It is a pity that discussion of global warming has fallen victim to politics: the political right wing (and only the political right wing) has taken to denying it entirely. Any discussion of the topic really needs to consider the following:-
*Is there evidence that the climate of the world is actually warming? And is there reason to believe this is a long-term trend?
*If so, is this phenomenon in any way unusual or unexpected?
*If so, can it definitely be attributed to human activity?
*That being the case, can anything be done about it?
Not being a scientist, I cannot answer these questions, but can only make a contribution as a historian:-
1. The world's climate has always been in a state of flux. Indeed, in the long term, the whole of recorded human history may only be taking place in an interval between Ice Ages
2. It appears likely that the collapse of the Egyptian Old Kingdom (the pyramid-builders) around 2,200 BC was due to climate change. There is evidence for a serious long-lasting drought in Egypt, which seems to be linked with unusually cold sea temperatures in the Atlantic at the time.
3. There have been signficant changes even in the last 2,000 years. It appears that there was a cold spell lasting a few centuries at the end of the Roman Empire, followed by warmth in Viking and Norman times, and then a "little ice age" starting in the early 14th century, with particularly cold decades in the 1590s, 1670s and early 19th century. The early 20th century saw warmer weather, but until recently, some scientists were anticipating the coming of another cold period.
4. There have been many sudden climatic changes caused by natural disasters. For instance, the human race was almost wiped out around 74,000 years ago, by the gigantic eruption of Toba on Sumatra, Indonesia. Probably only a few thousand humans in total survived the catastrophe. Again, the sudden disappearance of the Clovis people in North America, together with the extinction of the largest mammals (mammoths etc) around 15,000 years ago seems to be linked with dramatic climate change, perhaps caused by an asteroid impact.
5. It is now widely accepted that the collapse of Minoan civilisation in ancient Crete around 1600 BC was linked with the eruption of Santorini and a resultant tsunami. An event like this would not only cause massive loss of life and economic devastation, but a complete breakdown of morale and confidence, as the survivors thought their gods had abandoned them.
6. The real end of the classical world occured not with the fall of Rome to the Goths (410 AD) or the deposition of the last western Roman emperor (476), but in the reign of the Byzantine emperor Justinian (527-565). Not only did he attempt a reconquest of Italy, which resulted in vastly more damage to the city of Rome that the Goths and Vandals ever did, but there was also a devastating outbreak of plague (542) and in 535-6 reports by chroniclers of "darkened skies" and other strange phenomena which, when combined with evidence from tree-rings of very restricted growth, suggest a sudden deterioration of climate, possibly caused by major volcanic eruption (Krakatoa, perhaps?) or a meteorite strike. The Arabian peninsula was also badly affected.
7. More recently, the social and economic distress immediately after the Napoleonic Wars clearly resulted from the gigantic eruption of Tambora, and the resulting lurid skies that inspired Turner's paintings were no compensation for widespread crop failures and resultant mass starvation in 1816; "the year without a summer".
I would conclude that we have always been subject to climate change, sometimes with devastating effect