🌡️Climatology Unit 9 – Anthropogenic Climate Change

Key Concepts and Definitions

• Anthropogenic climate change refers to the human-induced alterations to Earth's climate system primarily caused by greenhouse gas emissions from fossil fuel combustion, deforestation, and other human activities
• Greenhouse gases (GHGs) are atmospheric gases that absorb and re-emit infrared radiation, trapping heat in the atmosphere and contributing to the greenhouse effect (carbon dioxide, methane, nitrous oxide, water vapor)
• Global warming is the long-term trend of rising average global temperatures, primarily attributed to the increasing concentrations of GHGs in the atmosphere
  • Since the pre-industrial era, global average surface temperatures have risen by approximately 1.1°C (2°F)
• Climate change encompasses not only rising temperatures but also changes in precipitation patterns, sea level rise, ocean acidification, and more frequent and intense extreme weather events
• Radiative forcing is a measure of the difference between incoming solar radiation and outgoing infrared radiation, quantifying the influence of various factors on Earth's energy balance
  • Positive radiative forcing (GHGs, solar activity) leads to warming, while negative radiative forcing (aerosols, volcanic eruptions) leads to cooling
• Climate sensitivity is the amount of global temperature change expected in response to a doubling of atmospheric CO2 concentrations, estimated to be between 1.5°C and 4.5°C (2.7°F to 8.1°F)
• Tipping points are thresholds in the climate system that, if crossed, can lead to irreversible and abrupt changes (melting of the Greenland ice sheet, collapse of the Atlantic Meridional Overturning Circulation)

Historical Context of Climate Change

• In the late 19th century, Swedish scientist Svante Arrhenius first proposed that increasing atmospheric CO2 levels could lead to global warming
• Guy Stewart Callendar, a British steam engineer, connected rising CO2 levels to observed warming trends in the 1930s, although his findings were initially met with skepticism
• The Keeling Curve, a continuous record of atmospheric CO2 concentrations measured at the Mauna Loa Observatory in Hawaii since 1958, provided clear evidence of the rapid increase in CO2 levels
  • CO2 concentrations have risen from pre-industrial levels of ~280 ppm to over 410 ppm today
• The First World Climate Conference in 1979 recognized climate change as a serious problem and called for global cooperation in research and monitoring
• The Intergovernmental Panel on Climate Change (IPCC) was established in 1988 to assess the scientific, technical, and socio-economic information relevant to understanding climate change
  • The IPCC has released six comprehensive assessment reports (1990, 1995, 2001, 2007, 2014, 2021), each confirming the growing evidence of anthropogenic climate change
• The United Nations Framework Convention on Climate Change (UNFCCC) was adopted in 1992 to stabilize GHG concentrations and prevent dangerous anthropogenic interference with the climate system
• The Kyoto Protocol, adopted in 1997, set legally binding emissions reduction targets for developed countries, recognizing their historical responsibility for GHG emissions
• The Paris Agreement, adopted in 2015, aims to limit global warming to well below 2°C above pre-industrial levels and pursue efforts to limit the increase to 1.5°C

Causes of Anthropogenic Climate Change

• The primary driver of anthropogenic climate change is the emission of greenhouse gases from human activities, which enhance the natural greenhouse effect and trap more heat in the atmosphere
• Burning fossil fuels (coal, oil, natural gas) for energy production, transportation, and industrial processes releases large amounts of CO2 into the atmosphere
  • Fossil fuel combustion accounts for over 75% of global CO2 emissions
• Deforestation and land-use changes contribute to climate change by reducing the Earth's capacity to absorb CO2 through photosynthesis and releasing stored carbon when vegetation is burned or decays
  • Deforestation is responsible for approximately 10% of global CO2 emissions
• Agricultural practices, such as livestock farming and rice cultivation, emit significant amounts of methane (CH4), a potent greenhouse gas
  • Methane has a global warming potential 28-36 times higher than CO2 over a 100-year period
• Industrial processes, such as cement production and the use of fluorinated gases in refrigeration and air conditioning, also contribute to GHG emissions
• Urbanization and the expansion of built environments can alter local and regional climate patterns through the urban heat island effect, where cities experience higher temperatures than surrounding rural areas
• Positive feedback loops can amplify the effects of climate change, such as the melting of Arctic sea ice, which reduces the Earth's albedo and leads to increased absorption of solar radiation
• Natural factors, such as variations in solar activity and volcanic eruptions, can also influence the Earth's climate, but their effects are relatively minor compared to human-induced changes

Greenhouse Gas Emissions and Sources

• Carbon dioxide (CO2) is the most significant anthropogenic greenhouse gas, accounting for about 76% of total GHG emissions
  • The primary sources of CO2 emissions are fossil fuel combustion, deforestation, and cement production
  • Atmospheric CO2 concentrations have increased by over 45% since the pre-industrial era
• Methane (CH4) is the second most important anthropogenic GHG, contributing around 16% of total emissions
  • Key sources of methane emissions include agriculture (livestock, rice cultivation), landfills, wastewater treatment, and fossil fuel production (natural gas, coal mining)
  • Methane concentrations have more than doubled since the pre-industrial era
• Nitrous oxide (N2O) accounts for about 6% of total GHG emissions and has a global warming potential 265-298 times higher than CO2 over a 100-year period
  • The main sources of N2O emissions are agricultural soil management (fertilizer use), industrial processes, and biomass burning
• Fluorinated gases, such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6), are powerful GHGs with global warming potentials thousands of times higher than CO2
  • These gases are used in various applications, including refrigeration, air conditioning, fire suppression, and industrial processes
• The global distribution of GHG emissions varies significantly, with developed countries historically responsible for the majority of cumulative emissions
  • However, emissions from developing countries, particularly China and India, have been rapidly increasing in recent decades
• Emissions from the transportation sector, including road vehicles, aviation, and shipping, have grown substantially due to increased global trade and travel
• The energy sector, which includes electricity and heat production, is the largest contributor to global GHG emissions, accounting for around 35% of total emissions

Observed Climate Changes and Impacts

• Global average surface temperatures have risen by approximately 1.1°C (2°F) since the pre-industrial era, with the warmest years on record occurring in the past decade
  • The rate of warming has accelerated in recent decades, with the period from 1981 to 2020 being the warmest 40-year period in at least the last 800 years
• Sea levels have risen by about 21-24 cm (8-9 inches) since 1880, with the rate of rise accelerating in recent decades due to thermal expansion of the oceans and melting of glaciers and ice sheets
  • Coastal communities are increasingly vulnerable to flooding, erosion, and saltwater intrusion
• Arctic sea ice extent has declined by about 13% per decade since 1979, with the summer minimum extent shrinking by over 40%
  • The loss of sea ice has significant implications for Arctic ecosystems, global climate patterns, and human activities in the region
• Glaciers and ice sheets are rapidly losing mass, contributing to sea level rise and altering regional water resources
  • The Greenland and Antarctic ice sheets have lost an average of 279 and 148 billion metric tons of ice per year, respectively, between 1993 and 2019
• Extreme weather events, such as heatwaves, droughts, heavy precipitation, and tropical cyclones, have become more frequent and intense in many regions
  • The frequency and intensity of heatwaves have increased, leading to increased heat-related mortality and impacts on agriculture and energy demand
• Precipitation patterns have shifted, with some regions experiencing more frequent and severe droughts while others face increased flooding risk
  • Changes in the timing and amount of rainfall can have significant impacts on water resources, agriculture, and ecosystems
• Ocean acidification, caused by the absorption of atmospheric CO2, has increased by about 30% since the pre-industrial era, threatening marine ecosystems and the livelihoods that depend on them
  • The average pH of the ocean surface has decreased by 0.1 units, equivalent to a 26% increase in acidity
• Biodiversity is under threat as species struggle to adapt to changing climatic conditions, with many facing increased risk of extinction
  • Climate change interacts with other stressors, such as habitat loss and overexploitation, to exacerbate biodiversity loss

Future Projections and Scenarios

• The IPCC has developed a range of future climate scenarios, known as Representative Concentration Pathways (RCPs), based on different levels of GHG emissions and mitigation efforts
  • RCP2.6 represents a stringent mitigation scenario, RCP4.5 and RCP6.0 are intermediate scenarios, and RCP8.5 is a high-emissions "business-as-usual" scenario
• Under the RCP8.5 scenario, global average surface temperatures are projected to rise by 2.6°C to 4.8°C (4.7°F to 8.6°F) by 2100 compared to the 1986-2005 average
  • This level of warming would have severe and widespread impacts on human and natural systems
• Even under the more optimistic RCP2.6 scenario, which assumes aggressive emissions reductions, global temperatures are still projected to rise by 0.3°C to 1.7°C (0.5°F to 3.1°F) by 2100
  • This highlights the importance of immediate and sustained action to reduce GHG emissions
• Sea levels are projected to rise by 0.26 to 0.82 m (0.85 to 2.7 feet) by 2100 relative to 1986-2005, depending on the emissions scenario
  • The rate and magnitude of sea level rise beyond 2100 depend heavily on the extent of ice sheet loss, which is still subject to significant uncertainty
• The frequency, intensity, and duration of extreme heat events are expected to increase, while cold extremes are projected to become less frequent
  • The number of hot days and heatwaves will likely increase, particularly in the tropics and mid-latitudes
• Precipitation patterns are projected to continue changing, with wet regions generally becoming wetter and dry regions becoming drier
  • The contrast between wet and dry seasons is expected to increase, with more intense precipitation events and longer dry spells
• The Arctic is projected to continue warming at a rate two to three times higher than the global average, leading to further reductions in sea ice extent and thickness
  • The Arctic Ocean may become essentially ice-free in summer before 2050 under high-emissions scenarios
• The ocean is projected to continue warming and acidifying, with widespread impacts on marine ecosystems and the communities that depend on them
  • Coral reefs are particularly vulnerable, with most projected to experience severe degradation even under intermediate emissions scenarios
• The risks associated with tipping points and abrupt changes in the climate system, such as the collapse of the West Antarctic Ice Sheet or the shutdown of the Atlantic Meridional Overturning Circulation, increase with higher levels of warming
  • These low-probability, high-impact events could have catastrophic and irreversible consequences

Mitigation Strategies and Policies

• Mitigation strategies aim to reduce GHG emissions and enhance carbon sinks to limit the extent of future climate change
• Transitioning to low-carbon and renewable energy sources, such as solar, wind, hydro, and geothermal power, is crucial for reducing emissions from the energy sector
  • Increasing the share of renewables in the global energy mix can significantly reduce CO2 emissions while providing co-benefits such as improved air quality and energy security
• Improving energy efficiency in buildings, transportation, and industry can help reduce energy demand and associated emissions
  • Implementing building codes, appliance standards, and fuel efficiency standards can drive efficiency improvements
• Carbon pricing mechanisms, such as carbon taxes and emissions trading systems (ETS), can create economic incentives for reducing emissions
  • By putting a price on carbon, these policies encourage businesses and individuals to invest in low-carbon technologies and practices
• Promoting sustainable land management practices, such as reforestation, afforestation, and reduced deforestation, can enhance carbon sequestration in terrestrial ecosystems
  • Protecting and restoring forests, wetlands, and other carbon-rich ecosystems can help mitigate climate change while providing co-benefits for biodiversity and local communities
• Encouraging sustainable transportation, including the adoption of electric vehicles, public transit, and active transportation (walking, cycling), can reduce emissions from the transportation sector
  • Investing in low-carbon transportation infrastructure and promoting compact, mixed-use urban development can support sustainable mobility
• Implementing policies to support research, development, and deployment of carbon capture, utilization, and storage (CCUS) technologies can help reduce emissions from hard-to-abate sectors
  • CCUS involves capturing CO2 from point sources or the atmosphere and either using it in industrial processes or storing it in geological formations
• Promoting sustainable consumption and production patterns, such as reducing food waste, adopting plant-based diets, and minimizing the use of single-use plastics, can help reduce emissions across various sectors
  • Encouraging circular economy principles, such as recycling, reuse, and remanufacturing, can minimize resource extraction and associated emissions
• International cooperation and agreements, such as the Paris Agreement, are essential for coordinating global efforts to reduce emissions and support developing countries in their mitigation and adaptation efforts
  • Strengthening the ambition of nationally determined contributions (NDCs) and ensuring their effective implementation is crucial for achieving the goals of the Paris Agreement

Adaptation Measures and Challenges

• Adaptation measures aim to reduce the vulnerability of human and natural systems to the impacts of climate change and enhance their resilience
• Developing and implementing early warning systems for extreme weather events, such as heatwaves, floods, and tropical cyclones, can help communities prepare and respond effectively
  • Improving weather forecasting and risk communication can minimize the loss of life and property during extreme events
• Enhancing the resilience of infrastructure, such as buildings, roads, and power grids, to withstand the impacts of climate change is essential for maintaining critical services and reducing economic losses
  • Incorporating climate risk assessments into infrastructure planning and design can help identify and address vulnerabilities
• Promoting sustainable water management practices, such as water conservation, efficiency improvements, and integrated water resources management, can help address water scarcity and quality issues exacerbated by climate change
  • Investing in water storage and distribution infrastructure, such as reservoirs and irrigation systems, can enhance water security
• Adapting agricultural practices to changing climatic conditions, such as adopting drought-resistant crop varieties, implementing efficient irrigation techniques, and diversifying crops, can help ensure food security
  • Promoting agroforestry and other climate-smart agriculture practices can enhance the resilience of farming systems while providing mitigation co-benefits
• Protecting and restoring ecosystems, such as wetlands, mangroves, and coral reefs, can help buffer coastal communities from sea level rise, storm surges, and erosion
  • Ecosystem-based adaptation approaches can provide multiple benefits, including biodiversity conservation, carbon sequestration, and livelihood support
• Developing and implementing heat action plans, particularly in urban areas, can help reduce the health risks associated with extreme heat events
  • Measures such as establishing cooling centers, improving building insulation, and increasing green spaces can mitigate the urban heat island effect
• Enhancing public health systems and surveillance to detect and respond to climate-related health risks, such as vector-borne diseases and respiratory illnesses, is crucial for protecting human health
  • Strengthening healthcare infrastructure and capacity, particularly in vulnerable communities, can improve resilience to climate-related health impacts
• Mainstreaming climate change considerations into development planning and decision-making processes can help ensure that adaptation measures are integrated across sectors and scales
  • Conducting vulnerability and risk assessments can inform the prioritization and implementation of adaptation actions
• Financing adaptation measures remains a significant challenge, particularly for developing countries with limited resources and competing development priorities
  • Mobilizing and scaling up climate finance, including through international mechanisms such as the Green Climate Fund, is essential for supporting adaptation efforts in vulnerable communities
• Adaptation efforts face various challenges, including limited institutional