Global warming refers to a gradual increase in the average surface temperature of the Earth. It is caused by the buildup of certain gases in the atmosphere, called greenhouse gases, which absorb and trap energy emitted from the Earth, causing temperatures to rise. These gases include carbon dioxide, methane, and nitrous oxide. They have been released into the air by human activities such as burning fossil fuels (oil, natural gas, and coal) for electricity and heat. Some of these gases were also produced by natural processes such as volcanic eruptions and by changes in the Sun’s brightness.
Since the start of the Industrial Revolution in the mid-18th century, global emissions of these greenhouse gases have increased significantly. The Earth is already warmer today than it was before the Industrial Revolution. The warming trend is likely to continue, unless human emissions decrease.
The climate change caused by human activity has many negative effects. For example, glaciers and ice sheets are melting, rivers and lakes are warming, and vegetation is changing in response to heat and drought. Heat waves are becoming more intense, and wildfires are starting and spreading faster. Climate change has also reduced water supplies, increased health-related problems in cities, and made it harder to grow crops.
The international community has made a commitment to limit the rate of global warming to less than 2 degrees Celsius over preindustrial levels, which is the target set by the United Nations Framework Convention on Climate Change. This goal is important because it can help to avoid crossing so-called tipping points, beyond which major planetary systems are expected to make irreversible changes. It is also important for the world’s poorest people, who have fewer resources to adapt to climate change and will be hit hardest by its consequences.