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Climate Change — Causes, Evidence, and Impacts
2.1 The Greenhouse Effect and Enhanced Warming
Natural Greenhouse Effect:
Without the greenhouse effect, Earth's average temperature would be -18°C instead of the actual +15°C — a difference of 33°C. Greenhouse gases (GHGs) trap outgoing longwave (infrared) radiation in the atmosphere, keeping the planet habitable.
Natural GHGs and their contribution:
- Water vapour (H₂O): Most abundant GHG; ~50% of natural greenhouse effect
- Carbon dioxide (CO₂): ~20% of natural greenhouse effect; long atmospheric residence (~200 years)
- Methane (CH₄): ~4% contribution; 80× more potent than CO₂ over 20-year period; 30× over 100 years
- Nitrous oxide (N₂O): ~5%; 273× more potent than CO₂ over 100 years
- Ozone (O₃): Tropospheric ozone; some warming effect
Enhanced (Anthropogenic) Greenhouse Effect:
Human activities since the Industrial Revolution (~1750) have increased GHG concentrations:
| GHG | Pre-industrial Level | 2024 Level | Source |
|---|---|---|---|
| CO₂ | 280 ppm | 425 ppm (+52%) | Fossil fuels (76%), deforestation (11%), cement (4%) |
| CH₄ | 722 ppb | 1,922 ppb (+166%) | Livestock, rice paddies, landfills, fossil fuels, wetlands |
| N₂O | 270 ppb | 335 ppb (+24%) | Agriculture (fertilisers), industrial processes |
| CFCs/HFCs | Near zero | Rising | Industrial processes; refrigerants |
2.2 Evidence of Climate Change
Temperature records: 2023 was the warmest year in 125,000 years (WMO) — 1.45°C above pre-industrial average. The 20 warmest years on record have all occurred in the past 22 years. Arctic warming 4× faster than global average (Arctic amplification).
Sea level rise: ~20 cm since 1900; rate accelerating to 3.7 mm/year in 2010s (2× the 20th-century average).
Glacial retreat: Glacier mass globally declining — 31 billion tonnes/year ice loss (2000–2019). Arctic sea ice has declined ~13% per decade since 1979.
Ocean changes: Ocean heat content at all-time high. Ocean pH dropped from 8.2 to 8.1 (26% more acidic). Coral bleaching events becoming annual.
Extreme weather: IPCC AR6 (2021) attributes increased frequency and intensity of: heat waves, heavy precipitation events, droughts in some regions, and tropical cyclone intensification (more Category 4–5) to climate change.
2.3 Impacts of Climate Change
On ecosystems:
- Shifting of ecosystems poleward and to higher altitudes (~6 km/decade for some species)
- Coral bleaching: 2016 and 2017 consecutive mass bleaching killed ~50% of Great Barrier Reef corals; 2024 fourth global mass bleaching event
- Permafrost thaw (Arctic) releasing stored CH₄ and CO₂ — positive feedback loop
- Loss of species that cannot adapt fast enough — accelerating 6th mass extinction
On human society:
- Food security: Wheat and maize yields declining ~1.8–5.6% per decade in warming regions; rice and wheat protein content declining
- Water stress: Accelerating glacial retreat threatens ~2 billion people dependent on glacier meltwater (Hindu Kush-Himalayan region, Andes)
- Displacement: Up to 200 million climate refugees by 2050 projected (World Bank)
- Health: Expanding range of malaria, dengue, cholera; heat mortality; air quality degradation
- Conflict: Climate change amplifying competition for water and agricultural land — identified as "threat multiplier" by military analysts
Tipping Points (critical thresholds):
- Amazon Dieback: At ~25% deforestation, Amazon may shift to savanna — releasing 90 billion tonnes CO₂ equivalent
- West Antarctic Ice Sheet collapse: Could raise sea levels by 3.3 m over centuries
- Greenland ice sheet melt: Could raise sea levels by 7 m over millennia
- Atlantic Meridional Overturning Circulation (AMOC): Risk of weakening — would severely cool Europe and shift global rainfall patterns
2.4 International Response to Climate Change
UN Framework Convention on Climate Change (UNFCCC, 1992):
- Adopted at Rio Earth Summit; entered force 1994
- All 197 countries are parties; establishes basic framework for climate action
- Principle of "common but differentiated responsibilities" (CBDR) — developed nations bear greater historical responsibility
Kyoto Protocol (1997):
- First legally binding GHG reduction treaty
- Annex I countries (developed) committed to reduce emissions 5% below 1990 levels by 2012
- USA never ratified; Canada withdrew; generally seen as partially successful but insufficient
Paris Agreement (2015):
- Adopted at COP21, Paris; 195+ countries; entered force November 2016
- Key commitments:
- Limit warming to 1.5°C (aspirational) and 2°C (maximum) above pre-industrial levels
- NDCs (Nationally Determined Contributions) — each country sets own targets; not legally binding
- $100 billion/year climate finance for developing countries (from 2020 — target not fully met)
- Loss and Damage fund established at COP27 (2022) for vulnerable nations
- 5-year NDC ratchet mechanism — countries must submit increasingly ambitious targets
India's commitments (Updated NDC, 2022):
- Net Zero emissions by 2070
- 50% electric power from non-fossil sources by 2030
- 45% reduction in emissions intensity of GDP by 2030 (vs 2005)
- 500 GW non-fossil power capacity by 2030
- 2.5–3 billion tonnes additional carbon sink through forest/tree cover by 2030