The Climate System

EESC 2100 Spring 2007

Lectures - Monday and Wednesday, 11:00 AM - 12:15 PM
Lab - Tuesday, 4:10 PM -7 PM

Climate Archives: The Climate Record of the Distant Past

I. Oxygen Isotopes in Seawater: How do we know how much ice was in the large glaciers 18,000 years ago?

• In the Northern hemisphere, "permanent" ice is located primarily on Greenland with a small amount of floating sea ice located in the Arctic Ocean (Fig 1)
• Glacial ice on Greenland today is several thousand meters thick (Fig 2)
• In the northern hemisphere, glacial maximum (18,000 years ago) ice sheets covered a major fraction of North America with ice several thousand meters thick (Fig 3)
• Water molecules: relative abundances of oxygen by stable isotope composition (Fig 4)
• Stable isotope composition permille delta notation (Fig 5)
• The stable isotope composition of rain and snow today shows large variability, especially with latitude (Fig 6)
• Polar snow and ice water molecules are very depleted in ¹⁸O (Fig 7)
• The ¹⁸O/¹⁶O ratio of global precipitation shows a strong correlation with precipitation, resulting in very "light" water molecules in polar ice sheets (See Fig 7)
• The mean delta ¹⁸O composition of "new" glacial ice during the glacial period was probably about -25 permille.
• Sea level was lower by 130 meters 18,000 years B.P. (Fig 8)
• Sea water today has a delta ¹⁸O concentration of 0 permille

  
  

  From the above information, calculate the change in isotopic composition you would expect to observe during the last glacial period.

  1. Would the glacial ocean have greater or lesser ¹⁸O/¹⁶O ratios than today?
  2. Assuming sea level dropped by 130 meters, and the corresponding volume of water was stored as ice in continental glaciers, what would the magnitude of change in ¹⁸O be for average ocean water?

• Deep sea sediment fossil carbonates actually record these past changes (Fig 9) in continental ice volume. The calcite (CaCO3) shells have ¹⁸O/¹⁶O in the mineral equilibrated isotopically with the sea water from which they precipitated.
• This record of the past volume of glacial ice is actually recorded in sediments about 3 km below the sea surface, near the equator, far away, both in space and time, from those ancient glaciers (Fig 10)

II. Why we study past climates (paleoclimatology)

A. We can measure many aspects of the climate system today (Fig 11) and, from these measurements, understand fundamental physical and chemical processes.

B. These measurements can be used as input to climate models and the complex processes and their feedbacks can be simulated (Fig 12)

C. We cannot instrument the past; the complex atmospheric processes can not be measured. The climate system must remain a black box (Fig 13)

D. However, the mysterious climate system of the past produces a record and from this record we learn about past climate systems. (Fig 14)

III. The kinds of paleoclimate archives

A. Marine sediments (Fig 15) accumulate slowly (typically 2-6 cm per 1000 years) but relatively continuously.

B. Lake sediments: Glacial-aged sediments from the Dead Sea rift valley (Fig 16).

1. Sediments were deposited in the Dead Sea rift valley basin between 20-50 kyr. (Fig 17)
2. The arrow of time points vertically up in sedimentary deposits that are undisturbed. (Fig 18)
3. Some sedimentary deposits have annual layers (seasonal cycles). (Fig 19)

4. Annual layers can provide detailed histories of a basin (a lake or region of the sea floor). (Fig 20)

C. Tree rings: annual growth layers in trees and shrubs.

1. Can be used to reconstruct atmospheric temperature, precipitation, sea surface temperature, or pressure indices (eg. ENSO SOI) depending on the tree species used and its site ecology.

2. Can precisely date droughts, volcanic eruptions, and other events in Earth history that might have been influenced by or caused changes in climate. For more on tree-ring research, visit Columbia University's Tree Ring Lab online.

D. Ice cores.

Boreholes drilled into polar ice sheets (Fig 21) have recovered long cores of ice which contain information about the temperature and chemistry of the atmosphere.

E. Peat deposits.

1. Contain pollen grains which record changes in vegetation and climate over time (eg. Malaspina pollen diagram).
2. Contain plant macrofossils which define the changes in vegetation and climate for the region (eg. Siberian macrofossil diagram).

IV. The Milankovitch Mechanism: Pacemaker of the ice ages

A. The Milankovitch or astronomical theory of climate change is an explanation for the changes in the seasons which result from changes in the earth's orbit around the sun. The theory is named for Serbian astronomer Milutin Milankovitch, who calculated the slow changes in the earth's orbit by careful measurements of the position of the stars, and through equations using the gravitational pull of other planets and stars. (See this excellent summary of the Milankovitch Mechanism by the NOAA Paleoclimatology Program).

B. There are three main components to Earth orbital variability (Fig. 22): Eccentricity (100,000 year period), Tilt (41,000 year period), and precession (23,000 and 19,000 year periods). Only variations in orbital tilt and precession significantly affect the amount of radiation received during a given season.

C. These orbital changes cause large changes (up to ±15%) in the amount of sunlight received during a given season. Here is how the northern hemisphere cooled and warmed over the 25,000 to 10,000 years ago due to these variations (Fig. 23).

D. We can reconstruct past variations in ice sheet size using measurements of oxygen isotopes in the calcite (the "O" in CaCO3) of shells (called foraminifera (Fig 24)) found in deep-sea sediments. As you learned above, seawater ¹⁸O variations over time can be linked to the growth and decay of ice sheets. During the last ice age sea level was ~130m lower than today reflecting about 3% of the ocean's volume. Consequently the ¹⁸O of the ocean was ~1.2 permille heavier (higher) than it is today. Measuring the ¹⁸O in foraminifera allows to reconstruct past variations in the size of the ice sheets over millions of years.

E. Here is a summary of the relationship between changes in polar ice sheet size (ice growth and decay) and orbital eccentricity, tilt, and precession (Fig. 25). The key variable is the amount of summer radiation at high northern latitudes. Most of the ¹⁸O variability can be related to direct radiation forcing by orbital variations. Periodic variations in the Earth's orbit have been the pacemaker for the ice ages - there have been many (~50-60) ice ages over the last several million years, not just one.

V. The climate record of ice cores

A. The great remaining ice sheets

1. Antarctica (Fig 26)
2. Greenland (See Fig 2)

B. Latitudinal balance of incoming versus outgoing radiation (Fig 27)

C. Ice cores

1. Result of snow accumulation
2. Snow contains air (Fig 28)
3. Some air gets trapped in bubbles
   • Bubbles then contain "fossil air."
4. Ice contains water and water contains isotopes of both hydrogen and oxygen
5. Factors controlling the behavior of hydrogen and oxygen isotopes
6. Geographical distribution of dO¹⁸ (See Fig 6)
7. Relation of dO¹⁸ values to temperature. (See Fig 7)

D. The glacial ice record

1. Behavior of snow as it turns to ice (see again Fig 28). Pockets of ancient air are trapped by closing pores in ice.
2. The flow of glacial ice (Fig 29)
3. The relation of age to depth in the ice core (Fig 30)
4. Records of past temperature and atmosphere CO2 AND CH4 variations preserved in glacial ice (Fig 31 and Fig 32). Note that variations in temperature and greenhouse gases (CO2 and CH4) appear to covary with temperature over both glacial and interglacial periods.

VI. Anthropogenic Greenhouse Gas increases and future climate change

A. The anthropogenic rise (Fig 33 in CO2 is ~1.5% each year due to fossil fuel combustion and deforestation.

B. The preindustrial CO2 level was ~280 ppm, it is now (1998) at 375 ppm By the time most of you are 30 years old it will be close to 460-470 PPM Doubling of atmospheric CO2 is expected by the year 2050, perhaps sooner depending on the emission scenario which plays out.

C. Here is a comparison of the CO2 variations over the full glacial to interglacial cycle of the last 140,000 years compared to the anthropogenic rise in CO2 just over the past 300 years (Fig. 34).

D. Based on what you learned about the past links between temperature and CO2 from the ice cores, would you predict that the world will become warmer with rising CO2 levels?

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Lecture text by James D. Hays and Peter B. deMenocal, 1998.

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