Global warming

Topic: Climate Change: Causes, Evidence and the Global Response · NCERT: Class 7, Ch 3 "Climates of India"; Class 9, Ch 3 "Atmosphere and Climate"; Class 11, Ch 4 "Climate"; Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"; Class 11, Ch 11 "World Climate and Climate Change"

Meaning

Global warming is the long-term rise in the Earth's average surface temperature. It happens mainly because greenhouse gases (gases such as carbon dioxide, methane and nitrous oxide that trap heat near the ground) build up in the air when people burn fossil fuels and cut forests. It matters because the warming melts ice, raises sea levels and brings more extreme weather. NCERT also warns that "once the global warming sets in, it will be difficult to reverse it."

Explanation

How it works (cause → process → result)

• Cause: more greenhouse gases (GHGs)
• Burning coal, oil and gas releases carbon dioxide.

• Cutting forests removes trees that would have taken in carbon dioxide.

• Process: more heat is trapped

• The Sun heats the Earth's surface. The surface then gives off heat back towards space.
• GHGs absorb this outgoing heat and hold it near the ground.

• More GHGs trap more heat. This is an enhanced greenhouse effect.

• Result: the planet warms

• The lower air, the land and the oceans all get warmer.

• Earth's energy imbalance (more energy coming in from the Sun than going back out to space) reached a new high in 2025 [2].

• Baseline: scientists compare today's temperatures with the 1850–1900 average. This is the pre-industrial period, before large-scale burning of fossil fuels.

The global temperature record

• NCERT's record (data up to 2000)
• The world's annual average near-surface air temperature is about 14 °C.
• 1901–1944: warmed by about 0.4 °C.
• 1944–1977: a short period of slight cooling.
• 1977–1999: warmed by another 0.4 °C.
• By 2000: about 0.6 °C warmer than the late 19th century.

• 1998 was the warmest year in NCERT's record. A strong El Niño (unusual warming of the eastern Pacific Ocean that heats the whole globe for a while) added to the heat that year.

• The current record

• IPCC AR6: the world was about 1.1 °C warmer in 2011–2020 than in 1850–1900.
• 2024 is the warmest year on record (WMO), at about 1.55 °C above pre-industrial. It was the first calendar year above 1.5 °C.
• 2025 was 1.44 °C (± 0.13 °C) above 1850–1900. It was the second or third warmest year in a 176-year record [2].
• Of WMO's eight datasets, two ranked 2025 second warmest and six ranked it third [2].
• 2015–2025 were the 11 warmest years on record [2].

• 2023, 2024 and 2025 were the top three warmest years in all eight global datasets [2].

• Ocean heat

• Ocean heat content (the heat stored in the top 2,000 m of the ocean) hit a record high in 2025. This beat the earlier record from 2024 [2].

• Each of the past nine years set a new record [2].

• The Paris trap

• One year above 1.5 °C does not mean the Paris Agreement (2015) goal has been broken.
• The 1.5 °C limit refers to a long-term average of about 20 years, not to a single year.

India's temperature record (IMD)

• 2024 was India's warmest year since 1901. It was +0.65 °C above the 1991–2020 average [3].
• 2025 was the 8th warmest year since 1901, at +0.28 °C [3].
• 2016–2025 was the warmest decade, with an average anomaly (difference from the long-term normal) of +0.32 °C [3].
• 10 of the 15 warmest years fell in 2011–2025 [3].
• Long-term trend (1901–2025): mean temperature is rising by +0.68 °C per 100 years [3].
• Days are warming faster than nights
• Maximum temperature: +0.89 °C per 100 years [3].
• Minimum temperature: +0.47 °C per 100 years [3].

• So daytime heat is rising almost twice as fast.

• The early-2025 winter

• January–February 2025 had an anomaly of +1.17 °C. This was the highest winter anomaly since 1901 [3].
• NCERT (Class 7) notes temperatures were 1–3 °C above normal, which gave a short, mild winter.
• Rabi crops (winter crops such as wheat) need a cool period to fill their grain, so they suffered.
• Small industries that depend on winter sales, such as woollens, lost income.

What warming does: effects and feedbacks

• Main effects (NCERT, Class 11 Ch 11; Class 9 Ch 3)
• Glaciers, ice caps and polar ice sheets melt.
• Sea levels rise and flood low coasts and islands.
• Floods, droughts, heatwaves and other extremes become more common.
• Biodiversity loss: many species cannot move or adapt fast enough.

• Health, farming and livelihoods suffer. Women and children are hit hardest because they often depend on farming, water and fuel-wood, and have fewer resources to recover.

• Sea-level rise

• It has two causes. Melting land ice (glaciers, the Greenland and Antarctic ice sheets) adds new water to the ocean. Thermal expansion means water takes up more space as it warms.
• Melting sea ice adds little, because it is already floating.
• Global sea level has risen about 20 cm since 1901.
• Satellites measured a rise of 11.1 cm between 1993 and 2023 [4].
• The yearly rate more than doubled, from about 2.1 mm a year in 1993 to 4.5 mm a year by 2024 [4].

• IPCC AR6 projections for 2100: about 0.3–0.6 m under low emissions and about 0.6–1.0 m under very high emissions. More than 1 m is possible if ice sheets collapse (low likelihood). NCERT's older single estimate is 48 cm by 2100, which falls inside this range.

• Arctic amplification

• The Arctic has warmed nearly four times faster than the global average since 1979 (Rantanen et al., 2022).
• It is driven by the ice-albedo feedback. Albedo is the share of sunlight a surface reflects. Snow and ice reflect most sunlight.
• Warming melts the ice, which uncovers dark ocean and land.
• Dark surfaces absorb more sunlight, which causes more warming and more melting.
• This is a positive feedback (a loop in which a change makes itself stronger).
• Arctic sea ice is smallest in September. September ice is shrinking by about 12.2% per decade compared with the 1981–2010 average [5]. NCERT gives about 12–13% per decade.

• September 2025 tied for the 10th-lowest minimum on record [6].

• Glacier retreat

• A glacier retreats when it loses more ice to melting than it gains from snowfall. Its front (the snout) moves back up the valley.
• Rivers fed by glaciers first carry more water, so floods become more likely.
• After "peak water" (the point where meltwater flow is highest before it starts to fall), dry-season flow drops.

• A GLOF (Glacial Lake Outburst Flood) happens when a moraine (a natural dam of loose rock and ice) holding back a glacial lake breaks.

• Tipping points

• A climate tipping point is a threshold. Once a system crosses it, the system changes suddenly and often cannot recover, even if temperatures later fall.
• Tipping elements include ice sheets, the Amazon, permafrost (ground frozen for at least two years), the AMOC and coral reefs.
• Coral bleaching happens when heat-stressed corals push out their colourful algae and turn white.
• The Global Tipping Points Report (2025) judged that warm-water coral reefs have likely already crossed their tipping point. They would be the first major system to do so (verify current).

Where it occurs

• The Arctic: warming fastest of all regions.
• The Northern Sea Route runs along Russia's Arctic coast, from the Barents Sea to the Bering Strait. It now stays ice-free for longer each summer, which shortens the Europe–East Asia journey compared with the route through the Suez Canal.

• Greenland's ice sheet (North Atlantic) is melting faster. Full melting could raise sea level by about 7 m over many centuries.

• Hindu Kush Himalaya (HKH)

• According to ICIMOD (2023) (International Centre for Integrated Mountain Development, Kathmandu), HKH glaciers melted 65% faster in 2011–2020 than in 2000–2010.
• They may lose 30–50% of their volume by 2100, even if warming is held at 1.5–2 °C.

• Three rivers are at risk. The Indus rises near Lake Mansarovar in Tibet. The Ganga rises from the Gangotri glacier at Gaumukh in Uttarakhand. The Brahmaputra rises from the Chemayungdung glacier near Mansarovar, where it is called the Yarlung Tsangpo.

• GLOF sites in India

• Chorabari Tal, Kedarnath (Rudraprayag district, Uttarakhand), June 2013: extreme rain burst the lake and about 6,000 people died (NCERT).

• South Lhonak Lake (North Sikkim), October 2023: the flood rushed down the Teesta and destroyed the Teesta III dam at Chungthang.

• Places threatened by sea-level rise

• Mega-deltas: the Ganga-Brahmaputra delta (Sundarbans, India and Bangladesh, Bay of Bengal); the Mekong (Vietnam, South China Sea); the Nile (Egypt, Mediterranean Sea).
• Small island states: the Maldives (Indian Ocean; most land is less than 1–2 m above sea level); Tuvalu and Kiribati (coral atolls in the Pacific Ocean).

• Indian cities: Mumbai (Arabian Sea coast, Maharashtra); Kolkata (on the Hooghly, near the delta, West Bengal); Chennai (Coromandel coast, Bay of Bengal, Tamil Nadu).

• Tipping-point regions

• Thwaites Glacier (Amundsen Sea, West Antarctica), nicknamed the "Doomsday Glacier". Warm ocean water is melting it from below.
• Amazon rainforest (South America, mainly Brazil). Parts of it could dry out and turn into savanna (dieback).
• Permafrost in Siberia, Alaska and northern Canada. As it thaws, it releases CO₂ and methane.
• AMOC (Atlantic Meridional Overturning Circulation, which includes the Gulf Stream) in the Atlantic Ocean. If it slows, Europe could cool and monsoons could shift.
• Coral reefs: the Great Barrier Reef, Lakshadweep and the Gulf of Mannar.

Don't confuse with

• Greenhouse effect: this is the natural trapping of heat by gases in the air, and it keeps the Earth warm enough for life. Global warming is the extra rise in temperature caused when human activity adds more GHGs.
• A single warm year vs the Paris 1.5 °C limit: 2024 crossed 1.5 °C as a single calendar year. The Paris goal refers to an average over about 20 years, so it has not been breached yet.
• Melting sea ice vs melting land ice: floating Arctic sea ice adds little to sea level when it melts. Ice from glaciers and from the Greenland and Antarctic ice sheets does raise sea level, as does thermal expansion.
• El Niño warming vs long-term warming: an El Niño heats the globe for a short time, as it did in 1998. Global warming is a steady trend over many decades.

Prelims Hooks

• 2024 is the warmest year on record (about 1.55 °C) and the first calendar year above 1.5 °C. 2025 was second or third warmest (1.44 °C), and 2015–2025 were the 11 warmest years [2].
• India (IMD): warmest year 2024 (+0.65 °C). 2025 was 8th warmest. The warmest decade was 2016–2025. Maximum temperatures (+0.89 °C per 100 years) are rising faster than minimum temperatures (+0.47 °C) [3].
• Sea-level rise more than doubled, from 2.1 mm a year (1993) to 4.5 mm a year (2024) [4]. Its two causes are melting land ice and thermal expansion.
• Arctic amplification means about 4 times the global warming rate since 1979. Its mechanism is the ice-albedo feedback. Arctic sea ice is smallest in September.
• Matching pairs: South Lhonak Lake (2023) – Sikkim – Teesta III dam. Chorabari Tal (2013) – Kedarnath, Uttarakhand. Thwaites Glacier – West Antarctica. AMOC – Atlantic Ocean.
• NCERT figures: world average temperature about 14 °C, warmest year in its record 1998, warming of about 0.6 °C by 2000. Trap: a single year above 1.5 °C does not breach the Paris goal.

Mains Points

• The evidence keeps getting stronger (GS-III Environment).
• Record air temperatures (2023–2025), record ocean heat (2025), sea-level rise of 4.5 mm a year and shrinking Arctic sea ice all point the same way.

• The long-term 1.5 °C limit is close, so cutting emissions and adapting to change are both urgent.

• The Himalaya is a "water tower" under threat (GS-III Disaster Management).

• Faster glacier melt brings meltwater floods and GLOFs now (Kedarnath 2013, South Lhonak 2023). Later, after peak water, dry-season flow in the Indus, Ganga and Brahmaputra will fall.

• Policy responses: mapping glacial lakes, early-warning systems, safety checks on hydropower dams, and NMSHE (National Mission for Sustaining the Himalayan Ecosystem, part of the National Action Plan on Climate Change).

• India's coasts and farms are exposed (GS-I and GS-III).

• Sea-level rise threatens Mumbai, Kolkata, Chennai and the Sundarbans.
• Warm winters (+1.17 °C in January–February 2025) [3] and faster-rising daytime heat hurt rabi wheat and small seasonal industries.
• Women and the poor suffer most. Tipping points, such as ice sheets, the AMOC and coral reefs, cannot be undone on human timescales, so delay is risky.

Related concepts

• Global temperature record
• Sea level rise
• Arctic amplification
• Glacier retreat in the Himalaya
• Climate tipping points

Read more

• Global warming: the record, the evidence and the risks