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Contemporary climate change includes both global warming — the ongoing increase in global average temperature — and its wider effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to Earth's climate.
The current rise in global average temperature is more rapid than previous changes, and is primarily caused by humans burning fossil fuels. Fossil fuel use, deforestation, and some agricultural and industrial practices add greenhouse gases, notably carbon dioxide and methane, to the atmosphere. Greenhouse gases absorb some of the heat that the Earth radiates after it warms from sunlight. Larger amounts of these gases trap more heat in Earth's lower atmosphere, causing global warming.
Due to climate change, deserts are expanding, while heat waves and wildfires are becoming more common. Increased warming in the Arctic has contributed to melting permafrost, glacial retreat and sea ice loss. Higher temperatures are also causing more intense storms, droughts, and other weather extremes.
Changes to the environment include the loss of biodiversity, and disruption to food production and freshwater access. The World Health Organization considers climate change to be the greatest threat to global health in the 21st century.