L1 - Global Energy Balance

Physics of the Atmosphere and Ocean

• Climate vs Weather: Climate is defined as the long-term average of weather over extended periods and can be characterized by statistical data such as temperature, humidity, precipitation, wind, and other atmospheric conditions. Weather, on the other hand, conveys the short-term atmospheric state.

  • Quote: "Climate is what you expect, weather is what you get!" This indicates that while climate gives a general guideline of conditions for a region, actual weather can vary dramatically from that expectation on a daily basis.

Recommended Reading

• Introduction to Physical Oceanography, Stewart, Academic Press: This text provides foundational knowledge on oceanographic principles.

• Atmospheric Science: An Introductory Survey, Wallace and Hobbs: A comprehensive overview of atmospheric science fundamentals.

• Ocean Circulation, Open University Course Team, Pergamon Press: Discusses the dynamics and complexities of ocean circulatory patterns.

• Atmosphere, Ocean, and Climate Dynamics, Marshall and Plumb, Elsevier: Explains the interplay between atmospheric processes and ocean dynamics in climate systems.

• Ocean Dynamics and the Carbon Cycle, Williams and Follows, Cambridge: Examines the interactions between ocean currents and the global carbon cycle.

• Meteorology Today, Ahrens, West: A modern take on meteorological concepts and processes vital for understanding weather phenomena.

Global Energy Balance

• The energy that drives the circulation of the atmosphere and ocean is primarily derived from the sun's radiation. Solar energy impacts climatic patterns by influencing temperature and atmospheric dynamics through various mechanisms including absorption, reflection, and reradiation.

Earth’s Geometry

• Shape: Close to a sphere, with slight flattening at the poles and a bulge at the equator due to Earth’s rotation.

• Mean radius: 6370 km, which is crucial for calculations involving gravitational forces and atmospheric pressure.

• Surface gravity field: 9.81 m/s², which defines the strength of gravitational pull on the planet's surface.

• Rotation period: 24 hours, affecting the diurnal cycle of temperature and atmospheric changes.

• Angular velocity: 7.29 x 10⁻⁵ s⁻¹, which defines how quickly the Earth spins on its axis.

• Atmosphere is thin, with 80% of its mass located below 10 km, significantly affecting weather and climate phenomena.

• Land covers 30% of Earth’s surface, primarily found in the northern hemisphere, impacting biodiversity and land use.

• Mountain heights rarely exceed 2 km, whereas ocean depths average 4 km, creating unique pressure and temperature gradients in various ecosystems.

Atmospheric Composition

• Nitrogen (N₂): 780,840 ppmv (78%), the most abundant gas in the atmosphere, playing a crucial role in maintaining biological processes.

• Oxygen (O₂): 209,460 ppmv (20%), essential for respiration in most life forms on Earth.

• Argon (Ar): 9,340 ppmv (0.9%), an inert gas with limited role in chemical processes.

• Carbon Dioxide (CO₂): 381 ppmv (0.04%), a significant greenhouse gas that contributes to the earth's warming and climate change.

• Other trace gases include Neon (Ne), Helium (He), Methane (CH₄), Krypton (Kr), Hydrogen (H₂), with Water vapor (H₂O) typically around 0.5%, yet highly variable and critical for weather processes.

• The importance of photosynthesis and the Nitrogen cycle highlights the interdependencies between atmospheric composition and biological activity.

Earth’s Radiation Budget

• Solar Constant: The amount of solar energy reaching Earth averages around 1361 W/m².

• Stefan-Boltzmann Law: This law states that the total radiation emitted by a body is proportional to the fourth power of its absolute temperature.
  Total emitted radiation = Stefan-Boltzmann constant | 5.67 x 10⁻⁸ W m⁻² K⁻⁴ .

• Planetary Emission Temperature: For Earth to maintain energy balance, it must radiate energy away that matches the incoming solar energy, leading to equilibrium.

  • Albedo (a): The fraction of incident solar energy that is reflected back into space, estimated at 0.3 for Earth, which influences the planet's overall temperature.

Atmospheric Absorption Spectrum

• Atmospheric opacity varies by wavelength:

  • Transparent in the visible spectrum, allowing sunlight to reach the surface.

  • Opaque in UV wavelengths, protecting life on Earth from harmful radiation.

  • Variable opacity across the infrared spectrum, critical for understanding heat retention and the greenhouse effect.

• The absorption is dominated by greenhouse gases (H₂O, CO₂) effectively absorbing terrestrial radiation while being present in smaller quantities, thus significantly impacting climate change.

Greenhouse Effect

• Radiation can escape into space only from certain altitudes in the atmosphere where greenhouse gas absorption is minimal, thereby compromising the radiative balance.

• With altitude, temperatures are generally colder; thus, these regions absorb less radiation, resulting in surface warming.

• The addition of greenhouse gases elevates the altitude from which radiation escapes, directly contributing to increased surface temperatures and the ongoing trend of global warming.

Atmospheric Layers

• Troposphere: Contains 85% of the atmosphere’s mass, retains most of the water vapor that influences weather, with temperatures typically decreasing with altitude.

• Stratosphere: Contains the ozone layer, which is crucial for regulating temperature changes and absorbing harmful ultraviolet radiation.

• Thermosphere: Characterized by high temperatures resulting from UV absorption; dominated by ionization processes affecting satellite communications and space weather phenomena.

Energy Transport

• The transfer of energy from the tropics (areas with energy surplus) towards polar regions (areas with energy deficit) is achieved through atmospheric and ocean currents, maintaining a global energy balance.

• Reference: The Earth’s Radiation Budget Experiment (ERBE) has demonstrated how measurable outgoing longwave radiation is influenced by latitude, providing insights into global climate patterns.

Summary of Key Points

• The atmosphere and oceans are intricately linked, forming thin fluid layers shaped by gravitational forces, Earth’s rotation, and solar radiation.

• Solar energy emitted by the sun (shortwave radiation) is absorbed and then re-emitted as longwave radiation by the Earth, impacting temperature and weather patterns.

• Greenhouse gases enhance Earth's temperature by contributing to the greenhouse effect, raising average surface temperatures above the expected planetary emission steady state.

• The vital role of heat transport enables the mitigation of energy deficits and surpluses across geographic latitudes, assisting in the regulation of global climate systems.