Comprehensive Study Notes on the Possible Effects of Climate Change

CONTEMPORARY CLIMATE CHANGE AND GLOBAL TEMPERATURE TRENDS

According to the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6, 2021), the global average surface temperature (GST) has risen approximately $1.1\,^\circ\text{C}$ above the 1850–1900 pre-industrial baseline, specifically during the decade of 2011–2020. This warming trend is characterized by the following specifics:

Non-Uniform Warming: Land areas have warmed by approximately $1.59\,^\circ\text{C}$ , significantly more than ocean surfaces, which have warmed by approximately $0.88\,^\circ\text{C}$ .
Historical Context: Each of the last four decades has been successively warmer than any preceding decade since 1850. The most recent decade was warmer than any multi-century period over the past 125,000 years.
Polar Amplification: The Arctic and Antarctic regions are warming at a rate two to three times faster than the global average. This is driven by positive feedback loops, primarily the ice-albedo feedback.
Upper Ocean Warming: The upper ocean (0–700 m) has been warming since the 1970s, with human influence identified as the primary driver.
Heatwaves: Since 1950, the frequency, intensity, and duration of heatwaves have increased globally. This trend is projected to continue even if warming is stabilized at $1.5\,^\circ\text{C}$ .

PRECIPITATION PATTERNS AND HYDROLOGICAL CHANGES

Global precipitation trends are spatially complex and lack a uniform global trajectory, unlike temperature. Key observed and projected changes include:

Observed Increases: Globally averaged land precipitation has likely increased since 1950, with accelerated rates since the 1980s. Increases have been recorded in mid-to-high latitude Eurasia, most of North America, south-eastern South America, and north-western Australia.
Observed Decreases: Precipitation has decreased over most of Africa, eastern Australia, the Mediterranean, the Middle East, and parts of East Asia.
Extreme Events: The frequency and intensity of heavy precipitation events have increased since the 1950s. This leads directly to increased magnitude and frequency of pluvial (surface water) flooding.
Projected Trends: High latitudes, the equatorial Pacific, and monsoon regions (including South and South-East Asia) are expected to become wetter. Conversely, the subtropics and dry regions like Mexico and the Mediterranean are projected to become drier, with intensified droughts.
Intensity Shifts: There is a global prediction of fewer weak storms and a shift toward more intense storms.

IMPACTS ON AQUATIC ECOSYSTEMS: FRESHWATER AND MARINE SYSTEMS

Freshwater Environments

Impacts on freshwater systems include physical transformations and biological stress:

Thermal Stratification: Lake surface waters warmed by $0.21\,^\circ\text{C}$ to $0.45\,^\circ\text{C}$ per decade (1970–2010). This leads to less frequent mixing and the loss of ice, which prevents inverse thermal stratification.
Oxygen Depletion: Dissolved oxygen in deep waters declined by $16.8\%$ between 1980 and 2017, increasing hypoxia and killing bottom-dwelling species.
Case Study: Lake Tanganyika: Sustained warming over the last 150 years reduced lake mixing, depressed algal production, and shrunk the oxygenated benthic habitat by $38\%$ , reducing fish yields.
Water Management Challenges: Floods redistribute pollutants and wastewater, while droughts reduce the ability of rivers to dilute contaminants. Warming also promotes harmful algal blooms and pathogens.
Ice Loss: Global river ice extent fell by $25\%$ (1984–2018). Ice duration is shrinking by over two weeks per year.
Biological Responses: Species are shifting poleward and upward to cooler waters. Cold-water specialists like bull trout in Idaho have lost $20\%$ of spawning habitat. Approximately $81\%$ of freshwater species populations are in decline due to deoxygenation and desiccation.
Wetland Loss: Freshwater wetland areas declined by approximately $35\%$ between 1970 and 2015.

Marine Environments

Physical drivers include rising sea surface temperatures (SSTs), which have increased by $0.88\,^\circ\text{C}$ since pre-industrial times.

Marine Heatwaves (MHWs): These are extreme seawater temperature periods relative to the long-term seasonal mean. They have doubled in frequency and increased by $54\%$ in intensity over the past century.
The "Blob" (2013–2015): An extensive MHW in the Northeast Pacific that caused toxic algal blooms, kelp forest collapse in California, and mass mortality of seabirds.
Coral Reef Crisis: Coral reefs are highly vulnerable to thermal stress. Under low-emission scenarios (SSP1-2.6), reefs will show structural degradation by 2050. Under high emissions (SSP5-8.5), near-total global loss of coral reefs is projected by 2100.
Singapore Case: In the 2024 MHW, $44\%$ of Singapore’s corals bleached with $5\%$ mortality. Deeper reef live coral cover (6–8 m) has dropped from $30\%$ in the 1980s to $5\%$ today.
Arctic Marine Life: The Arctic is warming at $3\,^\circ\text{C}$ compared to the global $1.1\,^\circ\text{C}$ . A $2/3$ decline in polar bear populations is projected by mid-century as ringed seals (prey) lose sea ice habitat.
Ocean Acidification: CO2 dissolves in the ocean forming a weak acid. Atmospheric CO2 has risen from 280 ppm to 425 ppm (2024). Surface pH has declined by $0.016$ – $0.020$ per decade in subtropics since the 1980s. This thins the shells of pteropods and coccoliths and reduces coral growth rates.

SEA LEVEL RISE (SLR) AND COASTAL IMPACTS

Global mean sea level has risen by approximately $20\,\text{cm}$ since 1901 and is accelerating. Projections suggest a rise of $26$ – $77\,\text{cm}$ by 2100 if warming is capped at $1.5\,^\circ\text{C}$ .

Causes: Thermal expansion (responsible for $50\%$ of SLR from 1971–2018), glacier melt ( $22\%$ ), ice sheets ( $20\%$ ), and land-water storage ( $8\%$ ).
Arable Land Loss: A $1\,\text{m}$ rise would result in the loss of $12$ – $15\%$ of Egypt’s arable land and inundate $11.5\%$ of Bangladesh's land area.
Urban Flooding: By 2050, over 570 coastal cities will face at least a $50\,\text{cm}$ rise, putting 800 million people at risk. Potential economic costs could reach $US\$1\,\text{trillion}$ .
Small Island Developing States (SIDS): Countries like the Maldives, Marshall Islands, and Tuvalu face potential complete submergence. Papua New Guinea reported that $25\%$ of its shoreline was inundated by 2005.

TERRESTRIAL ECOSYSTEM TRANSFORMATION

Climate change significantly alters biological timing and species distributions on land:

Phenological Asynchrony: Spring occurs earlier by $2.3$ – $5.2$ days per decade. This leads to mismatches, such as animals waking from hibernation before food sources (insects, leaves) emerge.
Species Extinction: The Golden Toad of Costa Rica was extinct by 1990 due to droughts. The BC melomys of the Torres Strait was declared extinct in 2016 due to island inundation and loss of $97\%$ of its food resources (herbaceous vegetation).
Pest Outbreaks: Warm weather in the 1990s halved the life cycle of spruce beetles from two years to one in Alaska. Projections suggest $41\%$ of major insect pests will increase damage as the climate warms.
Wildfires: Warming dries organic matter. In the 2020 Australian fires, over one billion native animals were killed.
CO2 Fertilization Limits: Increased CO2 can initially accelerate photosynthesis ("greening"). However, this is limited by soil nutrient depletion (e.g., in the Arctic) and the replacement of trees by less efficient sunlight-loving vines in tropical forests.

HUMAN IMPACTS AND SOCIO-ECONOMIC CONSEQUENCES

Food and Water Security

Crop Yields: In California, a $19\%$ increase in CO2 concentration (1985–2019) correlated with a $13\%$ decline in wheat yields. High CO2 levels also reduce protein, minerals, and vitamins in rice.
Water Stress: The American Southwest "megadrought" (since 2000) is the driest period in 1,200 years. Water disputes have led to political tension, such as the 1944 treaty dispute where Mexico failed to deliver its required $1.75\,\text{million acre-feet}$ of water to the US due to drought.
Economic Inequality: Climate change has increased the economic gap between developed and developing nations by $25\%$ since 1960.

Physical and Mental Health

Vector-Borne Diseases: Range expansions of Aedes mosquitoes increase the spread of Zika, Dengue (reproduction potential increased $17$ – $80\%$ in Central/South America since 1950), and Chikungunya.
Mortality and Displacement: Over 20 million people are internally displaced annually by weather events. The 2019–2020 Australian wildfires resulted in 33 direct deaths, 429 smoke-related deaths, and $US\$1.95\,\text{billion}$ in health costs.
Climate Anxiety: A 2021 study of 10,000 young people across 10 countries found that $45\%$ felt climate emotions (powerlessness, anger) impacted their daily lives.

Positive Impacts (Short-term/Localized)

Arctic Shipping: Routes like the Northern Sea Route can reduce travel distance by up to $40\%$ . The carrier MV Nordic Orion saved 1,000 nautical miles and 4 days of travel (worth $US\$200,000$ ) in 2013.
Extended Growing Seasons: Higher latitudes (Canada, Russia, Scandinavia) may see increased agricultural productivity for crops like corn and soybeans.

CASE STUDY: THE MALDIVES AND LOW-LYING ISLAND STATES

Geography: The state is an island chain $1,000\,\text{km}$ long with an average height of only $1.5\,\text{m}$ above sea level. It comprises 1,200 islands in 26 coral atolls.
Population Density: The capital, Mal, is the most densely populated city in the world, with 110,000 people on $2\,\text{km}^2$ .
Economy: Tourism accounts for $90\%$ of government income and $60\%$ of foreign exchange. However, the nation is in debt and spends $25\%$ of its GDP on diesel for energy.
Climate Threats: SLR could make islands uninhabitable by 2100. Freshwater shortages affect over $1/4$ of populated islands, requiring water shipments during long dry seasons.
Adaptation Strategies: Former President Mohamed Nasheed proposed a "sovereign wealth fund" to buy a new homeland in India or Sri Lanka. The artificial island Hulhumal was created to reach $2\,\text{m}$ high and house 160,000 people.

CASE STUDY: VULNERABILITY ON THE DECCAN PLATEAU, INDIA

Context: A semi-arid region in south-central India where $60\%$ of people rely on agriculture.
Vulnerability Factors: Agricultural liberalization since the 1990s reduced subsidies. Farmers shifted from drought-resistant sorghum and millet to water-intensive cash crops (sugarcane, cotton), leading to deep debt for borehole drilling.
Outcome: Despair from debt and environmental stress caused increasing suicides. Official figures recorded nearly 10,000 suicides (2011–2015) in Andhra Pradesh and Telangana, while some studies attribute up to 60,000 suicides in India to climate change impacts.

VARIATIONS IN CLIMATE IMPACTS AND VULNERABILITY

Impacts are not equal across the globe due to several factors:

Geographic Vulnerability: Coastal regions, hurricane-prone areas (North Atlantic, NW Pacific), and high mountain regions face higher physical risks. In the Alps, low-level ski resorts are failing due to snow loss.
Socio-economic Vulnerability: Developing countries have lower adaptive capacity. For instance, the 2010 wheat crisis caused massive protests in Egypt (25% poverty rate) while the UAE and Israel remained stable due to higher income per capita.
Governance Quality: Inadequate planning or corruption hinders adaptation. The "Levee Effect" describes how structural flood defenses (like on the Yellow River or Mississippi) can paradoxically increase damage by encouraging human settlement in flood-prone zones.
Gender and Caste: Social norms often amplify risk for women. In the 2004 Indian Ocean Tsunami, more women died as they were on the shore and societal norms did not prioritize survival training (swimming) for girls.

UNCERTAINTY IN CLIMATE PROJECTIONS AND MODELS

While human-caused warming is unequivocal, future impacts remain uncertain due to:

Feedback Processes:
- Cloud Feedback: High clouds trap heat (positive feedback), while high-latitude clouds shifting to water droplets reflect more energy (negative feedback). Their net effect is likely positive but remains the dominant driver of model differences.
- Ocean Feedback: There is uncertainty regarding whether oceans will maintain their rate of CO2 absorption as they warm.
- AMOC: The Atlantic Meridional Overturning Circulation slowed by $15$ – $20\%$ in the 20th century. A total collapse is a possible tipping point.
Natural Variability: Volcanic eruptions (cooling), solar cycles, and ENSO (El Nio/La Nia) can mask or amplify human-driven signals.
Data Limitations: Sparse coverage in deep oceans, deserts, and polar regions. Urban Heat Island bias in land stations. Argo floats (3,600 since 2000) have improved ocean data, but deep ocean data (below 2000m) is still limited.
Future Emissions: Human behavior, policy, and technology are unpredictable. Scientists use Representative Concentration Pathways (RCPs) and Shared Socioeconomic Pathways (SSPs) to model possible scenarios.
GCM Limitations: General Circulation Models (GCMs) work on resolutions of $250$ – $600\,\text{km}$ . They simplify small-scale processes like thunderclouds and turbulence through parameterization, which introduces approximation errors.

QUESTIONS & DISCUSSION

Question: How certain is the mechanism for climate change? Response: By 2001, the IPCC established warming, and by 2009, the evidence was deemed "unequivocal." Human activities leading to rising greenhouse gas concentrations are the established driver of the enhanced greenhouse effect.

Question: Does climate change affect everyone equally? Response: No. Developing countries face disproportionate impacts (climate injustice) despite lower emissions. Vulnerability is a function of exposure, sensitivity, and adaptive capacity, which are influenced by socio-economic status, gender, and geography.