🪨Biogeochemistry Unit 11 – Atmospheric Cycles and Global Change

Key Atmospheric Components

• Nitrogen ($N_2$) most abundant gas in the atmosphere at 78% by volume provides a stable, inert background for other atmospheric processes
• Oxygen ($O_2$) second most abundant gas at 21% by volume essential for aerobic respiration and combustion processes
• Water vapor ($H_2O$) variable concentration (0-4%) plays a crucial role in the Earth's energy balance as a potent greenhouse gas and in the hydrologic cycle
• Carbon dioxide ($CO_2$) trace gas (0.04%) but a significant greenhouse gas that helps regulate Earth's temperature
• Methane ($CH_4$) potent greenhouse gas with a global warming potential 28-36 times that of $CO_2$ over a 100-year period
• Ozone ($O_3$) trace gas in the stratosphere absorbs harmful UV radiation protecting life on Earth
  • In the troposphere, ozone is a pollutant and greenhouse gas formed by chemical reactions involving nitrogen oxides and volatile organic compounds
• Aerosols tiny solid or liquid particles suspended in the atmosphere can have cooling (sulfates) or warming (black carbon) effects on climate

Biogeochemical Cycles Overview

• Carbon cycle involves the exchange of carbon between the atmosphere, biosphere, oceans, and geosphere
  • Key processes include photosynthesis, respiration, decomposition, and weathering
• Nitrogen cycle the transfer of nitrogen between the atmosphere, biosphere, and geosphere
  • Processes such as nitrogen fixation, nitrification, denitrification, and ammonification are essential for plant growth and ecosystem productivity
• Phosphorus cycle the movement of phosphorus through the biosphere, geosphere, and hydrosphere
  • Weathering of rocks releases phosphorus, which is taken up by plants, passed through food chains, and eventually returned to the geosphere through decomposition and sedimentation
• Sulfur cycle the circulation of sulfur through the Earth's systems, including the atmosphere, biosphere, and geosphere
  • Processes like volcanic emissions, weathering, and biological activity (sulfate reduction) play important roles
• Interactions between cycles biogeochemical cycles are interconnected and influence one another
  • For example, the carbon and nitrogen cycles are linked through processes such as plant growth and decomposition
• Human impacts human activities (fossil fuel combustion, land-use change, agriculture) have significantly altered biogeochemical cycles
  • This has led to changes in atmospheric composition, climate, and ecosystem functioning

Greenhouse Effect and Climate Change

• Greenhouse gases (GHGs) absorb and re-emit infrared radiation, trapping heat in the atmosphere and warming the Earth's surface
  • Primary GHGs include water vapor, carbon dioxide, methane, and nitrous oxide
• Anthropogenic GHG emissions human activities (fossil fuel combustion, deforestation, agriculture) have increased atmospheric concentrations of GHGs since the Industrial Revolution
• Enhanced greenhouse effect the increase in the Earth's average temperature due to the rise in atmospheric GHG concentrations
  • This has led to observed changes in global climate patterns, such as rising sea levels, more frequent heatwaves, and shifts in precipitation patterns
• Climate feedback mechanisms processes that amplify (positive feedback) or diminish (negative feedback) the initial climate change response
  • Examples include the ice-albedo feedback (positive) and the ocean carbon sink (negative)
• Climate tipping points thresholds beyond which abrupt, irreversible changes in the Earth's climate system may occur
  • Examples include the collapse of the West Antarctic Ice Sheet or the shutdown of the Atlantic Meridional Overturning Circulation
• Impacts on ecosystems and society climate change affects biodiversity, ecosystem services, agriculture, human health, and infrastructure
  • Adaptation and mitigation strategies are necessary to reduce the risks and impacts of climate change

Human Impacts on Atmospheric Composition

• Fossil fuel combustion releases $CO_2$, $CH_4$, and other pollutants (nitrogen oxides, sulfur dioxide) into the atmosphere altering its composition and contributing to climate change and air pollution
• Land-use change (deforestation, urbanization) alters the Earth's surface properties affecting atmospheric processes such as carbon uptake, albedo, and evapotranspiration
• Agricultural practices (livestock farming, rice cultivation, fertilizer use) emit $CH_4$ and nitrous oxide ($N_2O$) two potent greenhouse gases
• Industrial processes release various pollutants (particulate matter, volatile organic compounds) that degrade air quality and affect human health
• Stratospheric ozone depletion caused by the release of ozone-depleting substances (chlorofluorocarbons) has led to the formation of the Antarctic ozone hole
  • The Montreal Protocol has successfully phased out these substances, and the ozone layer is slowly recovering
• Geoengineering proposed large-scale interventions to counteract climate change, such as solar radiation management or carbon dioxide removal
  • These techniques are controversial and may have unintended consequences

Feedback Mechanisms and Tipping Points

• Ice-albedo feedback as Arctic sea ice melts due to warming, the exposed darker ocean absorbs more solar radiation, leading to further warming and ice loss (positive feedback)
• Permafrost carbon feedback thawing permafrost releases stored carbon ($CO_2$ and $CH_4$) into the atmosphere, amplifying warming (positive feedback)
• Ocean carbon sink feedback as oceans absorb atmospheric $CO_2$, they become more acidic, reducing their capacity to take up additional $CO_2$ (positive feedback)
  • Conversely, warming oceans may also lead to increased phytoplankton growth, enhancing $CO_2$ uptake (negative feedback)
• Methane hydrate destabilization warming oceans and thawing permafrost can release methane hydrates, a potent greenhouse gas, further amplifying warming (positive feedback)
• Amazon rainforest dieback drought and deforestation may cause the Amazon rainforest to transition from a carbon sink to a carbon source, accelerating climate change (tipping point)
• Atlantic Meridional Overturning Circulation (AMOC) slowdown freshwater input from melting Greenland ice and increased precipitation can weaken the AMOC, affecting global climate patterns (tipping point)
  • A complete shutdown of the AMOC would have severe consequences for regional climates and ecosystems

Atmospheric Monitoring and Data Analysis

• Greenhouse gas measurements atmospheric $CO_2$, $CH_4$, and other GHG concentrations are monitored through a global network of stations (Mauna Loa Observatory)
  • Ice core records provide a longer-term history of atmospheric composition
• Satellite observations remote sensing data (NASA's Earth Observing System) provide global coverage of various atmospheric parameters (temperature, humidity, aerosols)
• Weather and climate stations measure surface temperature, precipitation, wind speed, and other meteorological variables
  • These data are used to track climate trends and validate climate models
• Atmospheric chemistry measurements monitor the concentrations of various pollutants (ozone, nitrogen oxides, particulate matter) to assess air quality and inform policy decisions
• Paleoclimate proxies (tree rings, coral reefs, lake sediments) provide indirect evidence of past climate conditions and help contextualize current climate change
• Data assimilation and reanalysis techniques combine observations and model simulations to create consistent, long-term datasets for climate research and applications

Climate Modeling and Predictions

• General Circulation Models (GCMs) complex numerical models that simulate the Earth's climate system by incorporating atmospheric, oceanic, and land surface processes
  • GCMs are used to project future climate change under different emission scenarios
• Earth System Models (ESMs) extend GCMs by including additional components (carbon cycle, vegetation dynamics, ice sheets) to provide a more comprehensive representation of the Earth system
• Regional Climate Models (RCMs) higher-resolution models that focus on specific regions, allowing for more detailed projections of local climate impacts
• Ensemble modeling running multiple model simulations with varying initial conditions or model physics to quantify uncertainty and improve the robustness of projections
• Downscaling techniques (statistical, dynamical) used to translate coarse-resolution GCM output to finer spatial scales relevant for impact assessments and adaptation planning
• Intergovernmental Panel on Climate Change (IPCC) assesses the scientific, technical, and socio-economic information relevant to climate change
  • IPCC reports provide policymakers with regular assessments of the state of knowledge on climate change, its impacts, and potential response strategies

Mitigation Strategies and Policy Implications

• Greenhouse gas emission reduction targets international agreements (Paris Agreement) aim to limit global warming to well below 2°C above pre-industrial levels by reducing GHG emissions
  • Nationally Determined Contributions (NDCs) outline each country's emission reduction pledges
• Renewable energy transition shifting from fossil fuels to renewable sources (solar, wind, hydro) to decarbonize the energy sector
  • Policies such as feed-in tariffs, renewable portfolio standards, and carbon pricing can incentivize the deployment of renewable energy technologies
• Energy efficiency improvements reducing energy consumption through technological advancements (LED lighting, insulation) and behavioral changes (public transport, eco-driving)
• Sustainable land management practices (afforestation, reforestation, agroforestry) can enhance carbon sequestration and mitigate climate change
  • Reducing deforestation and forest degradation (REDD+) is a key strategy for preserving carbon sinks and biodiversity
• Carbon capture and storage (CCS) technologies that capture $CO_2$ from point sources (power plants) and store it underground to prevent its release into the atmosphere
• Climate change adaptation measures (infrastructure upgrades, early warning systems, crop diversification) to reduce the vulnerability of communities and ecosystems to the impacts of climate change
• International cooperation and finance mechanisms (Green Climate Fund) to support developing countries in their mitigation and adaptation efforts and foster technology transfer