Lags
Heating and cooling takes time. Before electric refrigerators became common, people used iceboxes. Some iceboxes looked like
electric refrigerators, but the upper compartment contained a big block of ice. These blocks of ice were sold by icemen
who would carry them through the streets in the backs of their trucks. Whenever they saw a sign marked ICE they would stop and pick up a block of ice
with tongs and deliver it straight into their customer's icebox. This system worked only because of the time lag needed for the ice
to melt. The phase-change from solid-ice to liquid-water requires energy called latent heat supplied by the food in the bottom compartment. The transfer
of heat is what keeps the food cold. Heat transfer takes time, which is why the iceman came only once or twice per week.
In the north the longest day is usually June 21st, but the hottest day is usually in August, a lag of two months. In Chicago and Berlin summer heat is enough to melt all the ice
and snow from the previous winter. However, in the far north of Canada and Russia the heat in summer is enough to melt only the top one or two meters of ice. Below the surface of the ground there is
permafrost - ice that has not melted for thousands and perhaps millions of years. In Greenland and Antarctica there are ice caps or glaciers that have not melted completely since the middle Miocene,
about 12 million years ago. The time lag needed to melt these glaciers is longer than the interglacial periods, which like the present interglacial,
the Holocene, last only about 20,000 years or so.
Broecker, W.S., The end of the present interglacial: How and when?, Quat. Sci. Rev., 17, 689-694, 1998.
Kennett, James. A Review of Polar Climatic Evolution during the Neogene, Based on the Marine Sediment Record, in Paleoclimate and Evolution, (eds. Elizabeth Vbra et al.), Yale University Press, 1995.
Kukla, G., The last interglacial, Science, 287, 987-989, 2000.
What worries some scientists is that our present interglacial is different from all the others because of a new factor -
modern human technology. Human technology is a new factor, because during the last interglacial, over 100,000 years ago, modern humans were few in number and had done little to change the earth. There were no cities, no farming or forestry,
and no industry as we know it. Today humans have altered the land by making farms where there were once forests. Humans have altered the air by dispersing chemicals and by burning coal and oil. Humans have even altered the oceans.
Most of the changes caused by humans, anthropogenic change, tend to make the earth warmer and thus tend to cause glaciers to melt.
Recently an Antarctic ice shelf, known as
the Larsen B, collapsed sending ice fragments into the southern ocean. What concerns some scientists
is growing evidence that such dramatic event have ocurred in the past and will occur again in the future.
(See: Progress in Physical Geography.)
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Negative feedback
The following illustrates a negative feedback system: a winter storm causes a house to become cooler. Then the thermostat starts the heating unit which makes the temperature go in the opposite direction, so the house
maintains its temperature. The feedback is negative because the system moves in the opposite direction from the initial movement, tending to restore its equilibrium or balance.
Positive feedback
The following illustrates a positive feedback system: a large patch of snow slides down a hillside and combines with other large patches of snow.
As the pile of snow grows, its momentum increases so that nothing stops the avalanche. This is positive feedback because the system
tends to move in the same direction as the initial impulse and away from its original equilibrium state.
The system will continue on its runaway path until the energy feeding the movement has been converted to another form of energy or dissipated.
Thus, negative feedback dampens the effect of the driving force, while positive feedback amplifies it.
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