Climate change refers to any significant change in measures of climate (such as temperature, precipitation or wind) lasting for an extended period (decades or longer). Climate change may result from: natural factors, such as changes in the sun's intensity or slow changes in the Earth's orbit around the sun; natural processes within the climate system (e.g. changes in ocean circulation); and human activities that change the atmosphere's composition (through the burning of fossil fuels) and the land surface (e.g. deforestation, reforestation, urbanization, desertification, etc.). (Source: EPA / Photo: Flickr)