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Paleoceanography and Climate Change:

Text notes to accompany the lecture of
Dr. Andreas Schmittner

Climate vs. Weather

CLIMATE = the composite or generalization of weather conditions of a region, such as temperature, precipitation...throughout the year, averaged over a series of years.

WEATHER = State of the atmosphere, such as temperature, moisture, pressure etc. NOW in the present.

Importance

- Greenhouse gas debate.

- Impact of natural variability; questions of rates of change.

Fundamental Contributions of
Paleoceanographic / Paleoclimatic Studies

Science of past changes in ocean conditions. Let's look at some EVIDENCE for global warming:
- Arctic Sea ice has changed dramatically in YOUR lifetime
- North pole may one day be ice-free in the summer
- Glaciers are melting everywhere too
- Global mean temperature and average sea level have risen about 1.5deg. F in YOUR PROFESSORS' lifetime!

WHY has this occurred?
- CO2 in atmosphere has steadily increased - the "Mauna Loa curve" shows the ups and downs of the global vegetation "breathing" on the planet surface, but the average trend shows a strong increase from 1950s until present (from 310 ppm before the 1950s to 390 ppm today)
- CO2 is a "greenhouse" gas that traps heat on the planet

Mauna Loa Curve

Atmospheric CO2 concentration has steadily increased from 1958 to 1985, due mainly to burning of fossil fuels. We will double the present amount of atmospheric CO2 in the next century if we persist with present activities.



We can BO BACK IN HISTORY to look at climate changes. How fast has climate change been? Is it natural or normal?

Antarctic Ice Cores from Present Back to 150,000 Years Ago

- we have doubled atmospheric CO2 since 18,000 years ago

- there is 60 times more CO2 in the oceans than on land

- therefore, the oceans control the chemistry of the atmosphere

- therefore, we must understand the oceans in order to understand long-term climate

- Antarctic ice is ~2 miles thick
- snowfall adds to that - snowflakes are very porous - the snow falls, compacts (into ferns), air bubbles are trapped. Hence air bubbles trapped in snow and ice and retrievable through ice cores provide us with a sample of the air at the time when the snow fell --> sample of ancient air
- measure of CO2 from these air bubbles extends the Mauna Loa curve BACK through time
- not much change in 19th century CO2 and even further back to 10,000 years ago
- Look again at Dr. Schmittner's slide on proxy records

Ocean sediments give us the oldest record of all

Important long response time processes

Deep Ocean Circulation (1000 yr time scales)

Ocean/Atmospheric Chemistry (1000 yr)

Sea Ice Variations (years to 1000 yr)
- what would happen if the sea ice disappeared?
- ice is white, ocean is blue
- white reflects sunlight, blue absorbs it
- oceans would heat up unbearably without the ice

Continental Ice Sheets (Glaciers - 10,000 yr)

Landmass changes (continental drift, mountain building; 1,000,000 yr)

Atmosphere/Ocean Interaction

Question: What is the History of Continental Ice Sheets and Why is it Important?

Last Glacial Maximum - sea level lowered by 100 to 150 m.

Melt all modern glaciers, raise sea level by over 30 m.

How fast can all this happen?

"Ice Ages: Solving the Mystery"

See Dr. Schmittner's slides for an animation that shows the retreat of the Laurentide ice sheet that covered much of North America. The retreat occurred 18,000 to 8,000 years ago.

Question: What is the history of continental ice sheets?
What "library" do we go to?

Best "library" is a deepsea core

- example in class from 110 degrees W longitude at the Equator (Pacific Ocean)

- made of biogenic (or biogenous) sediments

- light/dark sequences = sediments responded to changes in climate through the years

Oxygen Isotopes

Isotopes of an element have same number of protons.

Differ by number of neutrons

Chemically identical, but molecular weight is a little HEAVIER (because of the extra neutrons)

Only processes that are mass dependent fractionate

Normal Oxygen has a molecular weight of 16 - we will concentrate on this molecule, plus the Oxygen 18 isotope (molecular weight of 18)

Oxygen Isotopes - Definition

d18O = (18O / 16Osample - 18O / 16Oref)
------------------------------------------ * 1000
18O / 16Oref
Changes in molecular weight are very small but we have mass spectrometers that can measure these changes.

During the ice age, the oxygen atoms in ocean water and in the shells of organisms became "heavier" due to a change in the content of the oxygen isotopes.

Fossils

Benthic foraminifers
- calcium carbonate secreting animals that live on the ocean bottom.
- oxygen to make shells comes from ocean water
- measure oxygen content of sediments in core (derived from these shells) to oxygen content of ocean at time sediments were laid

Planktonic foraminifers - calcium carbonate secreting animals that live near the ocean surface at different latitudes:



Coccoliths - calcium carbonate secreting algae that live near the ocean surface:



Radiolarians - silica-bearing animals that live near the ocean surface at different latitudes
Radiolarian occurrence can be tied to sea surface temperature



Deep Sea Cores

- data from upper 3 m of cores shown in diagram
- amount of isotopes in ocean changes with time but is recorded the same everywhere in deepsea sediments
- therefore, past volume of ice on planet can be measured anywhere
- isotopic records from cores taken from the North Atlantic, Equatorial Atlantic, Equatorial Pacific, and South Atlantic all look the same!
- Dr. Duncan showed one deepsea core of calcareous sediments (CaCO3 in fossilized foraminifers and coccoliths)
- also a core of dark colored, siliceous ooze (SiO2), made from radiolarian fossils

March of the Seasons

Eccentricity of Earth

As a tilted Earth revolves around the sun, changes in the declination of sunlight cause the succession of seasons: Milankovitch hypothesis

Orbit of Earth (eccentricity) is usually an ellipse but can change to a circle and then back to an ellipse, very 100,000 years or so: Milankovitch cycle

This can greatly impact our climate.

If such a small change on the order of 100,000 years can do this, just think what our yearly increases in atmospheric carbon dioxide can do to climate!!

Precession of Earth

- precession = wobbliness of Earth from gravitational pull of other planets - occurs every 23,000 years Northern Hemisphere
- Earth is closer to sun in winter (our winters are warmer)
- Earth is farther away from sun in summer (our summers are cooler)

Southern Hemisphere
- Earth is farther away from sun in winter (their winters are cooler)
- Earth is closer to sun in summer (their summers are warmer)

Occasional wobble of Earth will make these relationships be the exact opposite, but the total amount of energy is always the same, just distributed differently.

Tilt of Earth

Tilt of Earth's axis changes with time, every 41,000 years

All 3 - Eccentricity, Precession, Tilt - change how solar insolation is distributed on the planet. Amount of energy is always the same, however.

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