The document discusses the greenhouse effect and how human activities have contributed to climate change through increasing greenhouse gases. Natural factors like variations in solar output and volcanic eruptions can influence climate. The greenhouse effect refers to greenhouse gases trapping heat in the lower atmosphere, warming the planet. The enhanced greenhouse effect is a rapid increase in greenhouse gases from human activities like burning fossil fuels and deforestation, raising global temperatures. Key human activities are burning fossil fuels for energy, industry, transport, and land use changes from agriculture, urbanization and deforestation.