Earth's Energy Budget Notes

Earth's Energy Budget

Introduction

• The Earth's energy budget is a critical factor determining the globally averaged temperature, which in turn dictates the habitability of our planet. It maintains a delicate balance between incoming shortwave radiation from the sun and outgoing longwave radiation emitted from Earth.

• When the incoming and outgoing energy are in equilibrium, Earth's average temperature remains constant, ensuring a stable climate. However, imbalances in this budget lead to temperature changes, resulting in either warming or cooling trends over time.

Key Processes and Interactions

• Numerous individual processes and interactions contribute to the overall energy balance of the Earth. While each plays a role, some exert a more significant impact than others, influencing the planet's climate on a larger scale.

Solar Radiation

• Energy from the sun, a primary driver of Earth's climate system, is emitted as shortwave radiation due to the sun's extremely high temperature. This energy takes just over eight minutes to traverse the vast distance between the sun and Earth.

• Upon reaching Earth, solar radiation is experienced as a combination of visible light, which illuminates our world, and radiant heat, which warms the planet's surface and atmosphere.

Impact of Surface Characteristics (No Atmosphere)

• The characteristics of Earth's surface play a crucial role in how solar radiation is absorbed, reflected, and distributed. Bright surfaces such as snow, ice, and desert sands exhibit high albedo, reflecting a significant portion of incoming solar radiation back into space.

• Conversely, dark surfaces such as forests and asphalt absorb a greater proportion of solar radiation, retaining heat and contributing to localized warming. For example, the Sahara Desert experiences temperature fluctuations of up to 70^{\circ}F from day to night, while the Amazon Basin, with its dense vegetation, fluctuates less than 10^{\circ}F.

• Globally, it is estimated that about 30% of incoming solar energy is reflected back into space due to these surface characteristics, influencing the overall energy budget of the planet.

Earth's Longwave Radiation (No Atmosphere)

• In response to incoming solar radiation, Earth emits energy as longwave radiation due to its cooler temperature compared to the sun. Without an atmosphere, all outgoing longwave energy would pass directly into space, resulting in a significantly colder planet.

• Under such conditions, Earth's surface temperature would plummet to approximately 0^{\circ}F, rendering the planet uninhabitable for most known forms of life. The absence of an atmosphere would prevent the retention of heat, leading to extreme temperature fluctuations and a harsh environment.

Role of the Atmosphere

• The atmosphere plays a vital role in regulating Earth's energy balance and maintaining a habitable climate. Most atmospheric gases are transparent to incoming solar radiation, allowing sunlight to reach the surface relatively unimpeded.

• However, certain gases, such as ozone, play a critical role in absorbing harmful ultraviolet (UV) radiation from the sun, protecting the surface and living organisms from its damaging effects.

Greenhouse Gases

• Greenhouse gases, including water vapor, methane, and carbon dioxide, exhibit a unique property of being transparent to incoming shortwave radiation while being opaque to some wavelengths of outgoing longwave radiation.

• These gases absorb longwave energy emitted by Earth's surface and re-emit less energy than they absorb, effectively trapping heat within the atmosphere. This process is known as the greenhouse effect, which raises Earth's surface temperature to approximately 58^{\circ}F, allowing life to thrive.

Rising Greenhouse Gas Concentrations

• Since the early 1900s, human activities, such as the burning of fossil fuels and deforestation, have led to a significant increase in the concentrations of greenhouse gases, including carbon dioxide and methane, in the atmosphere.

• The proliferation of greenhouse gas molecules results in increased absorption of longwave energy, exacerbating the greenhouse effect and causing Earth's temperature to rise. This phenomenon is a primary driver of global warming and climate change.

Venus - A Runaway Greenhouse Effect

• Venus, Earth's neighboring planet, serves as a stark example of the consequences of a runaway greenhouse effect. With a surface temperature of around 872^{\circ}F, Venus is the hottest planet in our solar system.

• Scientists theorize that early in the history of our sun, Venus may have possessed a global ocean. However, as the sun grew stronger and its brightness increased, this ocean evaporated, saturating the atmosphere with water vapor.

• Water vapor, a potent greenhouse gas, amplified the greenhouse effect, leading to increased trapping of longwave radiation in Venus' atmosphere and a corresponding rise in temperature. The hotter temperature, in turn, caused the ocean to boil away, creating even more water vapor and further accelerating the warming trend.

• Over time, hydrogen escaped from the atmosphere, leaving the oxygen to chemically recombine with carbon, resulting in Venus' atmosphere becoming 96.5% carbon dioxide, a dense and heat-trapping gas.

Clouds

• Clouds exert a complex and multifaceted influence on Earth's energy budget, impacting both incoming solar radiation and outgoing longwave radiation.

• Clouds reflect approximately 24% of incoming radiation back into space, effectively cooling the Earth's surface. However, clouds, especially high clouds composed of ice crystals, also absorb longwave radiation emitted by the Earth, contributing to warming.

• Understanding the intricate balance between the cooling and warming effects of clouds is of paramount importance for accurately modeling and predicting future climate change scenarios.

Other Factors

• In addition to the factors mentioned above, various other elements contribute to Earth's energy budget. Aerosols, including dust and air pollution particles, can reflect, scatter, and absorb energy and heat, influencing regional and global climate patterns.

• Changes to land cover and land use, such as urbanization and agriculture, also have significant impacts on the energy budget by altering surface reflectivity and evaporation rates.

Conclusion

• Understanding the Earth’s energy budget is very important for determining the temperature of the Earth.

• It can be very complicated if you account for every individual process, but it always comes down to the basic balance of incoming shortwave energy and outgoing longwave energy.