I "swing" with Al Gore, but mainly because I have studied Global Warming and believe the scientists as opposed to the lobbyists in Washington.

First off, a message to anyone who watches "An Inconvenient Truth". While it is a good movie and gets many of its facts right remember, Al Gore is a Democrat. That means that a)it is going to be heavily biased against Republicans and all the data about how Republicans screwed up is over exaggerated (this does not mean that they did everything perfect, just that Gore is loyal to his party and his party does not get along well with Republicans); b)some of his data is exaggerated to make a point.

Now I want you all to know as you read this that the term "greenhouse effect" is not accurate. The reason that greenhouses get so hot is because they have no interaction with the outside air, and so the heat cannot be dissipated across a larger area. What is happening on Earth is that the Earth absorbs radiation from the sun and re-radiates it back out to space. Because of the greenhouse gases, that heat can't escape to space and is thus reflected back down to earth, re-radiated to space, etc.

First off, climate is a function of several ongoing process found in different levels of Earth's systems. There is the obvious atmosphere, where weather patterns have a huge impact on climate (climate is not exclusively weather, although weather is always a part of climate). There is also the hydrosphere, the systems of rivers, lakes, and oceans which affect climate (we'll get to the effect of oceans in minute). Then there is the biosphere which consists of all living things that exist above, on and in Earth. And finally there is the cryosphere, which is all solid mass on Earth.

Now, you are right that the Earth's climate does exist in cycles which repeat on a semi-regular basis, the problem with this idea as applied to modern day climate is the fact that according to the cycles we should be on our way back to an ice age, not approaching temperatures that are the highest they have been during our cycle after we had already begun getting colder. We have ways of measuring the natural levels of gases and other climate markers from the ice cores in Greenland and Antarctica along with other, more confusing methods (ancient packrat midden, tree rings). Dr. William Ruddiman of the University of Virginia found an anomaly in the ice core levels of Carbon Dioxide(CO2) and Methane. He found that around 8,000 years ago CO2 levels began rising, contrary to the natural fall of CO2 that has been observed in the cores. The rise of CO2 happens to correspond with the growing deforestation of 8,000 years ago when Europeans and Asian began domesticating non-indigenous plants where forests once were, and the rise of methane corresponds to the beginning of the cultivation of rice patties several thousand years later (rice patties are one of the biggest modern methane producers). These increasing amounts of these so-called "greenhouse gases" contributed to keeping the temperature warm at a time when it should have been getting colder. This is generally agreed upon by all scientists, just other scientists give different reasons for the rise in the gases. (Found in the march 2005 issue of Scientific American.)

In addition to the rise of gases, the Earth is nearing its point in an 11,000 year cycle where we should astronomically be colder than other time periods because we are at a time in several astronomical cycles which dictate that the earth is farther away from the sun and thus should be colder. Yet, we are at our warmest point in over a hundred thousand years. That is not usual.

Next, I will talk about the ocean. The ocean affects climate in several huge ways. First, the ocean can absorb CO2 and take it out of the atmosphere, thus helping reduce the build-up. Unfortunately, CO2 has built up so fast over the last two centuries that the oceans have not been able to keep up and don't contain the full capacity of CO2 that they could. Even more unfortunate is the fact that the warmer the oceans get, the less CO2 they can hold, the warmer it gets, etc. (this is a positive feedback loop where the cause creates an effect that increases the cause which increases the effect, etc. to infinity without the interference of other forces). The second effect of the oceans on climate is how they spread heat around. The current in the oceans act as a heat transfer from the equator to cooler regions of the ocean. Here is an example using the North Atlantic current: because heat rises, as the warmer water on top gets cooled off the water falls to the bottom of the ocean creating a vacuum. This vacuum sucks up the warmer water from the golf of mexico which leaves a vacuum that the cold water from the north gets sucked into. This works to transfer heat, and if this current is interrupted then the north falls into another ice age (this has happened before when one of the last great glaciers in Canada around the St Lawrence River flooded the current with freezing cold current, plunging the world into a minor ice age thousands of years ago). Rising ocean levels may also be a problem, not due to the melting of the ice (frozen and liquid water displace the same amount of volume) but rather because the warmer water gets the more it expands.

The cryosphere reflects light back into space, thus cooling us down, and due to human cities and deforesting earth is absorbing more heat than reflecting making it hotter (if you want a more detailed description ask, but I don't think it is incredibly important).

As to Humanity's affect on climate, I would like to point out the reports released by the Intergovernmental Panel on Climate Change (IPCC; http://www.ipcc.ch/index.htm ) state that climate change is affected by humanity (the question isn't if, but rather how much). This is a panel set up by the UN that contains scientists from over 130 nations that all agree that this change is not natural, and that humans are part of the cause.

If anyone has any questions over what I wrote I will be happy to explain/expand what I stated.