Climate change is a change in the statistical distribution of weather patterns when that change lasts for an extended period of time (i.e. decades to millions of years). Climate change may refer to a change in average weather conditions, or in the time variation of weather within the context of longer- term average conditions. Climate change is caused by factors such as biotic processes, variations in solar radiation received by Earth, plate tectonics, and volcanic eruptions. Certain human activities have been identified as primary causes of ongoing climate change, often referred to as global warming.