gives us a rate of deglaciation by measuring changes in ancient sea levels using radiocarbon and thorium/uranium dating.2,000-3,500 yeardiscrepancy...?
13History of Deglaciation Why the discrepancy?¹⁴C ages are younger than Th/U agesTh/U ages are more accurate when compared to tree ring data¹⁴C ages are too young!
14History of Deglaciation ¹⁴Carbon Dating¹⁴C production in atmosphere variesCosmic rays convert ¹⁴N into radioactive ¹⁴CWeaker magnetic fields = more ¹⁴CBased on half-life (decay) of ¹⁴Cmore ¹⁴C = less time has elapsedBut....extra ¹⁴C causes appearance of less elapsed time
15History of Deglaciation THE DEGLACIAL TIME PERIODNorth American ice sheet began to retreat 15,000 14C years agoReached a midpoint ~10,000 14C years agoEnded ~6,000 14C years ago.Timeline is supported by Radiocarbon dating of material found in, under, or atop moraines deposited by the iceSmaller Scandinavian ice sheet began retreating at the same time, but disappeared a few thousand years earlier
16TIMING OF ICE SHEET MELTING History of DeglaciationTIMING OF ICE SHEET MELTINGN. 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.
17THE DEGLACIAL RISE IN SEA LEVEL History of DeglaciationTHE DEGLACIAL RISE IN SEA LEVELLarger glaciers = more melt waterGenerally consistent with Milankovitch theoryRates of sea level rise changed dramaticallyBased on coral reef dataDeglacial Two-StepRapid rise from 20K to 14K years agoSlow from 14-12K yearsMore rapid rise after 12K years
18History 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.
19History of Deglaciation AN EARLY RAPID MELTING OF ICE SHEET!A pulse of unusually negative d18O values early in deglaciation.measured in planktic formaniferaThe 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.
20History of Deglaciation Climate records of Younger Dryas eventAll three graphs have correlating timeYounger Dryas
21History of Deglaciation The Younger DryasNorthern Hemisphere unexpectedly returned to near-glacial conditions.Interruption in general deglacial warmingAn Artic plant called Dryas reached EuropeReversal toward Artic vegetationEvidence comes from pollen records
22History of Deglaciation Last Glacial MaximumPolar water reached southward across the North Atlantic (45 N°)southern margin defined by polar front, area of fast transition to more temperate waters to south15,000 years ago (early deglaciation)polar front shifted northwest to point close to eastern Canadawarm water began to flow northward along European coast to moderate climatemoderate climate allowed trees to advance northward from prior full-glacial spot in far-southern Europe
23History of Deglaciation 13,000 years ago - 11,700 years ago (Younger Dryas period)polar front suddenly shifts back to the south almost reaches glacial positionNorth Atlantic Ocean cools, and Artic vegetation (Dryas) return to northern Europe11,700 years ago polar front quickly retreats north, and forest begin final push into north-central Europe
24Why the Younger Dryas oscillation? History of DeglaciationWhy the Younger Dryas oscillation?Wally Broecker (geochemist)lower N. Atlantic surface density prevented formation of deep watercriticized because the factor of global melting rate was slowing during Younger DryasThe cause of the Younger Dryas remains a mystery
25History of Deglaciation FeedbacksPositive feedbacks accelerated loss of iceBetween 17,000 and 14,000 years ago spikes in sea level rise caused by ice meltingIncreases in concentrations of greenhouse gasesIce sheets melted and CO₂ and methane levels increased at nearly the same timeIncreases in these greenhouse gases caused warming and more melting of ice
26Effects of Deglaciation Proglacial Lakesproglacial 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.
27Effects of Deglaciation Lake AgassizLargest pro-glacial lake innorth America.covered more than 200,000km2, at depths of 100 m ormore.
28Effects of Deglaciation A) Missoula flood deposits.B) ripples in the land, too large to be seen on the ground.
29Effects of Deglaciation flooding of coastlinesLarge-scale change onthe earths surface by deglacial rise of sea level.
30Increased insolation produced Monsoons Effects of DeglaciationIncreased insolation produced MonsoonsStronger summer monsoons near 10,000 years ago due to earth's configuration.summer insolation were 8% higher than today in N. HemisphereKutzbach Orbital Monsoonmodel simulations supported by geological observations.Lake levels higher in:- Arabia-North Africa- Southeastern Asia
31Effects of Deglaciation Timing14C dates for lake deposits in N. AfricaMatch the 10K insolation maximumWhen corrected for greater 14C production
32Effects of Deglaciation Upwelling in the Arabian SeaStrong Monsoon winds blowing across Somalia and eastern ArabiaEnhanced Coastal upwellingaltering the planktic foraminifera SpeciesUpwelling in Arabian Sea happened 9,000 years ago
33Effects of Deglaciation Climate evidenceEvidence for wet climate range from:Large dry river valleys in desertsfossil evidence includes:Grass pollen in lake depositsVariety of water loving animals
34Effects of Deglaciation Shifts in VegetationStrong summer insolation led to a northward shift from glaciers.Oaks (warm vegetation) transition from southeast US to Mid Atlantic StateSpruce (Cold Vegetation) transition from Central US to Northeastern CanadaModels versus Observed data, Spruce and Oak
35Effects of Deglaciation No Analog VegetationNo record of such vegetation in modern timesCaused by difference change in temperature and environmental variablesSpecific spruce tree can’t be found at Midwest USFrom Individual pollen record
36Vegetation-moisture feedback Effects of DeglaciationVegetation-moisture feedbackHigh 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.
37insolation reduced monsoons Effects of Deglaciationinsolation reduced monsoonsdecreased summer insolation expected to weaken summer monsoonsLake levels in N. Africa match well expected patterns.
38Effects of Deglaciation Peak Warmthwith atmospheric CO2 levels steady and high, glacial ice largely melted.summer insolation and vegetation changedchanges affected temperatures.insolation 5 percent higher warmed higher latitudes.displacement of high-albedo tundra by low-albedo spruce caused positive feedback.
39Effects of Deglaciation Cooling Down6,000 Years earth tilt and precession motion move Northern Hemisphere toward Aphelion position5% decrease overallTundra move southward replacing forest
40Evidence for cooling summer Insolation Effects of DeglaciationEvidence for cooling summer InsolationIce cap record from ice cores shows summer melting before 5,000 years agoGreenland diatoms from sea-ice were less before 5,000 years agoGlacier margin reappear 3,500 despite glacial margin significantly melted before 3,500 yearsGreatly affect diatom species that’s sensitive to temperature (great change in Norway’s southwest coast)
41Current 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 tiltDominant effect on higher latitudesDiminishing tilt with time => reduced seasonality => reduced insolation at the higher latitudesThe future according to general precessionDominant 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
42Current 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 nowFuture models indicate only further heating over the next 50,000 yrsAgricultural 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: