The Science of Climate Change: Causes, Evidence and Impacts

The greenhouse effect

The natural greenhouse effect: the Sun's shortwave radiation passes through the atmosphere and warms the Earth. The Earth emits longwave (infrared) radiation. Greenhouse gases (GHGs) absorb and re-emit part of it, warming the lower atmosphere. Without this effect, Earth's average temperature would be about −18°C instead of about +15°C.

The enhanced greenhouse effect: human activity since the Industrial Revolution has added GHGs. Atmospheric CO₂ has risen from about 280 ppm (pre-industrial) to over 420 ppm (2024–25), the highest in at least 800,000 years (ice core records).

Main greenhouse gases:

• Carbon dioxide (CO₂): fossil fuel burning, deforestation and cement. It is the largest contributor and stays for centuries.
• Methane (CH₄): paddy fields, livestock (enteric fermentation), landfills, and gas and coal leaks. It has about 28–30 times the warming potential of CO₂ over 100 years and about 80 times over 20 years, but lasts only about a decade.
• Nitrous oxide (N₂O): fertilisers and industry. About 265–273 times CO₂.
• Fluorinated gases (HFCs, PFCs, SF₆): refrigerants and industry. Thousands of times as potent as CO₂.
• Water vapour: the most abundant GHG, but it is a feedback, not a driver.

Global warming potential (GWP) compares gases with CO₂ over a set period. Emissions are expressed in CO₂-equivalent.

Aerosols: sulphate aerosols cool the climate by reflecting sunlight, which "masks" some warming. Black carbon warms it.

Evidence: IPCC findings

The IPCC (Intergovernmental Panel on Climate Change): set up in 1988 by the WMO and UNEP. It assesses science, but does not do research itself. It shared the 2007 Nobel Peace Prize.

The Sixth Assessment Report (AR6, 2021–2023):

• it is "unequivocal" that human influence has warmed the atmosphere, ocean and land;
• global surface temperature was about 1.1°C above 1850–1900 in 2011–2020;
• the remaining carbon budget for 1.5°C (50% chance) was about 500 Gt CO₂ from 2020, which would be exhausted in about a decade at current emissions;
• limiting warming to 1.5°C requires cutting global GHG emissions about 43% by 2030 and 60% by 2035 (from 2019 levels), and reaching net zero CO₂ by the early 2050s.

The Seventh Assessment cycle (AR7) is under way.

Recent records:

• 2024 was the warmest year on record, the first calendar year above 1.5°C over pre-industrial levels (WMO, about 1.55°C);
• a single year above 1.5°C does not mean the Paris goal is breached, because that goal refers to long-term (20-year) averages;
• 2023–2025 brought record ocean heat and sea surface temperatures.

Observed and projected impacts

Global impacts:

• Sea level rise: about 20 cm since 1900, now rising about 4 mm a year and accelerating. Its causes are thermal expansion and melting of glaciers and ice sheets (Greenland, Antarctica).
• Cryosphere: shrinking Arctic sea ice and glacier retreat.
• Extremes: more frequent and intense heatwaves, heavy rainfall, droughts and intense cyclones.
• Ocean acidification: the ocean's pH has fallen by about 0.1, which harms corals and shellfish.
• Ecosystems: species shifting poleward and uphill, coral bleaching, and the risk of tipping points such as Amazon dieback, permafrost thaw and ice sheet collapse.

Impacts on India:

• Heatwaves: longer and more intense. 2024 saw severe heat in north India; 2022 saw a March–April heatwave that cut the wheat harvest.
• Monsoon: overall rainfall shows no clear trend, but there are more extreme rainfall events and longer dry spells. Examples include the floods in Kerala (2018), Himachal (2023) and the Wayanad landslide (2024).
• Himalayan glaciers: retreating, which threatens water security. Glacial lakes are expanding, raising the risk of glacial lake outburst floods (GLOFs), as in Sikkim's South Lhonak (October 2023).
• Coasts: sea level rise threatens Mumbai, Kolkata, Chennai and the Sundarbans. The Arabian Sea is seeing more severe cyclones.
• Agriculture: heat stress lowers yields of wheat and rice; rainfed farming is at risk.
• Health: heatstroke, vector-borne diseases (dengue and malaria spreading to new areas), and wet-bulb temperature limits on outdoor work.
• Economy: the RBI (Report on Currency and Finance 2023) estimated that up to 4.5% of GDP could be at risk by 2030 from heat-related losses in labour hours.

The Germanwatch Climate Risk Index 2025 ranked India among the ten most affected countries over 1993–2022.

Key climate phenomena

El Niño and La Niña:

• El Niño is the warming of the central and eastern Pacific. It tends to weaken the Indian monsoon and raise global temperatures, as in 2023–24.
• La Niña is the cooling phase. It usually brings a stronger monsoon.
• Together they form ENSO (El Niño–Southern Oscillation).

Indian Ocean Dipole (IOD): a positive IOD (a warmer western Indian Ocean) can strengthen the monsoon even during El Niño, as in 2019.

Feedback loops:

• the ice–albedo feedback: melting ice exposes darker surfaces that absorb more heat;
• permafrost thaw, which releases methane;
• forest dieback.

These amplify warming.

Urban heat islands: cities are warmer than surrounding rural areas because of concrete, lack of vegetation and waste heat.

Analysis

Equity: climate change is a problem of cumulative emissions. India's historical contribution is about 4% of cumulative CO₂ since 1850. Its per capita emissions are about 2 tonnes of CO₂, well below the global average of about 4.7 tonnes. Yet India is among the most vulnerable, because of its large poor population, agricultural dependence, long coastline and the Himalayan system.

The challenge for India: pursue development while decarbonising, and adapt to impacts that are already unavoidable.

Common prelims traps

• Water vapour is the most abundant GHG, but a feedback.
• Methane has a higher GWP than CO₂ but a shorter lifetime.
• Aerosols (sulphates) have a cooling effect.
• El Niño generally weakens the Indian monsoon; a positive IOD can offset it.

Quick self-test

1. *What is a carbon budget?* The total CO₂ that can still be emitted while limiting warming to a given level.
2. *Why does sea level rise even without ice melt?* Thermal expansion of warming water.
3. *What caused the 2023 South Lhonak disaster?* A glacial lake outburst flood.

Practice: Mains question

*"Climate change is no longer a future threat for India but a present reality." Discuss with examples of recent extreme events. (250 words)*

Key terms

• Radiative forcing: the change in the Earth's energy balance caused by a factor such as CO₂, measured in watts per square metre. A positive forcing warms the planet.
• Climate sensitivity: the eventual warming from a doubling of CO₂. The IPCC's best estimate is about 3 °C, with a likely range of 2.5–4 °C.
• Feedback: a process that amplifies or dampens warming. The ice-albedo feedback and the water vapour feedback amplify it.
• Tipping point: a threshold beyond which change becomes self-sustaining and hard to reverse. Examples are ice sheet collapse, Amazon dieback and permafrost thaw.
• Carbon budget: the cumulative CO₂ that can still be emitted while keeping warming below a given limit.

Analysis: why this matters for India

India faces some of the steepest physical risks of any large economy:

• a long, densely populated coastline exposed to sea-level rise and stronger cyclones;
• dependence on the monsoon for agriculture and water;
• Himalayan glaciers that feed major rivers;
• rising heat stress on outdoor workers.

Climate science therefore informs India's agricultural planning, its urban heat plans and its disaster preparedness, as well as its negotiating position. Examiners expect answers that link physical science to development outcomes such as food security, health, livelihoods and migration, not just a list of greenhouse gases.

Practice: additional question

*Explain the role of feedback mechanisms in accelerating global warming, with examples relevant to the Indian subcontinent. (150 words)*