🍂Environmental Chemistry II Unit 10 – Climate Change: Greenhouse Gases & Forcing

Key Concepts

• Greenhouse effect traps heat in Earth's atmosphere due to greenhouse gases (carbon dioxide, methane, water vapor) absorbing and re-emitting infrared radiation
• Radiative forcing quantifies the change in energy balance of the Earth system due to external factors (greenhouse gases, aerosols, land use changes)
  • Positive radiative forcing leads to warming, while negative radiative forcing leads to cooling
• Climate sensitivity describes the amount of global temperature change in response to a given radiative forcing
• Feedback mechanisms can amplify (positive feedback) or dampen (negative feedback) the initial climate response to a forcing
  • Examples of positive feedbacks include ice-albedo feedback and water vapor feedback
• Anthropogenic activities, primarily fossil fuel combustion and land use changes, have significantly increased atmospheric greenhouse gas concentrations since the Industrial Revolution
• Global warming potential (GWP) compares the radiative forcing of different greenhouse gases over a specified time horizon (typically 100 years)
• Climate models simulate the complex interactions between the atmosphere, oceans, land surface, and cryosphere to project future climate changes based on different emission scenarios

Greenhouse Gases

• Carbon dioxide (CO2) is the most significant anthropogenic greenhouse gas, primarily due to fossil fuel combustion and deforestation
  • Atmospheric CO2 concentrations have increased from pre-industrial levels of ~280 ppm to over 410 ppm today
• Methane (CH4) is a potent greenhouse gas with a GWP 28-36 times that of CO2 over a 100-year time horizon
  • Key sources include agriculture (livestock and rice cultivation), landfills, and fossil fuel production
• Nitrous oxide (N2O) has a GWP 265-298 times that of CO2 over a 100-year time horizon
  • Primary sources include agricultural soil management (fertilizer use) and industrial processes
• Water vapor is the most abundant greenhouse gas, but its atmospheric concentration is not directly affected by human activities
  • However, as the atmosphere warms due to other greenhouse gases, it can hold more water vapor, leading to a positive feedback
• Ozone (O3) is a greenhouse gas in the troposphere, formed by chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight
• Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are potent greenhouse gases that also deplete the stratospheric ozone layer
  • The Montreal Protocol has successfully phased out the production of these substances

Radiative Forcing

• Radiative forcing is expressed in watts per square meter (W/m2) and represents the change in net irradiance at the tropopause
• Positive radiative forcing occurs when the energy absorbed by the Earth system exceeds the energy radiated back to space
  • Examples include increasing greenhouse gas concentrations and black carbon aerosols
• Negative radiative forcing occurs when the energy radiated back to space exceeds the energy absorbed by the Earth system
  • Examples include volcanic aerosols and increased surface albedo due to land use changes
• The total anthropogenic radiative forcing since 1750 is estimated to be 2.72 [1.96 to 3.48] W/m2 (IPCC AR6)
  • CO2 accounts for the largest contribution at 2.16 [1.90 to 2.41] W/m2
• Natural forcings, such as solar variability and volcanic eruptions, can also influence the Earth's radiative balance
  • However, their contributions are generally smaller and shorter-lived compared to anthropogenic forcings
• Effective radiative forcing (ERF) accounts for rapid adjustments in the atmosphere, such as changes in atmospheric temperature, water vapor, and clouds, in response to a forcing agent

Climate Feedback Mechanisms

• Ice-albedo feedback amplifies initial warming as rising temperatures lead to melting ice and snow, reducing surface albedo and increasing absorption of solar radiation
• Water vapor feedback is a powerful positive feedback, as a warmer atmosphere can hold more water vapor, which is itself a greenhouse gas
• Cloud feedback is complex and uncertain, as changes in cloud cover, altitude, and optical properties can have both warming and cooling effects
  • Low clouds generally have a cooling effect by reflecting solar radiation, while high clouds can trap outgoing longwave radiation
• Lapse rate feedback is a negative feedback that arises from the decrease in the vertical temperature gradient as the atmosphere warms
• Carbon cycle feedbacks involve changes in the uptake and release of CO2 by the oceans and terrestrial biosphere in response to climate change
  • Examples include increased soil respiration and reduced ocean CO2 solubility under warmer conditions
• Permafrost thaw feedback can release stored carbon as CO2 and CH4 as frozen soils warm and decompose
• Methane hydrate feedback involves the potential release of CH4 from hydrate deposits in ocean sediments and permafrost as temperatures rise

Measurement and Monitoring

• Atmospheric greenhouse gas concentrations are measured through a global network of monitoring stations, such as the Mauna Loa Observatory in Hawaii
  • Measurements are made using infrared gas analyzers, gas chromatography, and mass spectrometry
• Ice core records provide a long-term history of atmospheric greenhouse gas concentrations and climate variability over the past 800,000 years
  • Air bubbles trapped in the ice preserve samples of past atmospheric composition
• Satellite observations, such as NASA's Orbiting Carbon Observatory (OCO) missions, provide global measurements of atmospheric CO2 concentrations and their spatial and temporal variations
• Eddy covariance flux towers measure the exchange of CO2, water vapor, and energy between the atmosphere and terrestrial ecosystems
• Ocean acidification is monitored through measurements of seawater pH, carbonate ion concentration, and calcium carbonate saturation state
  • Autonomous sensors on buoys and floats, such as the Argo network, provide global coverage
• Proxy records, such as tree rings, corals, and lake sediments, provide indirect evidence of past climate variability and environmental conditions
• Climate models are evaluated against observations to assess their performance and improve their representation of key processes and feedbacks

Human Activities and Emissions

• Fossil fuel combustion for energy production, transportation, and industrial processes is the largest source of anthropogenic greenhouse gas emissions
  • Coal, oil, and natural gas combustion release CO2, as well as smaller amounts of CH4 and N2O
• Deforestation and land use changes release stored carbon as CO2 and reduce the capacity of terrestrial ecosystems to absorb CO2 from the atmosphere
  • Agriculture, particularly livestock production and rice cultivation, is a significant source of CH4 emissions
• Cement production involves the calcination of limestone, which releases CO2 as a byproduct
• Landfills and wastewater treatment emit CH4 due to the anaerobic decomposition of organic waste
• Fertilizer application in agriculture leads to N2O emissions from soil microbial processes, such as nitrification and denitrification
• Refrigeration, air conditioning, and industrial processes involve the use of fluorinated gases (F-gases), such as hydrofluorocarbons (HFCs), which are potent greenhouse gases
• Aviation and shipping contribute to greenhouse gas emissions through the combustion of fossil fuels and the formation of contrails and cirrus clouds

Environmental Impacts

• Global warming leads to rising sea levels due to thermal expansion of the oceans and melting of glaciers and ice sheets
  • Coastal communities and low-lying islands are particularly vulnerable to flooding and erosion
• Changes in precipitation patterns, including more frequent and intense droughts and floods, affect water availability and agricultural productivity
  • Shifts in the timing and amount of snowmelt can alter river flow regimes and water supply
• Extreme weather events, such as heatwaves, hurricanes, and wildfires, are becoming more frequent and severe as the climate warms
• Ocean acidification, caused by the uptake of atmospheric CO2, threatens marine ecosystems and organisms that build calcium carbonate shells and skeletons (corals, mollusks)
• Biodiversity loss and ecosystem degradation occur as species struggle to adapt to changing environmental conditions and face habitat loss and fragmentation
  • Phenological mismatches between interdependent species can disrupt ecological interactions
• Thawing permafrost and melting Arctic sea ice alter the energy balance, hydrology, and carbon cycling of high-latitude regions
• Human health is affected by climate change through the spread of vector-borne diseases, heat stress, air pollution, and food and water insecurity
  • Vulnerable populations, such as the elderly, children, and low-income communities, are disproportionately impacted

Mitigation Strategies

• Transitioning to renewable energy sources, such as solar, wind, hydro, and geothermal power, reduces greenhouse gas emissions from fossil fuel combustion
  • Improving energy efficiency in buildings, transportation, and industry also helps reduce energy demand and emissions
• Carbon pricing mechanisms, such as carbon taxes and cap-and-trade systems, create economic incentives to reduce emissions and invest in low-carbon technologies
• Afforestation and reforestation efforts increase carbon sequestration by creating new forests or restoring degraded ones
  • Sustainable forest management practices, such as reduced impact logging, can maintain carbon stocks and biodiversity
• Sustainable agriculture practices, such as reduced tillage, cover cropping, and precision fertilizer application, can reduce emissions and enhance soil carbon storage
• Waste reduction, recycling, and composting minimize methane emissions from landfills and reduce the demand for resource extraction and processing
• Carbon capture and storage (CCS) technologies aim to capture CO2 emissions from point sources, such as power plants and industrial facilities, and store them in geological formations or use them for enhanced oil recovery
• Lifestyle changes, such as adopting plant-based diets, reducing meat consumption, and using public transportation or cycling, can significantly reduce an individual's carbon footprint
• International cooperation and agreements, such as the Paris Agreement, set global targets and frameworks for reducing emissions and adapting to climate change impacts