1. What do we mean by "paleoclimate"? 2. What evidence exists for ice ages and ancient climate change? 3. What causes the climate to change? What we wish to learn Today: Past Climate Change and the Ice Ages

“Recent” climate change and variability… N.H. Temperature (°C) Year Mann et al. (1999) GRL 26:

…provides perspective on where we are headed IPCC Projections to N.H. Temperature (°C) Global Temperature (°C)

The great Aletsch glacier, Switzerland We KNOW the climate has changed recently

L.Thompson, in prep. Ice on Mt. Kilimanjaro Area (km 2 ) Year

Glacial Changes since last Ice Age Percent of Spruce in Total Trees Ice and Trees

Glacial Europe was treeless in the last ice age Vegetation in present day Europe is dominated by forest, with conifers in the north and deciduous trees in the south. At the glacial maximum, 20 thousand years ago, arctic tundra covered much of Europe south of the ice sheet, and only patches of forests remained near the southern coasts.

Climate was warm during the Age of the Dinosaurs (the Mesozoic) –Alligators lived in Siberia! –Dinosaurs lived north of the Arctic Circle in Alaska! Ancient Climates Mesozoic PreCambrian Paleozoic Cenozoic

Methods to establish past climate Isotopic Geochemical Studies: the study of rock isotopic ratios, ice core bubbles, etc. Dendochronology: the study of tree rings Pollen Distribution: the study of plant types and prevalence (e.g., Europe’s vegetation in the last ice age) Lake Varves: (like dendochronology, but with lake sediments) Coral Bed Rings: (like dendochronology, but with corals) Fossils: Studies of geological settings, etc. Historical documents: paintings of glaciers, etc.

Oxygen has three stable isotopes: 16 O, 17 O, and 18 O. (We only care about 16 O and 18 O) 18 O is heavier than 16 O (it has 2 extra neutrons). The amount of 18 O compared to 16 O is expressed using “delta” notation - the unit is “per mil” (parts per thousand) : Fractionation: Natural processes tend to preferentially take up the lighter isotope, and preferentially leave behind the heavier isotope. For most chemistry, the isotopes behave the same. Oxygen isotopes and paleoclimate  18 O ‰ = 18 O/ 16 O of sample - 18 O/ 16 O of standard 18 O/ 16 O of standard  1000

Isotope “fractionation” Oxygen isotopes are fractionated during evaporation and precipitation of H 2 O –H 2 16 O evaporates more readily than H 2 18 O –H 2 18 O precipitates more readily than H 2 16 O Oxygen isotopes are also fractionated by marine organisms that secrete CaCO 3 shells. The organisms preferentially take up more 16 O as temperature increases.

Ocean H 2 16 O, H 2 18 O Evaporation favors H 2 16 O H 2 18 O Precipitation favors H 2 18 O (1) Sea water is heavier than water vapor Land Ice (3) Snow and ice are depleted in H 2 18 O relative to sea water. (2) cloud water becomes more depleted in H 2 18 O as it moves inland or poleward… 18 O 16 O 18 O 16 O 18 O 16 O 18 O 16 O Fractionation effects

Ocean δ 18 O = 0 o/oo Land Ice Carbonate sediments also record the signal of the ocean, and the signal of temperature CaCO 3 δ 18 O = -10 o/oo Fractionation effects δ 18 O = -15 o/oo -6 o/oo δ 18 O = -20 o/oo -11 o/oo rain

Vostok Record  We can also show that the  18 O of precipitation is well correlated with temperature!  So, if we know the  18 O of water or ice, we know what the air temperature was at that time. (Note that hydrogen isotopes work the same way)

The Antarctic Ice Coring operation at Vostok station The Greenland Ice Coring operation at Summit station

The ice can be analyzed for its 18 O content to estimate temperature The air bubbles trapped in the ice can be analyzed for their carbon dioxide and methane content Ice Core Analyses

Ocean Sediment analysis Isotopes of organisms The “Ice Volume” effect Light isotope removed from ocean, locked into large ice sheets Remaining ocean water was +1.5‰ heavier in 18 O, as recorded in marine organism shells (CaCO 3 ) Ocean level was ~120 m lower than today Growing glaciers deep-sea foraminifera δ 18 O = - 35 δ 18 O = - 30 Ice δ 18 O = 1.5 δ 18 O = 0.0 Glacial Interglacial

Possible Causes of Climate Change Power: 4 x W 2 x W Long-Term 1.Solar Luminosity 2.Shifting Continents 3.Greenhouse gases Medium-Term 1.Orbital parameters 2.Greenhouse gases Short-Term 1.Greenhouse gases 2.Sunspots 3.Ocean currents

Evolution of our Sun‘s Luminosity Time (billions of years) Luminosity Snowball Earth ? Today

Shifting land masses (by plate tectonics) may have changed greenhouse gas concentrations, thus affecting climate Today’s configuration Past configurations

As the continents shift there is increased subduction and volcanic activity which increases CO 2 into the atmosphere That atmospheric CO 2 is then consumed in weathering reactions on continents, and eventually returned to the ocean. This is the long-term “weathering” control of climate.

Silicate weathering Ca 2 H 2 CO 3 Ca 2 ①CO 2 + H 2 O  H 2 CO 3 (carbonic acid) ②CaSiO 3 + 2H 2 CO 3  Ca HCO SiO 2 + H 2 O (silicate weathering) ③Ca HCO 3 -  CaCO 3 + H 2 CO 3 (carbonate precipitation & burial) Ca 3 SiO 3 From C. Poulsen’s lecture, 24 Sep CO 2 Net: CaSiO 3 + CO 2  CaCO 3 + SiO 2 Conversion of CO 2 gas to limestone! CO 2

Orbital forcing (Milankovitch) Shape (eccentricity, ~100K and 400K yrs) 2. Tilt (obliquity, ~41,000 yrs = 41K yrs) 3. Wobble (precession, ~23K yrs)

Interaction of orbital periods give different patterns of change. The magnitude of shifts in solar insolation are large enough to explain changes in climate

23 ky ~ 23 ky Tilt 41 ky Milankovitch Forcing Explains Ice Core Data Milankovitch Forcing Explains Ice Core Data 1000s of years Before Present ( kyr B.P. )

 18 O in Chinese caves and insolation D. Yuan et al., Science 304, s of years Before Present ( kyr B.P. ) GISP2 Ice Core Orbital forcing

What causes rapid and unpredictable changes in climate? order chaos order Greenland chaos Antarctica 1000s of years Before Present ( kyr B.P. )

Causes of Climate change A. Tectonic C. ??B. Orbital D. ??

Summary 1.Past changes in climate have been dramatic on Earth 2.The longest-term changes (100s Million years, Ma) are driven by shifting continents and interactions with greenhouse gases. 3.At medium time scales (1-10s Ma), changes are triggered by variations in orbital characteristics. Take-home point: “If you don’t like the climate, hang around awhile…”