Global warming is a natural cyclical phenomenon. The earth's average air temperature has increased 600 times over the last 900,000 years and then decreased again each time. These cycles of approximately 1,500 years can be seen in proxy records such as the Vostok Ice Core. 
Since the beginning of written historical records in Ancient Rome, there has been a warm period, followed by the cool period of the Dark Ages, followed by the Medieval Climate Optimum (when Greenland was colonized), a Little Ice Age (when European settlers abandoned Greenland), and since around 1850 a Modern Warm Period. All of this is natural, according to S. Fred Singer.
Variations in carbon dioxide levels correlate closely with the ups and downs of global air temperature, lagging behind by about 800 years. Although correlation is not causation, some scientists think that the variations in carbon dioxide are driven by the variations in air temperature.
When the Sun boasts a maximum of spots, cycle after cycle, Earth tends to be warmer than when its face is clear.  A lengthy period of cold weather coincided with the Maunder Minimum when hardly any sunspots were observed.
Since 1989, controversy has raged over the anthropogenic global warming theory that emissions of carbon dioxide and other "greenhouse gases" is responsible for much or most of the recent decades' warming. Adherents of this theory generally support the Kyoto Protocol, ostensibly intended to reduce emissions and thus prevent further warming.