Global warming is the long-term rise in the average temperature of the Earth's climate system. It is a major aspect of current climate change, and has been demonstrated by direct temperature measurements and by measurements of various effects of the warming. The term commonly refers to the mainly human-caused increase in global surface temperatures and its projected continuation. In this context, the terms global warming and climate change are often used interchangeably, but climate change includes both global warming and its effects, such as changes in precipitation and impacts that differ by region. There were prehistoric periods of global warming, but observed changes since the mid-20th century have been much greater than those seen in previous records covering decades to thousands of years.