Fossil fuels are natural resources formed over long periods in the Earth’s crust from the remains of deceased plants and animals. They are primarily used as a source of fuel. According to the United Nations, burning fossil fuels is responsible for over 80% of the carbon dioxide (CO₂) emissions caused by human activities. The extraction, combustion, and resulting emissions of fossil fuels harm the carbon cycle, disrupting the balance needed for climate stability and a healthy biosphere. Examples of fossil fuels include petroleum, coal, and natural gas, among others.