It's very hard to get neutral, objective facts and analysis on global warming because the political climate has so strongly polarized the debate on this issue. I will summarize arguments taken from Patrick Michaels and Robert Balling's book "Climate of Extremes". Michaels and Balling are professional climatologists who are semi-controversial because they agree man-made warming is occurring, but disagree that the end result will be catastrophic.
This is from their first chapter "A Global Warming Science Primer"
Quote: "Earth's mean surface temperature is doubtlessly warmer than it was 100 years ago. Get over it.
What matters is 1) how much it has warmed 2) how much of that warming is caused by human activity, and 3) how the relationship between that activity and present temperatures can be translated into a reliable estimate of future warming and it's effects."
They then go into an analysis of historical temperature data, taken from the United Nations Intergovernmental Panel on Climate Change (the IPCC), which they seem to regard as accurate and valid.
Figure 1.1 traces "temperature anomolies" from 1900 to 2006, and demonstrates a clear warming trend. There are in fact two distinct periods of warming, one from 1910 to 1945, and the other from 1975 to about 1998. In both of these warming trends, the actual rate of warming has been extremely steady: .320 degrees F plus or minus .038 per decade. This rate does not appear to be accelerating (and may have dropped off significantly in the last 10 years).
Michaels and Balling conclude that the first warming trend had more to do with changes in the sun, while the second one has resulted largely from human activities. The reason for this is that heating due to solar activity has a very different pattern from that due to greenhouse gases. Computer models predict that solar heating will heat the upper atmosphere as well as the lower atmosphere, while greenhouse gases heat primarily the lower atmosphere. They present data which shows that lower atmospheric heating, and upper atmospheric cooling, is indeed what is currently happening. In addition, models predict that greenhouse gases would warm winter temperatures more than summer temperatures, which is also occurring. They therefore conclude that since 1975, most warming has been due to human activity.
This has had the interesting side effect that the yearly temperature range has been narrowing. Both summer highs and winter lows are increasing over time, but winter lows are increasing faster, which is resulting in less temperature variation over the course of the year: the climate is becoming less variable. Finally, the daily temperature range is also narrowing for the same reason: in most places the daily low is warming faster than the daily highs. These are all signs of the effect of greenhouse gases.
Michaels and Balling then combine the model outputs with observed data and come up with a forecast for warming for the 21st century: 3.1 degrees F. They mention that this forecast should be treated with a great deal of caution, because the computer models are known to be imprecise (they predict more warming that we observe happening, esp. in the upper atmosphere, and also fail to predict the narrowing temperature ranges accurately).
They also mention that technically, global warming has stopped since 1998. They regard this as a temporary aftereffect of the 1998 El Nino, the largest on record. If you look at the century long trend line, it's visually obvious that the lack of warming since 1998 is within other temporary reverses that have frequently occurred. Computer models that assume that the warming trend is still continuing, but have been overlaid by the El Nino are more accurate at predicting monthly temperature variations since 1998. So the assumption is that warming will continue at some point in the near future, but at this point that's only an assumption.
Another interesting implication is that, if it's true that the lower atmosphere is warming faster than the upper, then increased precipitation is likely to occur. We are heading for more tropical conditions, not more drought. This still has the potential to do tremendous damage to the agricultural industry, as well as problems to populated areas due to flooding.
The bottom line is ammunition for both sides of the partisan debate. Global warming due to human activity is almost certainly occurring, yet the extent of the warming, esp. in the last 10 years or so, is somewhat in doubt. This could be considered good news: the worst damage may yet be preventable.