Climate change is a change in the statistical distribution of weather over periods of time that range from decades to millions of years. It can be a change in the average weather or a change in the distribution of weather events around an average (for example, greater or fewer extreme weather events). Climate change may be limited to a specific region, or may expand across the whole of earth. Global change occurs over a wide range of temporal and spatial scales. For example, changes in the configuration of the continents occur slowly over millions of years as a result of plate tectonics. As the distribution of land and ocean changes, so do oceanic and atmospheric circulation systems, hence altering earth's climate.