Multiple lines of evidence show changes in our weather, oceans, and ecosystems, such as:
Carbon dioxide is the primary greenhouse gas contributing to recent climate change. Carbon dioxide enters the atmosphere through burning fossil fuels, solid waste, trees, and other biological materials, and as a result of certain chemical reactions, such as cement manufacturing. Carbon dioxide is absorbed and emitted naturally as part of the carbon cycle, through plant and animal respiration, volcanic eruptions, and ocean-atmosphere exchange.
The earth's temperature depends on the balance between energy entering and leaving the planet’s system. When sunlight reaches the earth’s surface, it can either be reflected back into space or absorbed by the earth. Incoming energy that is absorbed by the earth warms the planet. Once absorbed, the planet releases some of the energy back into the atmosphere as heat (also called infrared radiation). Solar energy that is reflected back to space does not warm the earth.
Certain gases in the atmosphere absorb energy, slowing or preventing the loss of heat to space. Those gases are known as “greenhouse gases.” They act like a blanket, making the earth warmer than it would otherwise be. This process, commonly known as the “greenhouse effect,” is natural and necessary to support life. However, the recent buildup of greenhouse gases in the atmosphere has changed the earth's climate and resulted in dangerous effects to human health and welfare and to ecosystems. Without effective measures to reduce warming, experts predict a wide range of outcomes and impacts around the world.
In the United States, the transportation sector is the leading contributor to the greenhouse gas emissions inventory, representing about 29% of all GHG emissions.
Carbon dioxide, the most abundant greenhouse gas, may endure in the atmosphere between 300 to 1,000 years, according to the National Aeronautics and Space Administration.
Strategies that reduce GHG in the near term are vital to the potential for overall success in reducing emissions and achieving domestic and international mitigation targets.