What is an Ice Sheet?

@ Alchemy- There is much more at work than just the Milankovitch cycles when it comes to climate change. While humans do not affect the Milankovitch cycles, people affect albedo (reflection of solar radiation into space by ice sheets and glaciers) and atmospheric greenhouse gas concentration. By increasing the greenhouse gases in the atmosphere through the burning of fossil fuels, we are creating a barrier for heat radiating from the earth to return to space. This in turn causes higher surface and ocean temperatures, which increase the melting of polar ice caps. This leads to higher melting and more ice caps, in turn leading to more melting. You now have caused a mutation in the climate cycle; changing it from a negative feedback cycle to a positive feedback cycle. This has also happened in such a short period in the planets history that could mean unprecedented changes in an already capricious climate. My point is that nature is not benign; rather it has limits, and may even be somewhat fragile in certain respects.

@ Comparables- So if the earth's climate is controlled by these natural space cycles then what is the whole fuss about saving the arctic ice sheets and global warming. We should want to warm the earth if we are headed toward a period of glaciers and ice ages. I was always skeptical of global warming and never really thought it stood up to the tests of real science. I don't get why people think we have such an effect on the planet.

The statement that ice sheets around the poles is actually false. The earth has been going through cycles of ice ages and great thaws for most of its history. These circumstances are controlled by the regular shifts in the orbit of the earth called the Milankovitch cycles. The earth’s orbit moves from more elliptical to more circular in cycles that run approximately 410,000 years. Furthermore, the axial tilt of the earth shifts by almost two degrees and back every 41,000 years. This was a hypothesis, until 1975 when deep ocean ice cores were able to prove Milankovitch's theory by analyzing oxygen isotopes locked up in prehistoric fossils in the ocean sediment.

Essentially what all of this means is that the earth goes through periods of glaciation because of the change in distance from the sun and the obliquity of the earth in relation to the sun. These variations cause normal periods where polar ice sheets nearly reach the tropics to nearly no ice caps at all every 130,000 to 650,000 years. If you were to ignore the anthropogenic effects on climate, the Earth would be at the point where it begins to move toward a glacial period.