Climate changes on all time scales because there are different mechanisms operating over different periods of times which "force" the climate. Most of these mechanisms are entirely natural in origin, but over the last 20 years, concerns have grown that a man-made change in the composition of the atmosphere may be causing global warming.
In general, mechanisms of climate change alter the amount of energy stored by the climate system and in particular the atmosphere. The more energy stored the warmer the climate. The longest periods of global climate change occur over hundreds of millions of years, in response to episodes of continental drift and mountain building. Within such long term cycles there exist much shorter climatic fluctuations over tens and hundreds of thousands of years, driven by changes in the Earth's orbit around the Sun, and modulated by changes in ocean circulation and atmospheric composition. Over the shortest time scales of centuries, decades and even individual years, global climate is influenced by changes in the amount of energy from the Sun and the effects of major volcanic eruptions. The actual state of the global climate at any point in time represents an aggregate response to all the mechanisms which influence it.
How the global climate responds to this "climate forcing" will depend upon the different response times of the various parts of the climate system. The overall response time of the global climate will then be determined by the interactions between the parts. As mankind is releasing more greenhouse gases into the atmosphere for example, the atmosphere is slowly trapping more heat over the years and enhancing the Earth's natural greenhouse effect. The oceans in contact with the atmosphere however, respond much more slowly to changes in atmospheric heating, since they require a much greater quantity of energy to warm them up. A temperature increase of the oceans due to global warming may take hundreds of years. This will moderate and slow the response of the climate system to global warming, giving mankind a little breathing space with which to tackle the problem.