Global warming exists and is happening but it is probably due to the natural changing of the Earth's temperature than any man-made factor. I believe this current warming trend is a normal warming period much like the Earth has had in the past. In the early 1400's, before the climatological event known as the Little Ice Age, ships were able to sail around Greenland, which is impossible today, and the maximum extent of pack ice in the Southern Ocean was much less than it is today. By the late 1400's though a period of global cooling occured which dropped world temperatures between .5-1 degree centigrade. This caused the pack ice around both poles to expand and for harsh winters in Europe, Asia and North America. Ever since the 1500's we have slowly been getting warmer as the temperatures tried to normalize with pre-Little Ice Age temperatures. Even before this event we can see small changes in the Earth's temperature before, this often explains the cycle of droughts and wet periods in the equitorial regions. My final thought on Global Warming is to say that it is definitely happening but it is more likely, in my mind, that it is caused by natural enviromental factors than my man-made/artificial factors.
While it is quite true that the global climate has changed in the last 1000 years - The golden age of the vikings in Scnadinavia was probably caused by some exceptional warm centuries - the problem is the tempo in which the changes are happening. The Icecap in Greenland has given us and indication of the developments in temperature in the last 250.000 years, that shows some very speedy changes, but not anything like the effect the last 2000 years where the temperature has been on a constant rise unlike anything seen before
