Hem, you just pushed one of my buttons! (I'm a paleobotanist who studies the effects of ancient climate, among other things...)

Okay, current Earth first. The rising temperatures of the present day are accompanied by an increasing CO₂ concentration of the atmosphere. The carbon in CO₂ can come from various sources, and there are two (important) isotopes, C-12 and C-13, and the different sources contain different amounts of each (there's C-14 also, but it's unimportant as far as this goes). Based on the change in the isotopic ratio of carbon dioxide in our atmosphere, we can identify the source of the rising CO₂ levels: fossil fuels.

Now, carbon dioxide is a greenhouse gas, but is a very *weak* greenhouse gas, not enough to cause the climate changes we have been seeing on our own. However, according to computer climate models - which are *notoriously* difficult to test - the minor warming caused by increased CO₂ will cause the evaporation of water from the oceans. Water vapor is a much more powerful greenhouse gas, and this positive feedback, according to the models, will magnify the small temperature increase induced by adding CO₂ into the atmosphere. This will *not* produce a run-away greenhouse effect, i.e. the Earth will *not* boil away all of its oceans and become like Venus, but it could warm up more than the CO₂ increase alone would indicate. As a result, we can assume that human CO₂ emissions are warming the climate, but of the climate warming that we observe, there are many other possible explanations, the simplest being an increase in the brightness of the Sun (which we know is variable). How much of the current climate warming is due to human activity? That depends on which of the numerous climate models you believe - and, as I said, they're untested, so from a scientific standpoint, we don't actually know yet. What does this mean for the future? That depends on how much of current warming is due to human activity - as well as how the other factors that we have no control over, like solar luminosity, change.

Now, geologic influences - over the Earth's history, the primary source of CO₂ in the Earth's atmosphere has been volcanic activity. There are two major sinks, these being carbon burial (which is what creates fossil fuels), and carbonate precipitation, i.e. lime stones and dolomites. The Earth actually does have a Venusian atmosphere of CO₂ locked away in carbonate rocks. Like most other gasses, CO₂ also is more soluble in water the colder the water gets, so simply warming the climate will itself increase CO₂concentrations because it will come out of solution in the oceans. This is also why warm temperatures raise rates of carbonate precipitation. An increase in volcanic activity will add CO₂ to the atmosphere, and presumably warm the climate. As lime stones get subducted, their CO₂ will be released into the atmosphere as the subducting slabs melt. Thus, there is a geological carbon cycle just as a biological one.

On Ice ages - the distribution of ice on the continents over the Earth's history has generally been due to rearrangements of continental positions. Anything that cuts off pole-equator circulation will create ice. A continent over the pole is ideal for this: Antarctica has been ice-covered for fifteen million years, and there is really no way that any amount of global warming, short of a vast increase in solar intensity (such as has never happened), would be able to melt the entire Antarctic ice sheet. In the Northern Hemisphere, there is an ocean over the pole, but North America and Eurasia are both drifting northward and in doing so cutting off the Arctic Ocean. At present, the Earth is in a marginal state, such that small changes in the shape of the Earth's orbit (aka Milankovitch cycles) can change whether there is ice on the northern continents or not. According to some models, CO₂ emissions might affect this, and prevent the next ice age (although not the following one - CO₂ does get removed from the atmosphere geologically, after all). The model in question however does not take into account the fact that the continents are still drifting north; as a consequence of that, every successive ice age has been a little bit deeper. There have been twenty so far, since we entered this meta-stable state 1.81 million years ago, the Pliocene-Pleistocene boundary. At some point, the Arctic Ocean will be sufficiently cut off that the Northern Hemisphere will go into a permanent ice age, just as Antarctica has. This will not, of course, mean that all of North America will go under the ice, just as southern South America, Africa, and Australia, are ice-free today (except in the mountains, of course).

As for human remediation of greenhouse gas emissions, it is possible that we have already set in mot