☁️Atmospheric Physics Unit 7 – Climate Systems and Global Change

Key Concepts and Definitions

• Climate system consists of the atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere
• Global change refers to long-term changes in the Earth's climate system, including temperature, precipitation patterns, and sea level rise
• Radiative forcing quantifies the change in energy fluxes caused by changes in external drivers (greenhouse gases, aerosols, solar radiation)
• Albedo measures the reflectivity of a surface, with higher values indicating more reflection of solar radiation (snow, ice)
• Greenhouse effect traps heat in the atmosphere due to gases that absorb and emit infrared radiation (water vapor, carbon dioxide, methane)
• Climate sensitivity describes the change in global mean surface temperature in response to a doubling of atmospheric CO2 concentration
• Positive feedback amplifies the initial change (ice-albedo feedback, water vapor feedback)
• Negative feedback dampens the initial change (increased CO2 uptake by oceans and vegetation)

Earth's Energy Balance

• Incoming solar radiation (shortwave) is the primary energy source for the Earth's climate system
• Outgoing longwave radiation emitted by the Earth's surface and atmosphere balances the incoming solar radiation
• Greenhouse gases absorb and re-emit longwave radiation, trapping heat in the atmosphere
• Changes in the Earth's orbit (Milankovitch cycles) affect the distribution and intensity of solar radiation reaching the Earth's surface
  • Eccentricity: shape of the Earth's orbit around the sun (varies between nearly circular and slightly elliptical)
  • Obliquity: tilt of the Earth's axis relative to its orbital plane (varies between 22.1° and 24.5° over a 41,000-year cycle)
  • Precession: wobble of the Earth's axis (completes a full cycle every 26,000 years)
• Surface albedo influences the amount of solar radiation absorbed by the Earth's surface (oceans, land, ice)
• Clouds have a complex effect on the energy balance, reflecting solar radiation (cooling) and trapping longwave radiation (warming)

Atmospheric Composition and Structure

• Troposphere is the lowest layer of the atmosphere, where most weather phenomena occur
  • Contains approximately 80% of the atmosphere's mass and 99% of its water vapor
• Stratosphere is the second layer of the atmosphere, characterized by stable temperatures and the presence of the ozone layer
• Mesosphere is the third layer of the atmosphere, where meteors burn up and the coldest temperatures are found
• Thermosphere is the fourth layer of the atmosphere, characterized by high temperatures due to absorption of solar radiation by oxygen and nitrogen
• Exosphere is the outermost layer of the atmosphere, where atoms and molecules escape into space
• Atmospheric composition includes nitrogen (78%), oxygen (21%), argon (0.9%), and trace gases (water vapor, carbon dioxide, methane)
• Greenhouse gases (water vapor, carbon dioxide, methane, nitrous oxide) absorb and emit infrared radiation, contributing to the greenhouse effect

Climate Drivers and Feedbacks

• External climate drivers include changes in solar radiation, volcanic eruptions, and human activities (greenhouse gas emissions, land use change)
• Internal climate variability arises from natural processes within the climate system (El Niño-Southern Oscillation, North Atlantic Oscillation)
• Positive feedbacks amplify the initial climate change (ice-albedo feedback, water vapor feedback, permafrost carbon feedback)
  • Ice-albedo feedback: melting ice and snow expose darker surfaces, which absorb more solar radiation, leading to further warming
  • Water vapor feedback: warmer air holds more water vapor, a potent greenhouse gas, leading to additional warming
• Negative feedbacks dampen the initial climate change (increased CO2 uptake by oceans and vegetation, blackbody radiation feedback)
• Tipping points are thresholds beyond which the climate system undergoes rapid and irreversible change (collapse of the West Antarctic Ice Sheet, shutdown of the Atlantic Meridional Overturning Circulation)
• Carbon cycle describes the exchange of carbon between the atmosphere, oceans, land, and biosphere
  • Sources: fossil fuel combustion, deforestation, respiration
  • Sinks: photosynthesis, ocean absorption, geological processes

Global Circulation Patterns

• Hadley cells are large-scale atmospheric circulation patterns in the tropics, characterized by rising motion near the equator and descending motion in the subtropics
• Ferrel cells are mid-latitude atmospheric circulation patterns, characterized by rising motion in the subpolar regions and descending motion in the subtropics
• Polar cells are small-scale atmospheric circulation patterns in the polar regions, characterized by descending motion over the poles and rising motion in the subpolar regions
• Jet streams are narrow bands of strong winds in the upper troposphere that flow from west to east, influencing weather patterns and storm tracks
• Ocean currents transport heat and moisture around the globe, influencing regional climates (Gulf Stream, Kuroshio Current)
• El Niño-Southern Oscillation (ENSO) is a coupled ocean-atmosphere phenomenon characterized by fluctuations in ocean temperatures and atmospheric pressure across the equatorial Pacific Ocean
  • El Niño: warm phase, associated with weakened trade winds and increased sea surface temperatures in the eastern Pacific
  • La Niña: cold phase, associated with strengthened trade winds and decreased sea surface temperatures in the eastern Pacific

Climate Models and Projections

• Climate models simulate the interactions between the atmosphere, oceans, land surface, and ice, based on physical, chemical, and biological principles
• General Circulation Models (GCMs) represent the Earth's climate system using a three-dimensional grid, with typical horizontal resolutions of 100-300 km
• Earth System Models (ESMs) incorporate additional components, such as the carbon cycle, vegetation dynamics, and atmospheric chemistry
• Climate projections estimate future changes in temperature, precipitation, sea level, and other variables under different greenhouse gas emission scenarios
• Representative Concentration Pathways (RCPs) describe possible future greenhouse gas concentration trajectories, ranging from aggressive mitigation (RCP2.6) to high emissions (RCP8.5)
• Uncertainty in climate projections arises from natural variability, model limitations, and future greenhouse gas emissions
• Ensemble modeling involves running multiple simulations with different initial conditions or model formulations to assess the range of possible outcomes

Human Impact on Climate

• Anthropogenic greenhouse gas emissions, primarily from fossil fuel combustion and land use change, are the main drivers of current climate change
• Deforestation and land use change alter surface albedo, carbon storage, and evapotranspiration, contributing to climate change
• Urbanization creates urban heat islands, where temperatures are higher than in surrounding rural areas due to reduced vegetation, increased surface absorption, and anthropogenic heat sources
• Aerosols, such as sulfates and black carbon, can have cooling (reflecting sunlight) or warming (absorbing sunlight) effects on the climate, depending on their properties and location
• Ozone depletion, caused by the release of chlorofluorocarbons (CFCs) and other ozone-depleting substances, allows more harmful ultraviolet radiation to reach the Earth's surface
• Ocean acidification occurs when atmospheric CO2 dissolves in seawater, lowering the ocean's pH and affecting marine ecosystems (coral reefs, shellfish)
• Sea level rise results from thermal expansion of seawater and melting of land-based ice (glaciers, ice sheets), threatening coastal communities and ecosystems

Mitigation and Adaptation Strategies

• Mitigation strategies aim to reduce greenhouse gas emissions and enhance carbon sinks to limit the magnitude of future climate change
  • Renewable energy sources (solar, wind, hydro, geothermal) reduce reliance on fossil fuels
  • Energy efficiency measures in buildings, transportation, and industry reduce energy consumption and associated emissions
  • Carbon pricing, through taxes or cap-and-trade systems, incentivizes emissions reductions
• Adaptation strategies help communities and ecosystems cope with the impacts of climate change that are already occurring or are unavoidable
  • Infrastructure improvements, such as sea walls and flood barriers, protect against rising sea levels and extreme weather events
  • Sustainable land management practices, such as agroforestry and conservation agriculture, enhance resilience to climate variability and change
  • Early warning systems and emergency response plans help communities prepare for and respond to climate-related disasters
• Nature-based solutions, such as reforestation, wetland restoration, and green infrastructure, provide both mitigation and adaptation benefits
• Climate finance mechanisms, such as the Green Climate Fund, support mitigation and adaptation efforts in developing countries
• International agreements, such as the Paris Agreement, set global targets for emissions reductions and climate action