🌡️Intro to Climate Science Unit 11 – Recent Climate Change Observations

Key Climate Indicators

• Atmospheric carbon dioxide ($CO_2$) concentrations have increased from pre-industrial levels of ~280 ppm to over 410 ppm in 2020
  • Measured directly by instruments (Mauna Loa Observatory) and indirectly through ice core records
  • Primary driver of anthropogenic climate change due to its greenhouse gas properties
• Global average surface temperature serves as a fundamental metric of climate change
  • Combines land surface air temperature and sea surface temperature measurements
  • Shows a clear warming trend since the late 19th century, with accelerated warming in recent decades
• Sea level rise reflects the integrated effects of climate change on the ocean and cryosphere
  • Measured by tide gauges and satellite altimetry
  • Driven by thermal expansion of seawater and melting of land-based ice (glaciers, ice sheets)
• Arctic sea ice extent has declined significantly since the start of satellite observations in 1979
  • Minimum extent occurs in September, with a decreasing trend of ~13% per decade
  • Serves as a sensitive indicator of warming in the polar regions and has implications for Arctic ecosystems and global climate feedbacks
• Ocean heat content has increased substantially, particularly in the upper 700 m of the ocean
  • Measured by a network of floats (Argo) and ship-based observations
  • Represents the majority of the excess energy accumulated in the Earth system due to greenhouse gas forcing
• Glacial mass balance provides a direct measure of the impact of climate change on land-based ice
  • Assessed through field measurements, remote sensing, and mass balance modeling
  • Most glaciers worldwide are losing mass, contributing to sea level rise and affecting water resources in mountain regions

Observed Temperature Changes

• Global average surface temperature has increased by approximately 1.0°C since pre-industrial times
  • Warming has not been uniform, with greater temperature increases over land and in the Arctic
  • Most of the warming has occurred in the past 40 years, with the six warmest years on record taking place since 2014
• Satellite and weather balloon observations show warming of the lower troposphere and cooling of the stratosphere
  • Consistent with the pattern expected from greenhouse gas forcing
  • Stratospheric cooling due to ozone depletion and increased greenhouse gases
• Nighttime temperatures have increased faster than daytime temperatures
  • Leads to a decrease in the diurnal temperature range
  • Has implications for human health, agriculture, and ecosystems
• Warming is evident across all continents and ocean basins
  • Strongest warming trends observed in the Arctic, with temperatures rising more than twice the global average rate
  • Significant regional variations, influenced by local factors and natural climate variability (El Niño-Southern Oscillation)
• Heatwaves have become more frequent, intense, and longer-lasting in many regions
  • Defined as periods of abnormally hot weather, often exceeding historical thresholds
  • Have severe impacts on human health, agriculture, and infrastructure
• Urban heat island effect amplifies warming in cities
  • Caused by the built environment, reduced vegetation, and anthropogenic heat sources
  • Can exacerbate the impacts of heatwaves and air pollution on urban populations

Sea Level Rise and Ice Melt

• Global mean sea level has risen by ~21-24 cm since 1880, with about a third of that rise occurring in the last 25 years
  • Measured by tide gauges and satellite altimetry (TOPEX/Poseidon, Jason series)
  • Rate of sea level rise has accelerated from ~1.4 mm/year over the 20th century to ~3.6 mm/year in recent decades
• Thermal expansion of seawater accounts for about one-third of observed sea level rise
  • Occurs as the ocean absorbs excess heat from the atmosphere
  • Contributes to sea level rise even in the absence of ice melt
• Melting of glaciers and ice caps has contributed ~60% of the total sea level rise since 1993
  • Glaciers in the European Alps, Himalayas, Andes, Rockies, Alaska, and Africa are retreating rapidly
  • Many glaciers are projected to disappear by the end of the 21st century, affecting water resources and tourism
• Greenland and Antarctic ice sheets are losing mass at an increasing rate
  • Measured by satellite gravimetry (GRACE), altimetry (ICESat), and mass balance calculations
  • Greenland ice loss is primarily driven by surface melting and iceberg calving, while Antarctic ice loss is mainly due to ice shelf thinning and ice flow acceleration
• Permafrost thaw can contribute to sea level rise and amplify warming through the release of greenhouse gases
  • Permafrost contains large amounts of organic carbon, which can be released as $CO_2$ and methane upon thawing
  • Thawing can also lead to ground subsidence and infrastructure damage in Arctic regions
• Sea level rise is not uniform globally, with some regions experiencing higher rates due to ocean circulation, gravitational effects, and land subsidence
  • Low-lying coastal areas, small islands, and deltas are particularly vulnerable to the impacts of sea level rise (flooding, erosion, saltwater intrusion)
  • Adaptation measures include coastal protection, accommodation, and managed retreat

Extreme Weather Events

• Heatwaves have become more frequent, intense, and longer-lasting in many regions
  • Defined as periods of abnormally hot weather, often exceeding historical thresholds
  • Have severe impacts on human health, agriculture, and infrastructure (2003 European heatwave, 2010 Russian heatwave)
• Heavy precipitation events have increased in frequency and intensity in many areas
  • Warmer atmosphere can hold more moisture, leading to more intense rainfall
  • Can cause flooding, landslides, and damage to crops and infrastructure (2017 South Asian floods, 2019 Midwest U.S. floods)
• Droughts have become more intense and widespread in some regions
  • Influenced by changes in precipitation patterns, evapotranspiration, and land use
  • Can lead to water scarcity, crop failures, and increased wildfire risk (2011-2017 California drought, 2018-2020 Southern Africa drought)
• Tropical cyclones have shown trends towards greater intensity and slower movement
  • Warmer ocean temperatures provide more energy for cyclone development
  • Slower-moving storms can cause more damage through prolonged wind, rainfall, and storm surge (Hurricane Harvey 2017, Typhoon Haiyan 2013)
• Wildfires have increased in frequency, size, and severity in many regions
  • Driven by a combination of climate change, land management practices, and human activities
  • Can have devastating impacts on ecosystems, air quality, and human communities (2019-2020 Australian bushfires, 2018 California wildfires)
• Compound events, where multiple extreme events occur simultaneously or in succession, can amplify impacts
  • Examples include heatwaves coinciding with droughts, or tropical cyclones followed by flooding
  • Can overwhelm disaster response capacities and lead to cascading effects on society and the environment

Ecosystem and Biodiversity Impacts

• Species' ranges are shifting poleward and to higher elevations in response to warming temperatures
  • Observed in a wide range of taxa, including plants, insects, birds, and mammals
  • Can lead to mismatches between species and their habitats, food sources, or pollinators
• Phenological changes, such as earlier spring events and longer growing seasons, are occurring in many ecosystems
  • Examples include earlier flowering, leaf-out, and migration times
  • Can disrupt ecological interactions and increase vulnerability to late frost events
• Coral reefs are experiencing more frequent and severe bleaching events due to ocean warming and acidification
  • Bleaching occurs when corals expel their symbiotic algae under stress, often leading to coral mortality
  • Can have cascading effects on reef biodiversity and the communities that depend on them for food and livelihoods
• Biodiversity loss is accelerating, with climate change exacerbating other pressures such as habitat loss and overexploitation
  • Extinction risks are increasing for many species, particularly those with limited dispersal abilities or narrow climate tolerances
  • Loss of biodiversity can affect ecosystem functioning, resilience, and the provision of ecosystem services
• Invasive species are expanding their ranges and becoming more prevalent in some regions as a result of changing climatic conditions
  • Warmer temperatures and altered precipitation patterns can create new opportunities for invasive species establishment
  • Can have negative impacts on native biodiversity, ecosystem functioning, and human activities (agriculture, forestry)
• Ecosystem carbon dynamics are being altered by climate change, with potential feedbacks to the climate system
  • Warmer temperatures can increase soil respiration and permafrost thaw, releasing stored carbon
  • Drought and wildfire can reduce ecosystem carbon uptake and cause large-scale tree mortality
  • Changes in ecosystem carbon balance can affect the global carbon cycle and future climate change

Human and Economic Consequences

• Climate change impacts on agriculture threaten food security and livelihoods
  • Crop yields are affected by changes in temperature, precipitation, and extreme events
  • Livestock production is impacted by heat stress, disease, and changes in forage quality and availability
• Water resources are increasingly stressed by changes in precipitation patterns, glacial melt, and sea level rise
  • Reduced water availability and quality can affect drinking water supplies, sanitation, and irrigation
  • Increased risk of flooding and drought can damage infrastructure and disrupt water management systems
• Human health is affected by a range of climate-related factors
  • Heat stress and heatwaves can cause illness, exacerbate pre-existing conditions, and increase mortality
  • Vector-borne diseases (malaria, dengue) can expand their ranges as temperatures warm and precipitation patterns change
  • Air quality can be degraded by increased ozone formation, wildfire smoke, and allergens
• Coastal communities and infrastructure are vulnerable to sea level rise, storm surges, and coastal erosion
  • Flooding and saltwater intrusion can damage buildings, roads, and other critical infrastructure
  • Displacement of coastal populations can lead to social and economic disruption
• Climate change can act as a threat multiplier, exacerbating existing social, economic, and political stresses
  • Impacts on food and water security, health, and livelihoods can contribute to poverty, migration, and conflict
  • Unequal distribution of impacts and adaptive capacities can worsen existing inequalities and vulnerabilities
• Economic impacts of climate change are projected to be significant and widespread
  • Direct costs include damages from extreme events, reduced agricultural productivity, and infrastructure impacts
  • Indirect costs arise from supply chain disruptions, reduced labor productivity, and shifts in consumer behavior
  • Adaptation and mitigation efforts also entail costs, but can provide long-term benefits and avoid some of the worst impacts

Detection and Attribution Methods

• Detection involves identifying significant changes in the observed climate that are inconsistent with natural variability alone
  • Uses statistical methods to compare observed changes with the range of variability simulated by climate models
  • Considers multiple lines of evidence, such as temperature trends, sea level rise, and changes in atmospheric composition
• Attribution seeks to determine the relative contributions of different factors to the detected changes
  • Separates the influences of human activities (greenhouse gas emissions, land use change) from natural factors (solar variability, volcanic eruptions)
  • Uses climate models to simulate the response to individual forcings and compare with observations
• Fingerprinting techniques are used to identify the unique patterns of change associated with different forcings
  • Each forcing (greenhouse gases, aerosols, solar) produces a distinct spatial and temporal pattern of temperature change
  • By comparing these patterns with observed changes, the relative contributions of different forcings can be estimated
• Optimal fingerprinting is a widely used attribution method that combines climate model simulations with observations
  • Calculates the scaling factors needed to match the model-simulated response patterns to the observed changes
  • Provides a quantitative estimate of the contribution of each forcing to the observed change, along with uncertainty ranges
• Attribution studies have been conducted for a wide range of climate variables and extreme events
  • Temperature changes, sea level rise, and ocean acidification have been robustly attributed to human influence
  • The human contribution to specific extreme events (heatwaves, heavy precipitation) is assessed using event attribution methods
• Uncertainties in detection and attribution arise from observational limitations, climate variability, and model uncertainties
  • Incomplete spatial coverage and short record lengths can limit the ability to detect and attribute changes
  • Natural climate variability can mask or amplify the response to external forcings, making attribution more challenging
  • Climate model uncertainties, such as in the representation of clouds and the carbon cycle, affect the simulated response to forcings

Future Projections and Uncertainties

• Future climate change projections depend on the evolution of greenhouse gas emissions and other forcing agents
  • Emissions scenarios, such as the Representative Concentration Pathways (RCPs), are used to explore a range of possible futures
  • Higher emissions scenarios (RCP8.5) lead to greater warming and more severe impacts compared to lower emissions scenarios (RCP2.6)
• Global average surface temperature is projected to continue rising throughout the 21st century
  • The magnitude of warming depends on the emissions scenario, with a likely range of 1.5-4.5°C by 2100 relative to pre-industrial levels
  • Warming will be greater over land and in the Arctic, with increased frequency and intensity of heatwaves and other extreme events
• Sea level rise is projected to accelerate, with a likely range of 0.3-1.1 m by 2100 depending on the emissions scenario
  • Thermal expansion and melting of glaciers and ice sheets will continue to contribute to sea level rise
  • Rapid ice sheet loss, particularly in Antarctica, could lead to sea level rise exceeding the likely range in high emissions scenarios
• Changes in precipitation patterns are projected to vary regionally, with increased contrast between wet and dry regions
  • Many mid-latitude and subtropical dry regions will likely experience reduced precipitation, while high latitudes and some tropical regions will likely see increases
  • Heavy precipitation events are projected to become more frequent and intense in most regions
• Extreme events, such as heatwaves, droughts, and tropical cyclones, are expected to become more frequent and severe
  • The intensity and frequency of these events will depend on the magnitude of warming and regional factors
  • Compound events, where multiple extremes occur simultaneously or in succession, are also projected to increase
• Tipping points and irreversible changes in the climate system cannot be ruled out, particularly under high emissions scenarios
  • Examples include the collapse of the West Antarctic Ice Sheet, the shutdown of the Atlantic Meridional Overturning Circulation, and the dieback of the Amazon rainforest
  • These low-likelihood, high-impact events could have far-reaching consequences for the global climate and human societies
• Uncertainties in future projections arise from multiple sources
  • Future emissions trajectories and the effectiveness of mitigation policies are a major source of uncertainty
  • Climate model uncertainties, such as in the representation of clouds, the carbon cycle, and ice sheet dynamics, affect the projected response to forcing
  • Internal climate variability can modulate the long-term trend, particularly at regional scales and over shorter time horizons
  • Incomplete understanding of certain feedback processes, such as permafrost thaw and wildfire emissions, adds to the uncertainty in future projections