10 June, 1998

Well Hello:

Shall we finally get caught up on climate change? Even in my own department at Niles North this is a contentious and politically charged topic due to the question of man's role in global warming. Is the warming we see natural or caused by the addition of carbon dioxide by man? Or is it a combination of these things. What are some of the ideas about what drives climate change? (The following is from Brenda Hall and What Drives Glacial Cycles by Wallace S. Broecker and George H. Denton Scientific American January 1990. Another good reference is The Ice Age World by Bjorn Andersen).

In the 1920's and 30's a Yugoslav astronomer formalized the idea of astronomically driven changes. This pacemaker has three parts:

1. The first is the tilt of the earth's spin axis. Currently it is 23.5 degrees from the vertical. But it fluctuates from 21.5 degrees to 24.5 degrees and back every 41,000 years. The greater the tilt the more extreme the seasons - hotter summers, colder winters.

2. A second weaker factor is the shape of earth's orbit. Over a period of 100,000 years, the orbit stretches into a more eccentric ellipse and then grows more circullar again. As the eccentricity (the ovalness) increases the difference in the darth's distance from the sun at the orbit's nearest and farthest points grows, intensifying the seasons in one himishpere and moderating them in the other. At present, the earth is farthest from the sun during the southern Hemisphere's winter; as a result, southern winters are a little colder, ans summers a little warmer than their northern counterparts.

3. The third astronomical fluctuation governs the interplay between the tilt and the eccentricity effects. This is the precession, or the wobble of the earth's spin axis which traces out a complete circle on the background of the stars about every 23,000 years. the precession determines whether summer in a given hemisphere falls at a near or a far point in the orbit - in other words, whether tilt seasonality is enhanced or weakened.

When these two controllers of seasonality reinforce each other in one hemisphere, they oppose each other in the opposite hemisphere.

Milankovitch calculatd that these three factors work together to vary the amount of sunshine reaching the high northern latitudes in summer over a range of some 20% - enough he argued to allow the great ice sheets to grow during intervals of cool summers and mild winters.

During the 1950's, C. Emiliani of the University of Chicago found that the ratio of oxygen18 to oxygen 16 in single celled, shelled marine organisms called foraminifera varied. It is know understood that the ratio of oxygen isotopes in seawater closely tracks the proportion of the world's water that is locked up in glaciers and ice sheets. A kind of meteorological distillation

accounts for the link. 18 O is a little heavier, so as water evaporates from warm oceans the 18 O preferentially returns to the oceans in precipitation, and glaciers and ice sheets are enriched in 16 O. What ultimately falls as snow on ice sheets and mounntain glaciers is depleted of 18 O. So the oceans become enriched in 18 O; the more the ice builds up, the higher the 18 O in the marine sediments. Oxygen isotopes are important as geothrmometers.

What Emiliani found was that the pattern of 18 O ratio in marine sediment cores rose and fell in a pattern that roughly matched the Milankovich cycles. The ratio of 18O/16O is compared to the 18O/16O ratio of mean ocean water (or SMOW).

Over the last 800,000 years, the global ice volume has peaked every 100,000 years (the eccentricity variation). Wrinkles superimposed on each cycle (surges and retreat of ice volume) have come at intervals of roughly 23,000 and 41,000 years (the precession and tilt frequencies).

Another idea is that of the reorganization of the ocean's deep salty currents and the warm, less saline surface currents. This conveyor belt supposedley shows the main oceanic heat flow. If these systems are disrupted, climate will alter. The importance of these currents is illustrated by the fact that Greenland today is covered by a large ice sheet on the same latitude as northern Scandinavia, which is essentially unglaciated and has a significantly warmer climate.

Climate is complicated and dynamic. The greatest controversy seems to arise when we begin to tease out the role of humans in climate change.

Tomorrow an overview of the purpose of the work this year.


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