As greenhouse gases accumulate in the atmosphere, they trap the sun’s heat close to Earth and warm the planet. Developed or industrialized countries, including the United States, have been releasing greenhouse gases such as carbon dioxide into the atmosphere for more than a century by burning fossil fuels for power, heat, transport, and industrial activity. In recent decades, rapid economic growth in major developing countries such as Brazil, China, and India has led to significant increases in their greenhouse gas emissions as well. Greenhouse gases trap the sun’s heat close to the earth, causing the planet to warm. Some of these gases occur naturally, which is why the earth is warm enough to sustain life as we know it. Increasing the concentration of these gases in the atmosphere via man-made emissions is causing more heat to be trapped, raising average global temperatures—a phenomenon known as global warming—which leads to changes in the planet’s climate.

Climate change poses risks not only to the environment but also to the security and livelihood of people in the United States and around the world. These effects include rising sea levels, greater heat extremes, more intense precipitation events, deeper droughts, stronger storms, and bigger wildfires. If current trends continue, growing plentiful and affordable food for a rising global population could become more difficult; populations in low-lying areas—including many of the world’s major cities—could be forced to move; and more extreme weather could threaten the health of billions of people.

Domestic and international policy changes can reduce greenhouse gas emissions, but, if countries creating significant emissions fail to act, the overall level of warming will increase. Countries have reached multiple international agreements on climate change in recent years, including the 2015 Paris Agreement, which aims to reduce emissions globally to keep warming well below 2°C and, given the grave risks of climate change, to strive for 1.5°C. However, reducing greenhouse gas emissions worldwide will not be easy. Modern economies depend on fossil fuels for electricity and transportation. Output from alternative sources such as solar and wind energy is growing but not enough to fully replace fossil fuels yet, and currently available measures to increase energy efficiency are costly and time-consuming. Climate change, moreover, is a difficult issue for policymakers. The questions of how to cut emissions and prepare for climate repercussions, and who should bear the costs of doing so, have few simple answers.