We have had other threads on this topic before and I’ll try to post links to the most recent extensive previous one later. The IPCC link that SentientMeat posted, along with the EPA pages are a good place to start for a current summary of the science. There have also been reports by the National Academy of Sciences (NAS) and statements by the councils of the American Geophysical Union and by the American Meteorological Society. Some of the oil companies, most notably BP and Shell, have agreed that human-caused climate change poses a real potential threat that requires starting to take action. In fact, BP has already implemented a Kyoto-sized cut in emissions, 8 years early and they claim to be saving money in the bargain.
The basic facts are these:
(1) The CO2, methane, and other “greenhouse gas” levels in the atmosphere are rising and this rise is definitively due to human activity, especially the burning of fossil fuels (with land use changes and such contributing some too).
(2) The basic physics of how these gases trap heat and should lead to global warming is well-understood and was recognized over 100 years ago by Arrhenius who even made a calculation of the size of the effect which is not to far from current best estimates. The greatest source of uncertainty is in various feedback effects that can magnify or reduce this warming…For example, the rise in temperature causes more water to evaporate into the atmosphere and since water vapor itself traps more heat, this will tend to cause a positive feedback (more warming). More water vapor can also mean more condensation to form clouds. Whether clouds cause net warming or cooling depends on lots of things including their height, location around the globe, size of the water droplets in them, etc. and this is still not very well-understood. Another positive feedback is that as the earth warms, the size of glacier and snowpacks shrink which means less solar radiation gets reflected, causing further warming.
(3) Climate models predict that the human-caused rise in greenhouse gas levels will lead to a (globally-averaged) warming of 2.5 to 10 degrees F over the period 1990 to 2100. The uncertainty in the amount of warming reflects both the uncertainties in the feedback effects described above, as well as uncertainty in what our future emissions will be (and, to a lesser degree, the amount of the gases that will be taken back out of the atmosphere by the oceans and land).
(4) The warming will have various other likely associated effects in various parts of the world. One is a sea level rise of some fraction of a meter by 2100, due mainly to the thermal expansion of the water. Others include more extreme precipitation events, both floods and droughts, and general changes in weather patterns. There is also the potential of more dramatic effects…For example, sea level rises of 5 meters or so would occur if most of the land ice on Greenland and Antarctica were to melt, flooding various low-lying areas around the world. Unfortunately, the process by which this melting occurs is highly nonlinear and is only just beginning to be understood.
(5) In the 20th century, there was a warming of about 1 F, with a dramatic spike occurring over the last ~40 years. It is now considered likely that most of the warming seen over the latter part of the century is due to man. So, this is basically the beginning of the expected human-caused alteration of the climate.
It is important to note that the theory of human-caused global warming rests on several independent lines of evidence. Thus, although there is at any one time always controversies brewing over some aspects, it is not like the entire theory rests, for example, on that “hockey stick” plot of the temperatures over the last 1000 years. Even if the warmth at the end of the 20th century turns out not to be the highest it has been in the last 1000 or 2000 years does not mean that the whole science of global warming is wrong. (In fact, perversely, the idea that the earth’s climate system is more variable than previously recognized could be an indication that it is more sensitive to various perturbations than was thought, which could mean that the amount of warming that could occur due to the perturbation we are putting on it could be currently being underestimated.)
The goal of the so-called “skeptics,” who are few in number in the field and often have strong fossil fuel and/or conservative/libertarian think-tank connections is to cherry-pick the few papers (or sometimes, even parts of papers) that might support their view while ignoring the large body that does not. And, when a particular challenge, such as the one to the “hockey stick” graph is brewing, to make people believe that the whole theory of human-caused climate change rests on this one piece of evidence and the entire structure will come crashing down if this piece of evidence turns out to be wrong.
The goal of the IPCC, NAS, and the councils of the various professional societies is to summarize the state of the science at the time by looking at the entire body of peer-reviewed work.
