Warming trends tend to fluctuate throughout the millennia. The 'greenhouse effect' is caused by human-generated carbon dioxide, changes in the Earth's orbit around the Sun, volcanic emissions and solar activity -- yes, these statements are all true but these factors are not the only causes of global warming.
Large scale studies on global climate change show that since the last Ice Age (which occurred about 12,000 years ago) temperatures were relatively stable. But smaller scale studies show that there actually are some fluctuations. For example the temperatures increased between 0,08-0,22°C since 1979. Recent studies show that temperature fluctuations are more frequent throughout time than first thought. And in addition to that there was no 'human-generated carbon dioxide' in the distant past and these fluctuations were still there.
So the conclusion is that global warming is caused by natural and external forces. Human activities are just accelerating those natural processes. We can't say that it is an absolute threat. But nonetheless global warming is a problem because there are many species that are taking it pretty hard (e.g. amphibians) - they can even extinct.