Global warming is the long-term rise in the average temperature of the Earth's climate system. It is a major aspect of climate change, and has been demonstrated by direct temperature measurements and by measurements of various effects of the warming. The terms global warming and climate change are often used interchangeably. However, speaking more accurately, global warming denotes the mainly human-caused increase in global surface temperatures and its projected continuation, but climate change includes both global warming and its effects, such as changes in precipitation. While there have been prehistoric periods of global warming, many observed changes since the mid-20th century have been unprecedented over decades to millennia.