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What causes an Ice Age?
Question Date: 2003-02-05
Answer 1:

You've asked a very complex question about a topic that still is far from being fully understood so bear with me if my answer isn't a complete or clean picture.

The temperature of the earth's surface, atmosphere, and oceans (what I will call the earth system) is based on a balance of the amount of heat that comes into the Earth system and the amount that leaves. In general, there is a balance between incoming heat (mostly radiation from the sun) and outgoing heat (radiation from the sun that is reflected and heat that is emitted) so the earth system doesn't heat up or cool down on average. So let's pause here to note that there are two factors here, radiation going in and radiation going out and let's look at these separately.

Radiation in: Most of the radiation entering the earth system is from the sun. Some of the radiation from the sun is heat radiation (technically thermal infrared radiation), but most is ultraviolet, visible, and near-infrared radiation that either is absorbed by the earth system or reflected back into space. The exact amount of radiation from the sun that reaches the earth depends on the nature of the earth's orbit around the sun (such as its distance) which can change over hundreds to hundreds of thousands of years***.

Radiation out: Radiation leaves the earth system either by being reflected, or emitted. Radiation can be reflected or absorbed by any of the components of the earth system: oceans, surface, or atmosphere. Everything you see is related to this reflected radiation because it is this very reflected light that enables you to see. Thus, when you see glints of light from the ocean, the brightness of a snowfield or sand dune, or even the deep green of a tree, you are seeing light that has been reflected and may be on its way back into space.

Emitted radiation is the heat that you feel coming off of different objects. When the surface absorbs radiation from the sun, it becomes hot and slowly re-radiates that energy as heat. As an example, a dark rock will absorb a lot of heat during the day when the sun is shining on it and then will continue to radiate that heat into the cool night.

Ok, so how is all of this related to ice ages? As I said above, the earth system doesn't tend to get warmer or cooler because there is a balance between the radiation going in and the radiation going out. If you start to absorb more radiation than you loose (for instance, by putting more heat absorbing greenhouse gases into the atmosphere so more heat is absorbed), the earth system will warm up a little. If you reflect more radiation into space (for instance, by increasing the amount of very reflective clouds and glaciers), the earth system will cool down a little.

The problem with this is that there are an enormous number of complex factors involved in reflecting, absorbing, and emitting radiation from the earth system and to make things even messier, many of these factors change depending on what other factors are doing. As an example, if you increase greenhouse gases as I mentioned above, warmer temperatures may cause more moisture to evaporate from the ocean, creating more clouds which will reflect more radiation into space and cool the earth.

With all of this complexity, you might think that there is no way to figure out why ice ages occur when they do. Fortunately, we can look at historical records from ice that was laid down in glaciers or sediment that settled in lakes or on the ocean floor during previous glacial periods. These records tell us something about conditions during glacial times in a manner similar to the way tree rings tell us about conditions for the tree through its history. Again, I emphasize that there are many factors involved, but one of the strongest associations that has been found is that ice ages come at times related to the earth's orbit and how much the earth is tilted on its axis. The earth's orbit determines how much light reaches the earth at different times of year and the tilt of its axis determines how intense the summers and winters are. As the orbit and tilt gradually change, the amount of "radiation in" changes and therefore, the earth's average heat may change. During periods when the radiation going in causes the Earth System to be cooler, an ice age may begin. This is still a very confusing topic with a lot of components and I encourage you to continue looking for information.

Finally, let me ask you to think about what would happen if the Earth were moved closer to the sun? What would happen if it were moved farther away? Think about conditions on Venus and Mars and the amount of radiation from the sun reaching those planets. Is this what you think the earth would be like if it were closer to or farther?

Answer 2:

Over the past 1 million years or so, climate has fluctuated between Ice Ages and interglacials. Ice Ages, or periods of extensive glaciation, last approximately 100,000 years, and the interglacials, or warm periods, about 10,000 years. We are currently in an interglacial period. These cycles of cold and warm are the results of differences in solar insolation, or solar radiation, that are correlated with the earth's orbital relationship to the sun.

There are three components of the earth's rotation around the sun: eccentricity, obliquity, and precession.

Eccentricity is the shape of the earth's orbit around the sun and it can range from almost circular to slightly elongate. Cycles of eccentricity (from circular to elongate and back to circular again) take about 98,000 years. Obliquity is the variation in the tilt of the Earth's axis and it changes in cycles of 40,000 years. Precession is the change in the orientation of the earth's axis. It is something like the wobble of the axis of a spinning top. Cycles of precession require about 20,000 years.

When the eccentricity, obliquity, and precession are such that continents at high latitudes in the northern hemisphere have cold summers (less solar insolation) for an extended period of time, the glaciers present there begin to grow. As the glaciers expand southward, they become continental ice sheets, covering expansive areas under one or two miles of ice.

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