Key Points at a Glance

Climate change: Global average temperature has risen 1.1°C above pre-industrial levels (2024 data, WMO). CO₂ concentration reached 425 ppm in 2024 vs 280 ppm pre-industrial. 2023 was the warmest year in recorded history (1.45°C above average). The Paris Agreement (2015) set targets of limiting warming to 1.5°C (aspirational) and 2°C (maximum) above pre-industrial levels.

Ozone layer depletion — PYQ 2023 (10 marks): The ozone layer (stratosphere, 15–35 km altitude) absorbs 97–99% of the Sun's harmful UV-B and UV-C radiation. Ozone-depleting substances (ODS): CFCs (chlorofluorocarbons), HCFCs, halons, carbon tetrachloride — mainly from refrigerants, aerosols, fire extinguishers. The Antarctic Ozone Hole was discovered in 1985 (British Antarctic Survey). The Montreal Protocol (1987) — called the most successful international environmental agreement — has reduced ODS by 99%.

Biodiversity loss: Earth is experiencing its 6th mass extinction — current extinction rate is 100–1,000× the natural background rate. The IUCN Red List (2024) has assessed 157,190 species; 44,016 are threatened (28%). Three major causes: habitat destruction (most significant — 80% of threatened species impacted), overexploitation (hunting, fishing), invasive species.

Desertification is the degradation of drylands due to human overuse and climate change. Affects 40% of Earth's land (dryland areas). Around 3.2 billion people are affected. Annual cost: ~$490 billion in lost productivity. UNCCD (UN Convention to Combat Desertification, 1994) is the global framework. Rajasthan relevance: Western Rajasthan faces severe desertification — dunes advancing 0.5–1.5 km/year before check dams and agroforestry.

Plastic pollution: Over 400 million tonnes of plastic produced annually (2023). Only 9% of all plastic ever produced has been recycled; 22% mismanaged. Microplastics (< 5 mm) found in deepest ocean (Mariana Trench), Arctic snow, human blood, and placenta. The Global Plastics Treaty (2024) — UN member states negotiating a binding treaty to end plastic pollution. India's single-use plastic ban (2022).

Deforestation: The world loses approximately 4.7 million hectares of forest annually (net, after reforestation). The Amazon Rainforest (Brazil) has lost 18–20% of its original cover since 1970 — concern about reaching a tipping point (~25%) beyond which the forest collapses into savanna (dieback). REDD+ (Reducing Emissions from Deforestation and Forest Degradation) is the UN mechanism for forest conservation.

Ocean acidification: Oceans absorb ~30% of all CO₂ emissions. This forms carbonic acid (H₂CO₃) → ocean pH has dropped from 8.2 to 8.1 (0.1 unit = 26% more acidic since pre-industrial times, due to logarithmic scale). Dissolves coral skeletons (CaCO₃) → coral bleaching and reef death. Threatens 25% of all marine species that depend on coral reefs.

Sea level rise: Global mean sea level has risen ~20 cm since 1900; rising at 3.7 mm/year in the 2010s. Two causes: thermal expansion (60–70%) of ocean water as it warms; ice melt from glaciers, Greenland, and Antarctica (30–40%). Pacific Island nations (Tuvalu, Kiribati, Maldives) face existential threat. India: Mumbai, Kolkata, Chennai, Kochi vulnerable.

Glacial retreat: Global glaciers losing ~31 billion tonnes of ice annually (2000–2019 data). Himalayan glaciers (India's "Water Tower") are retreating at accelerating rates; key example: Gangotri Glacier retreating ~22 m/year. UN 2025: International Year of Glaciers' Preservation.

Transboundary air pollution: PM2.5 (fine particulate matter <2.5 µm) is the most lethal air pollutant — causes cardiovascular and respiratory disease; ~7 million deaths/year globally (WHO). India's National Clean Air Programme (NCAP) targets 40% reduction in PM2.5/PM10 by 2026 (vs 2017 baseline). Delhi's AQI regularly breaches 400+ in winter (hazardous threshold: 300+).

Key international environmental agreements: