Climate of Past—Clues to Future ► Climate has changed in past  Humans not present  So, why worry about present – ► Earth goes through cycles ► Always recovers ► Earth warming faster than previously ► More CO 2 in atmosphere than in previous 600 k yrs

Examples of Past Climate Change ► Glacial periods  See major cycles  Smaller cycles on large cycles ► How do we know?  Rocks  Fossils

Other methods for determining climate of the past ► Ice Cores ► Tree Rings ► Isotopes

Gathering Ice Cores Researchers choose between weatherports (the semi-cylindrical structures consisting of tubular metal frames overlaid with insulatory tent materials) or tents. Weatherports equipped with plywood floors and oil heaters, shared with up to 15 other scientists

Ice core drilled at Russian Vostok station in East Antarctica reaches depth of more than two miles and provides information about climate cycles over hundreds of thousands of years. (Schematic courtesy of John Priscu research group, Montana State University) Todd Sowers, LDEO – Columbia University Vostok ice core drilling site, Antarctica

Ice forms in contact with atmosphere Voids in ice Voids contain air from atmosphere With burial, voids closed, gas trapped Raynaud, 1992

Ice Cores ► Show annual layerss  Winter – darker  Summer Lighter ► Must be able to date  Count layers to get years ► Core must be complete  Isotopes  Ash layers

Determining Climate From Cores ► Look at thickness of layers  Thick lighter layers = longer summers ► Examine CO 2 in cores  CO 2 content changes with climate  Impacts greenhouse effect ► More dark layers during cold periods  More wind blowing, more dust ► We will concentrate on CO 2

► Thin cut of a polar ice core sample as seen through two polarising filters. The dark areas are gas bubbles enclosed in the ice (Image: W Berner/University of Bern) NOAA

CO 2 & Glacial Periods ► More CO 2 during warm periods  Less during ice ages  Reflect greenhouse effect ► Cycles are about k years  Shorter cycles in between ► Oceans warmer hold less CO 2 ► Oceans warmer release methane ► More plants and decay

Vostok Ice Core Record

► From 120,000 to about 20,000 years ago, there was a long period of cooling temperatures, but with some ups and downs of a degree or two. This was the last Great Ice Age. From about 18,000 or 19,000 years ago to about 15,000 years ago, the climate went through another warming period to the next interglacial, - the current one. ► Fig 1 and Greenland ice cores indicate climate oscillations lasting 7, ,000 years during the Last Great Ice Age ( kBP). These are known as � Heinrich events � and are also evidenced by ocean sediments. A more detailed ice core analysis shows an occasional abrupt change of climate during the last interglacial (the Eemian, at 120 kBP), changing by as much as 10K during only years. Such changes may be due to switchings of flows in the northern Atlantic. Similar changes have been observed at the end of the last glaciation. at the end of the last glaciationat the end of the last glaciation ► Fig 1 also shows that carbon dioxide and methane (main greenhouse gases) occur in higher concentrations during warm periods; the two variables, temperature and greenhouse gas concentration, are clearly consistent, yet it is not clear what drives what. The correlation coefficient is 0.81 between CO2 content and apparent temperature, on the whole. During deglaciation the two varied simultaneously, but during times of cooling the CO2 changed after the temperature change, by up to 1000 years. This order of events is not what one would expect from the enhanced greenhouse effect. ► Finally, Fig 1 shows that high concentrations of dust occur at the same times as the colder periods. The most likely reason is that the ice sheets were more extensive during colder periods, and therefore the sea level lower, thus there would have been more exposed, bare land. Dust concentration, mean temperature (as estimated from the oxygen isotope ratio), CO2 and CH4 concentrations plotted against time, estimated from the analysis of an ice core drilled at the Russian station, Vostok, on the Antarctica plateau

Past climates ► hange1/current/lectures/kling/paleoclimate/i ndex.html hange1/current/lectures/kling/paleoclimate/i ndex.html hange1/current/lectures/kling/paleoclimate/i ndex.html

Dendrochronology-Tree Rings ► Dating of past climate change through tree rings  Ist used early 20 th century ► Wide rings = wet periods  Narrow rings = dry periods ► New wood grows between old wood and bark.  In spring, moisture plentiful, tree producing new growth cells. ► first new cells are large, ► as summer progresses their size decreases  in fall, growth stops and cells die,  no new growth appears until next spring.

Extending Dates ► Trees unknown aged matched with tree sequences of known age ► Can extend ageing to the past ► Oldest known living thing are Bristlecone Trees ► By overlapping samples extend dating to 9000 yrs ► Problem: Ring width depends on environmental factors  If environmental factors constant, no ring variability  The more variable the environment the more the tree rings vary

Problem with Ice Cores and Tree Rings ► Cannot see to far into past  Ice cores 650,000 yrs  Tree rings 9000 yrs ► Earth is 4.6 billion years  Missing a lot of history of climate ► Must use another techniques

What is an isotope ► Element with same number of protons but with different number of neutrons ► Examples: O18, O16, C12, C13, C14

Oxygen Isotopes ► O 2 isotopes: 16 O (99.789%), 17 O (0.037%), 18 0 (0.204%) ► 18 O\ 16 O ratio built into shells, minerals. Foraminifers especially useful ► Important for Quaternary & Late sediments ► First developed in 1955 (Emiliani) Copyright Cushman Foundation

Foraminifera in Sediments ► Drill in ocean to retrieve cores of sediments ► Pick the foraminifera out ► Analyze oxygen isotopes ► Foraminifera must be well preserved ► Core must be dated  Forams can be used for dating  Other microfossils  Volcanic ash

Compare oxygen isotope ratio to a standard ► 18 O/ 16 O dependent on temp & ratio in water ► Deviation of ratio compared to standard  More common standard SNOW = Standard Mean Ocean Water

► Evaporation, lighter 16 O removed, heavier 18 O remains in water ► Precipitation, heavier 18 O returned to oceans near coast ► Vapor depleted in 18 O ► Also temp dependent, polar regions more 16 O locked in ice  Oceans depleted in 16 O, enriched in 18 O ► Warmer period, more 16 O in water ► Therefore, changes in 18 O reflect changes in ice volume, sea level ► Decreased temperature also causes more 18 O in shells.

► Oxygen, carbon, sulfur, strontium vary through time ► Ocean mixing time about 1000 yrs, variations considered instantaneous ► Timing of variations must be established