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 term commonly refers to the mainly human-caused increase in global surface temperatures and its projected continuation, though there are also natural causes of global warming. Global warming is linked to an increase in the concentrations of greenhouse gases in the atmosphere, which trap heat that would otherwise escape from Earth. Scientists have determined that human activities, such as the burning of fossil fuels and changes in land use (such as deforestation), are the primary sources of the increased greenhouse gas concentrations in the atmosphere. These human activities are resulting in higher average global temperatures and changes in global climate