🌦️Atmospheric Science Unit 15 – Climate Change and Global Warming

Basics of Climate Science

• Climate defined as long-term average weather patterns and variability over time
• Influenced by factors such as atmospheric composition, ocean currents, and solar radiation
• Earth's climate system consists of atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere
  • These components interact through complex feedbacks and processes
• Climate varies naturally on timescales ranging from decades to millions of years
  • Caused by changes in Earth's orbit, solar output, volcanic eruptions, and other natural factors
• Paleoclimatology studies past climates using proxy data (tree rings, ice cores, sediments)
• Climate models simulate Earth's climate system using mathematical equations and physical principles
  • Used to understand past, present, and future climate change

Greenhouse Effect and Key Gases

• Greenhouse effect traps heat in Earth's atmosphere, making it habitable
  • Caused by greenhouse gases (GHGs) that absorb and re-emit infrared radiation
• Key GHGs include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and water vapor
  • CO2 is the most important long-lived GHG, with concentrations increasing due to human activities
• Atmospheric CO2 levels have risen from pre-industrial levels of ~280 ppm to over 410 ppm today
• Methane is a potent GHG with a shorter atmospheric lifetime than CO2
  • Sources include wetlands, agriculture (livestock and rice cultivation), and fossil fuels
• Nitrous oxide is a long-lived GHG produced by soil bacteria, fertilizers, and industrial processes
• Water vapor is the most abundant GHG but acts as a feedback rather than a driver of climate change

Historical Climate Patterns

• Earth's climate has varied naturally throughout its history due to changes in forcing factors
• Ice ages have occurred periodically over the past 2.6 million years
  • Driven by variations in Earth's orbit (Milankovitch cycles) and amplified by feedbacks
• Last Glacial Maximum (LGM) occurred ~20,000 years ago, with global temperatures ~5°C cooler than today
• Holocene epoch (last 11,700 years) has been relatively stable, allowing human civilizations to flourish
• Medieval Warm Period (950-1250 CE) and Little Ice Age (1450-1850 CE) show regional climate variability
• Paleoclimate data (proxy records) provide context for current climate change
  • Show that current warming is unprecedented in at least the last 2,000 years

Human Impact on Climate

• Anthropogenic activities have significantly altered Earth's energy balance, causing global warming
• Burning of fossil fuels (coal, oil, natural gas) releases CO2 into the atmosphere
  • Deforestation and land-use changes also contribute to CO2 emissions
• Industrial processes and agriculture emit other GHGs (methane, nitrous oxide)
• Atmospheric CO2 levels have increased by over 45% since the start of the Industrial Revolution
• Human activities have caused ~1°C of global warming above pre-industrial levels
  • Warming is likely to reach 1.5°C between 2030 and 2052 if emissions continue at the current rate
• Climate change impacts are already observed, including sea-level rise, ice sheet and glacier melt, and more frequent and intense heatwaves, droughts, and extreme precipitation events

Current Climate Trends

• Global average surface temperature has increased by ~0.2°C per decade since 1970
  • 2011-2020 was the warmest decade on record, with each decade since 1980 warmer than the previous one
• Arctic sea ice extent and thickness have declined rapidly, with summer minimum extent decreasing by ~13% per decade since 1979
• Greenland and Antarctic ice sheets are losing mass at accelerating rates, contributing to sea-level rise
• Global mean sea level has risen by ~21-24 cm since 1880, with the rate of rise accelerating in recent decades
• Ocean heat content has increased, leading to thermal expansion and contributing to sea-level rise
  • Oceans have absorbed ~90% of the excess heat trapped by greenhouse gases
• Precipitation patterns are changing, with wet regions generally becoming wetter and dry regions becoming drier
  • Heavy precipitation events are becoming more frequent and intense in many areas

Future Climate Projections

• Climate models project continued warming and changes in the climate system throughout the 21st century and beyond
• The magnitude of future climate change depends on the level of GHG emissions and the sensitivity of the climate system to those emissions
• Under a high-emissions scenario (RCP8.5), global average surface temperature is projected to increase by 2.6-4.8°C by 2100 compared to pre-industrial levels
  • A low-emissions scenario (RCP2.6) could limit warming to 0.3-1.7°C
• Sea level is projected to rise by 0.4-0.8 m by 2100 under RCP8.5, with the possibility of multi-meter rise in the following centuries if ice sheet loss accelerates
• Arctic sea ice is projected to become nearly ice-free in summer by mid-century under high-emissions scenarios
• Changes in precipitation patterns, with increased risk of drought in some regions and more intense rainfall events in others
• Increased frequency, intensity, and duration of heatwaves and other extreme weather events
  • Risks to human health, agriculture, and ecosystems

Climate Change Mitigation Strategies

• Mitigation involves reducing GHG emissions and enhancing carbon sinks to limit the magnitude of future climate change
• Transitioning to low-carbon energy sources (renewable energy, nuclear power) and improving energy efficiency
  • Phasing out fossil fuels and reducing emissions from industry, transportation, and buildings
• Implementing carbon pricing mechanisms (carbon taxes, emissions trading schemes) to incentivize emissions reductions
• Promoting sustainable land management practices (afforestation, reforestation, reduced deforestation)
  • Enhancing natural carbon sinks (forests, soils, wetlands)
• Developing and deploying negative emissions technologies (direct air capture, bioenergy with carbon capture and storage)
• Encouraging behavioral changes (reducing energy consumption, shifting to plant-based diets, sustainable transportation)
• International cooperation and agreements (Paris Agreement) to coordinate global mitigation efforts

Adaptation and Resilience

• Adaptation involves adjusting to the impacts of climate change that are already occurring or are expected to occur in the future
• Building resilience to climate-related hazards (sea-level rise, extreme weather events, droughts)
  • Improving infrastructure (flood defenses, drought-resistant crops, early warning systems)
• Developing adaptation plans and policies at local, regional, and national levels
  • Incorporating climate risk assessments into decision-making processes
• Enhancing ecosystem-based adaptation (protecting and restoring coastal wetlands, mangroves, and coral reefs)
• Promoting sustainable water management practices (water conservation, efficient irrigation, rainwater harvesting)
• Supporting vulnerable communities and regions (small island states, developing countries) in adapting to climate change impacts
• Fostering knowledge sharing and capacity building for adaptation and resilience
• Integrating adaptation and mitigation strategies for co-benefits and synergies