Global Climate Change Unit 5

Pleistocene

start: 2.6 million years ago; end: 11,000 years ago = "glacials" (ice ages) with brief warm "interglacial periods"

Paleoclimatology

the study of past climates Gases can be analyzed from air trapped in ice cores. Air bubbles trapped in the ice cores provide a record of past atmospheric gases Ice core records prove that current levels of CO2 & CH4 are higher than any previous level in the past 800,000 years

Plants (& algae) preferentially absorb 12C in photosynthesis

• Almost 99% of all C atoms in world are 12C The ratio of 13C /12C is called an isotopic fingerprint • Plants absorb 12C from air faster than 13C ¬ 12C is lighter and diffuses into leaf more quickly than 13C ¬ Rubisco binds more quickly with 12C Plants have more 12C than the atmosphere or ocean (water) Fossil fuels have more 12C than atmosphere or ocean (water) (because they are formed from ancient plants)

C isotope ratios (12C /13C) in foram shells from ocean sediments reveal past climates

• If climate is good for PS, level of 12C in ocean is low because algae use 12C for PS • If level of 12C is low in ocean water, it will also be low in shells of foraminifera • Foraminifera shells persist as fossils on the ocean floor • C isotope ratios in shells indicate when warm temperatures allowed abundant photosynthesis or cold temperatures restricted plant growth

Studying natural climate recorders: annual growth rings in trees (dendrochronology)

• Study annual growth in long-lived temperate trees • Light-colored ring forms during active growth (spring/summer) • Darker ring forms as growth slows (fall) -because wood forms more slowly • Width of rings indicates good or bad years for growth (temperature & rainfall) • Data collected from live trees by drilling and extracting a pencil-sized core sample

Earth has a geographic pattern of 18O/16O ratio in water

1) Overall, rain is carried from equator to poles -evaporation at equator, release as wind pushes clouds towards poles (2) 18 O is heavier than 16O so it falls out first (closer to equator) (3) water in ice (near poles) has more 16O than water near equator

When ice melts into ocean, O16 in the ocean increases

4) 18O/16O ratio in water shifts when Earth temperature shifts (5) During warm periods, the ocean has less 18O (more 16O) - because melting ice added O16 to ocean (6) During cold periods, the ocean has more 18O (less 16O) - because more 16O is trapped in frozen water (ice)

Drilling Ice Cores in Greenland

A scientist at the US Geological Survey National Ice Core Lab (Denver CO) removes an ice core for analysis; Close-up view of layers within an ice core; - Arrows indicate lighter summer layers

How can climate be studied from earlier than 800,000?

By using proxies

CaCO3 forms two types of crystal arrangements depending on the pH of water: Calcite (low pH); Aragonite (high pH)

Calcite shells: dominate in low pH - means atmosphere has high CO2, warm climate Aragonite shells: dominate in high pH - means atmosphere has low CO2, cold climate

During Ice Ages -Ocean has more 18O

Colder earth means colder temperature near equator; rain with 18O condenses & drops into ocean closer to equator. Rain with 16O falls towards poles on ice sheets where it stays. 18O levels in ocean water increase, 16O levels in ice increase. Marine fossils with high 18O indicate glacial periods on earth - Paleontologists study deviations from expected proportion of 18O in CaCO3 or SiO2 in shells.

Drilling Ice Cores in Antarctica

Dome C and Vostok ice cores from Antarctica directly measure CO2 Atmospheric CO2 varied between 180-300 ppm for the past 800,000 years of earth's history

Past climates (atmospheric CO2 level) are also revealed by studying changes in ocean pH

Ocean pH decreases when atmospheric CO2 increases because ocean absorbs CO2, forming carbonic acid • pH of ocean reflects the equilibrium in flux of CO2 between ocean and atmosphere

Pollen deposits indicate kinds of plants & climates

Pollen coat resists microbial/chemical decay - persists in chronological layers in sediment Identify species, kinds of vegetation (forest, grassland etc.) infer climate Must rule out long distance transport of pollen by wind or water in order to match pollen to vegetation/climate of a region

Lake sediments can be windows to past climates

Scientists study preserved structures (shells, pollen) to determine what kind of climate existed at the time those organisms lived & died - oldest sediments are in lowest layers (chronological sequence)

Study of Elk Lake sediment (Minnesota) shows climate changing throughout the Holocene

Sediments in layers (varves) that mark seasons for the last 10,400 years Most sediment produced in the lake (diatom shells, sediment, CaCO3), but sand (quartz) is also blown into the lake; indicates how windy it was.

Why is earth warming now? Is it Milankovitch cycles?

These cycles explain how natural long-term climate changes on Earth are caused by changes in the position of the Earth in comparison to the Sun - explain periodicity of Ice Ages in the past - predict how natural causes (Earth's motion in space) continue to affect future climates - these cycles don't match rapid global warming now

Proxy

a preserved physical characteristic of the past that stands in for direct measurement, allows scientists to reconstruct past climates • Growth rings on trees, corals (how fast did they grow?) • Fossilized pollen (what kind of vegetation grew?) • Isotope ratios in fossils -How much 18O in ocean sediment, pollen, shells? -How much 13C in ocean sediment, pollen, shells? • Crystal structure of CaCO3 in shells (varies with ocean pH)

Holocene:

relatively stable climate period for the last 11,000 years = rise of agriculture, modern human civilization made possible because of climate stability

Last Glacial Maximum (LGM) = 20,000 years ago;

ice covered land in North America as far south as New York City

Coral growth rings and isotope ratios show evidence of past climate change

# of growth rings = age; width of rings indicates climate ; O-isotope ratio reflects the 18O /16O ratio of seawater

Oxygen isotope ratios used to determine past climate

(these are stable isotopes; no radioactive decay) As climate changes, so does the relative amount of 18O vs 16O in ocean water. O-isotope ratios in shells from marine organisms (eg. forams), indicate the O-isotope ratio in water from the time period. Water with 18O is heavier; it condenses faster to fall as rain.