Maarg Manthan · Topic 16.1

Impacts of Climate Change: Acid Rain, El Niño, La Niña, Ozone Depletion, Ocean Acidification

Environment & Ecology › Climate Change Impacts and Mitigation · Topic 16.1

Climate change affects the atmosphere, oceans, ice, ecosystems and societies. Global mean sea level has risen about 20 centimetres since 1900 and is rising faster now, by roughly 3 to 4 millimetres a year, because of ocean warming and melting glaciers and ice sheets. Heat waves, heavy rain, droughts and strong cyclones are becoming more frequent or intense. The El Nino-Southern Oscillation (ENSO) is a natural cycle of the tropical Pacific: during El Nino, the surface of the central and eastern Pacific warms and the trade winds weaken, often causing drought in India, Indonesia and Australia and floods in Peru; during La Nina, the cold phase, the trade winds strengthen, and India's monsoon is usually good. Climate change may alter ENSO's strength and effects. Acid rain, ozone depletion and ocean acidification are separate but linked environmental problems: acid rain comes from sulphur dioxide and nitrogen oxides, ozone depletion from CFCs and similar chemicals, and ocean acidification from the ocean absorbing CO2, which harms corals and shell-forming life. Other impacts are glacier loss, coral bleaching, species shifts, lower crop yields in many regions and risks to water, health and livelihoods.

5 min read · UPSC Prelims · Environment & Ecology · Climate Change Impacts and Mitigation

At a Glance

Sea level rise About 20 cm since 1900
El Nino Warm Pacific, weak monsoon risk
La Nina Cool Pacific, good monsoon
Acid rain SO2 and NOx
Ocean acidification CO2 absorption
Exam link UPSC Prelims, Environment & Ecology: Climate Change Impacts and Mitigation

Where Does It Fit?

This is the first topic of the macro on climate change impacts and mitigation. It collects the main physical impacts and links to earlier topics on acid rain, ozone and acidification, before the topics on health and mitigation. Previous topic: Global Warming Potential (GWP). Next topic: Global Warming and Human Health: Mosquito-Borne Diseases, Ozone Depletion Effects.

What Are El Nino and La Nina and How Do They Affect India?

Aspect	El Nino versus La Nina
Meaning	Phases of ENSO, a natural cycle in the tropical Pacific; neutral conditions lie between them
El Nino	Warm phase: sea surface temperature in central and eastern equatorial Pacific rises about 0.5 C or more; trade winds and Walker circulation weaken
La Nina	Cold phase: eastern Pacific cools; trade winds strengthen
Frequency	Irregular, every 2 to 7 years; El Nino usually lasts 9 to 12 months
Effect on India	El Nino years often bring weaker monsoon and drought, though not always; La Nina is linked with stronger rain
Global effects	El Nino: drought in Australia, Indonesia, southern Africa, floods in Peru, warmer global temperature; La Nina often the reverse
Monitoring	Oceanic Nino Index of 0.5 C or more for El Nino and minus 0.5 C or less for La Nina

What Other Impacts Are Seen?

Sea level rise: threatens coastal cities and deltas such as Sundarbans and Mumbai
Heat and extremes: heat waves, floods and cyclones become more intense or frequent
Glaciers: Himalayan glaciers are retreating, affecting river flow
Agriculture: heat stress and erratic rain cut yields of wheat and rice in many places
Ecosystems: coral bleaching, species moving to cooler areas
Water: changes in monsoon and snow melt alter water availability
Acid rain: damages lakes, forests and monuments
Ozone depletion and acidification: harm health and marine life, though they have different causes

What Else Should You Remember?

ENSO: El Nino-Southern Oscillation, coupling of ocean and atmosphere in the tropical Pacific
Walker circulation: east-west atmospheric circulation over the tropical Pacific that weakens in El Nino
Southern Oscillation: seesaw of pressure between Tahiti and Darwin
Peru: El Nino was named by fishers off Peru who noticed warm water around Christmas
Monsoon link: El Nino years such as 2009, 2014, 2015 and 2023 had deficient or erratic rain, but 1997 saw a normal monsoon because of a positive Indian Ocean Dipole
Indian Ocean Dipole: temperature difference between western and eastern Indian Ocean that can offset El Nino effects
El Nino Modoki: warm anomaly in the central Pacific
Climate change and ENSO: scientists expect more extreme El Nino and La Nina events but with uncertainty
Recent events: a strong El Nino in 2023-24 brought record global temperatures
Sea level components: thermal expansion of water, glacier melt and ice sheet loss
Greenland and Antarctica: ice sheets are losing mass and are the largest long-term risk
Arctic sea ice: summer extent has fallen sharply since 1979
Tipping points: thresholds beyond which change becomes abrupt or irreversible, such as ice sheet collapse
Cyclones in Arabian Sea: have increased in intensity in recent decades
Extreme rainfall: warm air holds more water, causing heavier downpours and floods
Drought: greater evaporation raises drought risk
Wildfires: hotter, drier conditions increase fire risk
Migration and conflict: climate stress can add to displacement
Acid rain, ozone, acidification: treated in detail in earlier topics on pollution
Economic losses: disasters cause large damage to infrastructure and livelihoods
Link to previous: see Global Warming Potential (GWP)
Link to next: see Global Warming and Human Health: Mosquito-Borne Diseases, Ozone Depletion Effects
Link to acid rain: see Smog, Ozone Depletion and Acid Rain
Link to acidification: see Dead Zones and Ocean Acidification
Caution: sea level and event statistics are updated every year, so quote the source
Exam hint: describe ENSO phases with sea surface temperature, wind change and monsoon effect

Why Does It Matter for the Exam?

Warm Pacific: El Nino
Cool Pacific: La Nina
Index: Oceanic Nino Index
Current affairs link: El Nino 2023-24
Common errors: saying El Nino always causes drought in India

As an analytical point, ENSO is a natural cycle, but global warming raises the baseline temperature so that El Nino years set new records. Impacts like sea level rise and glacier loss are slow and long-lasting, so adaptation and mitigation both matter.

Why Is This Topic Important?

Cycle ENSO
Trend Warming
Effect Extremes

What Came Next?

The next topic covers global warming and human health. Next topic: Global Warming and Human Health: Mosquito-Borne Diseases, Ozone Depletion Effects.

Key People and Terms

El Nino Warm Pacific phase
La Nina Cool Pacific phase
ENSO Pacific climate cycle
ONI Oceanic Nino Index
IOD Indian Ocean Dipole
Tipping point Threshold of abrupt change

Exam Corner

Points to Remember

El Nino: warm Pacific, weak trade winds.
La Nina: cool Pacific.
Cycle every 2 to 7 years.
Index threshold: 0.5 C.
Sea level up about 20 cm since 1900.

Do Not Confuse With

El Nino and La Nina Warm versus cool Pacific
ENSO and IOD Pacific versus Indian Ocean cycle
Ozone depletion and global warming Different causes and effects

Memory Hook

El Nino warms and weakens the winds, La Nina cools and strengthens; both shape the monsoon.

Mains Angle

Questions ask for ENSO and impacts. These points help.

ENSO Phases
Impacts Sea, heat, rain
Links Pollution problems

A Question You May Face

An original practice question, not a past paper question.

“Explain El Nino and La Nina and discuss their impacts on the Indian monsoon, along with other impacts of climate change.”

How to Answer

ENSO Phases.
India Monsoon.
Other impacts Sea level.
Links Acid, ozone.

GS Relevance

Prelims: ENSO phases, impacts. GS Paper 1 and 3: climate impacts.

Frequently Asked Questions

What is El Nino?

El Nino is the warm phase of the El Nino-Southern Oscillation, in which central and eastern equatorial Pacific sea surface temperature rises about 0.5 degrees Celsius or more above normal and the trade winds weaken. It recurs every two to seven years and often brings drought to India, Indonesia and Australia, floods to Peru, and higher global temperatures.

What is La Nina?

La Nina is the cold phase of the El Nino-Southern Oscillation, with cooler than normal sea surface temperatures in the central and eastern equatorial Pacific and stronger trade winds. It is often linked with a good monsoon in India, more rain in Australia and Indonesia, and cooler global temperatures than El Nino years.

How does El Nino affect the Indian monsoon?

Many El Nino years bring weaker or deficient monsoon rainfall and drought because the shift in atmospheric circulation weakens the monsoon flow, but the link is not perfect. The Indian Ocean Dipole can offset the effect, as in 1997 when the monsoon was normal. Forecasters watch both ENSO and the Dipole.

How is climate change raising sea levels?

Sea level is rising because ocean water expands as it warms and because melting glaciers and the Greenland and Antarctic ice sheets add water. Global mean sea level has risen about 20 centimetres since 1900 and the rate has increased to roughly 3 to 4 millimetres a year, threatening coasts, deltas and islands.

How are acid rain, ozone depletion and ocean acidification linked to climate change?

They are separate problems with different causes, but they overlap. Acid rain comes from sulphur and nitrogen oxides, ozone depletion from CFCs and related chemicals, and ocean acidification from absorption of carbon dioxide by oceans. The last is directly linked to rising CO2, while ozone loss and climate change interact through temperature and circulation changes.

PYQ Practice — Statement Analysis

1 El Nino involves warming of the eastern Pacific.

True

With weakened trade winds.

2 La Nina usually brings drought to India.

False

It is linked with good rain.

3 ENSO events recur every 2 to 7 years.