Orbital- Scale Climate Changes GEOG 401: Climatology Dr. John Abatzoglou
Ice Core Sampling Typically performed at top of ice dome where less lateral spreading occurs Diffusion issue can make high- resoluion sampling not feasible
Ancient Ice Annual cycle of precipitaion lays down a record, buried by subsequent snow Air bubbles trapped in each layer Provide info on: Global Temperatures (d- 0-18) Global Ice Volume (d- 0-18) Wind PaUerns (deposits of dust) Global VegetaIon
Ice Core from Vostok, AntarcIca Recall how we deduce temperature in ice core data? 9
What would the delta- 0-18 record look like from AntarcIca?
The Last 5.5 my Ice Volume Lisiecki and Raymo (2005) PosiIve delta O18 values mean that your sample is enriched in the heavy O isotope; negaive delta O18 values mean it s depleted in the heavy O18. As climate cools, marine carbonates record an increase in δ 18 O, enriched in heavier isotopes (light Oxygen locked away in ice) Warming yields a decrease in δ 18 O of marine carbonates. Good proxy for ice volume and temperature
RadioacIve isotopes: a proxy for age RadioacIve isotopes: logarithmic decay over Ime Rare isotopes, release energy through decay process Half- life: Ime required for 50% of the original atoms to decay from Carbon- 14 to Nitrogen- 14 Carbon 14 Sources: cosmic- rays in stratosphere, and release in Nuclear bombs! Half life = 5730 years If you know the raio of 14- C to 14- N, you can then deduce age of given object per mil
Carbon DetecIves Carbon Isotopes: understand fast/slow carbon cycle contribuions C- 12: most abundant carbon isotope plants preferenially take up C- 12 in photosynthesis (biota signal) C- 13: typical of inorganic carbon C- 14: radioacive isotope (decays): forms naturally in stratosphere through UV (proxy for solar acivity). Was also produced during nuclear tesing in 50s. Used extensively in radiocarbon daing as it has a half- life of 5730 years. δ 13 C, expressed again in per mil
Large δ 13 C, means high fracion of inorganic carbon Small δ 13 C, means higher fracion of living/once living carbon
Carbon Reservoirs During Last Glacial Max Atmospheric CO2 100ppm lower than present Ice sheet coverage and paleoecological evidence (i.e. pollens) tell us that there was much less carbon in the terrestrial/soil components Upper ocean equilibrates w/atmosphere, meaning carbon sink was deep ocean
Isotopic Record During cold periods Uptake of carbon by deep ocean and decrease in terrestrial carbon- 12 Oceans become depleted in delta- C- 13 and enriched in delta- 0-18
Delta- C- 13 record of Atmosphere Typically delta- C- 13 decreases during cooling Fossil Fuel (from long- dead plants) contain less C- 13 than standard air, thus decreasing atmospheric d- O- 13 concentraions Through delta- C- 14 of atmospheric observaions, we can also derive that added carbon to atmosphere is very depleted in C- 14
Orbital changes: Milankovitch Theory Serbian astrophysicist in 1920 s who suggested solar energy changes and seasonal contrasts varied with small variaions in Earth s orbit Changes in shape of the earth s orbit around sun: Eccentricity (100,000 years) Wobbling of the earth s axis of rotaion: Precession (23,000 years) Changes in the Ilt of earth s axis: Obliquity (41,000 years)
Eccentricity (100,000yr) The more eccentric the orbit, the greater variability in solar radiaion over the annual cycle Note the orientaion Of the axis Ilt
Obliquity (41,000 yrs) Change in tilt of axis 1. Weak Ilt means summers at high laitudes receive less radiaion than during a period of high Ilt 2. Cooler condiions tend to coincide with decreases in Earth s Ilt as cooler summers can t melt the past winter s snow.
Precession (23,000 yrs) Wobble
RadiaIon and the Monsoonal CirculaIon Red= 11kya NH perspecive
Proxy data from caves Stalagmite contain calcite and formed via groundwater deposits. Oxygen isotope records can provide informaion on precip:evap raio NegaIve delta- O- 18 records suggest stronger monsoon since heavy oxygen isotope tends to occur in only very dry years (E>>P)
Local Paleoclimate Records (Aside) Changes is d- O- 18 indicaive of changes in jet- stream, moisture source
Local Paleoclimate Records Depleted in oxygen 18 during wet epochs Enriched in oxygen 18 during dry epochs
Isotope DetecIves Isotopes of Local Water Source of Water for Beverages vs. Local Water Incorporated into proteins in body that contain signature of source
Cave records Hemispheric Changes in monsoon strength P>E P<E Larger delta- 0-18 = drier condicons
As a funcion of temperature
FormaIon of Glaciers Summer temperatures at high laitudes are criical Summer insolaion at 65 degrees laitude is a driver Warmer summers at high laitudes means increased melt thereby reducing global albedo thereby warming surface temperatures thereby increasing melt PosiIve feedback of oceanic carbon uptake
Orbits for Glacial Advance Cooler summers in the NH means that ice/snow are less likely to melt and thus retain for growth during winter. GROWTH>MELT N S Key component here is to de- emphasize seasonality in the northern hemisphere summer (cool summers) N S
10 Mya, too warm to start ice- albedo feedbacks
How much should orbital cycles alter global temperatures? w/o feedbacks, not much Few degrees C, redistribuion of heat globally/seasonally No direct change in greenhouse, solar constant or albedo ObservaIons suggest glacial- interglacial temperature change is about 6-8C
Chicken before the Egg? Records suggest that CO2 lags temperature change by 100-1000 years What this tells us: 1. DeglaciaIon is not inicated by CO2 but by orbital cycles 2. CO2 amplifies the changes which cannot be explained by orbital cycles alone increased dust during glaciaion may increase producivity and drawdown CO 2 Ocean- solubility feedbacks (+ feedbacks) CO2 amplified glacial- interglacial transiions