The global climate changes. With all due apologies to Al Gore, this fact is the only consensus you'll find in the scientific community. Whether it is currently being driven by natural causes, or humanity, is a subject of fierce debate. I personally have come down on the side of natural causes, and here are the reasons why.
The primary energy source for all natural processes on Earth is the Sun. The amount of energy received from the Sun by the Earth varies in response to complex variables. First, the Sun does not always emit the same amount of energy, thus increasing and decreasing the amount of energy available for absorption by the Earth. Not all of the energy that reaches the Earth is absorbed either, changes in the Earths orbit and tilt will alter this value.
Secondly, changes in the cloud cover will increase or decrease the energy absorbed. Recent experiments in Europe have shown that sunspot activity could have a profound effect on cloud formation in the Earth's atmosphere, which has never been accounted for in climate modeling.
The "Greenhouse Effect", while a commonly used term, is also misunderstood. There are three ways for energy, or heat, to be transferred; conduction, convection and radiation. The sun transmits heat to the Earth by radiation, convection occurs in most ovens, in which heat is transferred by air warmed with gas or electricity, and conduction is the direct contact between a hot and cold body. When heat is radiated to the Earth from the Sun, it is absorbed and re-emitted. The "Greenhouse Effect" implies that the heat is absorbed by the gases in the atmosphere, and then re-emitted back to the Earth. Thus, the more "Greenhouse gases" that are present, the more heat is re-emitted back to the Earth. Unfortunately, this is not how a greenhouse works, greenhouses retain heat by the prevention of convection. The hot air inside the greenhouse is prevented from rising, thus trapping the heat inside. To cool the greenhouse, all you have to do is open a ceiling panel. In nature, there are no ceiling panels, so there is nothing to prevent the hot gas from rising and emitting the excess heat into space. While some of the heat is undoubtedly re-emitted back to Earth, the majority will probably be lost.
If, however, some heat is re-emitted back to Earth by gases in the atmosphere, then more gases would imply more heat. So, just how big of an effect does man made carbon dioxide have on the concentration of gas in the atmosphere? The technical term is "point [email protected]#% over infinity". In actual numbers; carbon dioxide is less than one half of one percent, and is within the variation of natural levels.
There is also one other phenomena which is not well publicized, increases in carbon dioxide concentration generally come after increases in temperature. This is because the oceans are a carbon dioxide "sink". This means that as the temperatures rise, more carbon dioxide is released from the oceans, which implies that high carbon dioxide concentrations are an effect of high temperatures and not a cause. As temperatures fall, the carbon dioxide concentration drops as more is dissolved in the water.
Finally, increases in ocean temperature. It has been stated that suspected increases in sea temperatures are an indication of Global Warming. For this to be the case, the ocean would have to be warmed by the air above it, which would require an astronomical amount of energy. It is more likely that recent discoveries of underwater volcanic activity, in amounts orders of magnitude higher than expected, are the major contributors to increases in ocean temperatures.
The most commonly cited evidence that global climate change is tied to human activity is the recently released IPCC executive summary. This summary, the report containing the actual data has not been released yet, is said to be a general consensus of all work on climatology to date. Owing to its "peer-reviewed" nature, it is said to be unbiased. Again, this is not quite accurate. While the separate reports that make up the overall document are peer-reviewed, the executive summary is written by non-scientist bureaucrats and will often directly contradict the scientific reports. The goals pushed by these summaries can often be linked directly to the political agenda of the government they work for. Probably the best discussion of the discrepancies between the actual reports that make up the IPCC, and the summaries written, is given by Dr. Richard S. Lidzen of MIT in a Wall Street Journal editorial written on July 2, 2006. The editorial is titled, 'Don't Believe the Hype', and addresses the phenomena of over-hyping the dangers of global warming that seems to be prevalent in most discussions.