if it's warmer than normal or colder than normal, it's caused by a complex of factors, usually indicated as "weather".
Weather is about upper air jet streams, air masses of different properties interacting, causing frontal systems and changes in pressure all in a big-merry-go around. Oh yes, and the sun is the star player, always in the limelight, ultimately directing that merry-go-around together with gravity, centrifugal and coriolis forces, water cycles and, and...
Usually, there are seasonal patterns, also directed by the star. However, sometime they get disturbed. A big spoil sport, for instance, are abnormal warm or cold equatorial oceanic surface currents, known as the El Nino (warm) and La Nina (cold) twin. Currently we have a moderately strong El Nino going which causes the 2007 predictions to be the warmest year in history.
Also, currently a big and very loyal player is missing, which is highly unusual, The Central Siberia high pressure area. Because of this the generally SW Atlantic flow is exectionally strong causing an abnormal but not unique warm winter in Europe. But as this flow turns southwards again it also causes an extreme cold period in the East. There is a severe shortage of heaters in Qatar. Apparantly the impact of the missing Siberian static high area in combination with El Nino has clear global implications.
The current El Nino is predicted by Theodore Landscheidt in 2003. He also predicted other El Nino's fairly correctly. Elements in his hypothesis are gravitation interaction of the planets to the sun causing variation in sunspot cycles causing variations in solar particle streams, causing changes in cloudiness causing changes in sunlight reaching the ocean, causing ocean warming changes causing El Nino's.
And then there is global warming erm..yes! No? But how about yesteryear when the weather was at the other extreme in many places?