# External Forcing in Climate Science In addition to its own natural cycles, Earth's climate is also shaped by powerful influences from outside the climate system. These are called **external forcings**, and they are long-term drivers that can slowly change the planet's climate over centuries or thousands of years. This section examines the most significant external forces, including greenhouse gases from human activity, changes in the Sun's energy output, and gradual shifts in Earth's orbit. ## Definition of External Forcing External forcing refers to factors **outside** the climate system that influence its behavior. These factors alter Earth's energy balance, leading to long-term climate changes. They generally operate over longer time frames (centuries to millennia) and contribute to gradual climate shifts. ## Examples of External Forcing Factors ### Greenhouse Gas Concentrations * **Mechanism:** Increased levels of greenhouse gases (such as carbon dioxide $\mathrm{CO_2}$ and methane $\mathrm{CH_4}$) due to human activities. * **Effect:** Enhance the greenhouse effect, trapping more heat and causing global warming. ### Solar Variability * **Mechanism:** Changes in solar output over longer time scales. * **Effect:** Impacts Earth's climate by altering the amount of incoming solar radiation. This is related to the spectrum emitted by the Sun. ### Orbital Variations (Milankovitch Cycles) Earth's orbit around the Sun undergoes periodic variations, governed by . These variations affect the distribution of solar energy received by Earth. The three components of Milankovitch cycles are: * **Eccentricity:** The shape of Earth's orbit, which changes from nearly circular to slightly elliptical over approximately 100,000 years. * **Axial Tilt (Obliquity):** The angle of Earth's axis relative to its orbital plane, which varies between about 22.1° and 24.5° over a cycle of approximately 41,000 years. * **Precession:** The wobble of Earth's axis, similar to a spinning top, which completes a full cycle approximately every 26,000 years. These effects operate on long time scales, extending up to thousands of years. ### Aerosols and Pollution * **Definition of Anthropogenic Aerosols:** A suspension of particles or droplets in the air (such as dust, mist, fumes, smoke) originating from industrial sources or combustion processes. * **Effect:** Can cool the climate by reflecting sunlight back into space. Natural aerosols from volcanic eruptions also provide a cooling effect.