Climate change

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 11 "World Climate and Climate Change"

Meaning

Climate change is a long-term change in the average weather of one place or of the whole Earth. One hot summer or one failed monsoon is not climate change. Climate has changed naturally all through Earth's history; Class 11 NCERT calls it "a natural and continuous process". Since the industrial era, though, greenhouse gas emissions from human activity have become the main cause of warming. This is why climate change is now both a geography topic and a policy issue.

Explanation

Two kinds of drivers: natural and human

• Natural drivers (Class 7 NCERT) caused climate change over past millennia:
• changes in the Earth's orbit
• changes in solar output (how much energy the Sun gives out)

• volcanic activity

• Human drivers have been at work since the industrial era:

• burning and other human activities release greenhouse gases (gases that trap heat in the atmosphere)
• more trapped heat warms the planet

• these emissions are now the main cause of warming

• The key difference is speed and size:

• Natural change was slow and took thousands of years.
• Today's rise in CO₂ is faster, and it has gone above the natural range seen in ice cores.

Climate proxies: how we read past climates

• Climate proxy means an indirect record of past climate. Thermometers have existed for only about 150 years, so scientists use natural "recorders" for older times.

Proxy	What it records
Glacial landforms (moraines, U-shaped valleys, cirques)	How far glaciers advanced and retreated
Sediments in glacial lakes	Warm and cold periods
Tree rings	Wet years (wide rings) and dry years (narrow rings)
Historical records	Floods, droughts, harvests, frozen rivers
Ice cores	Past air (CO₂, methane) and temperature
Pollen	Which plants grew, and so how warm or wet it was
Corals	Sea-surface temperature
Cave stalagmites (speleothems)	Monsoon strength, read from oxygen isotopes
Ocean sediments	Global ice volume and temperature

• How oxygen isotopes work:
• Water with light oxygen (O-16) evaporates more easily than water with heavy oxygen (O-18).
• In an ice age, much of this light water gets locked up in ice sheets, so the oceans are left richer in O-18.
• Tiny sea shells record this ratio. A high O-18 reading means a cold period with large ice sheets.

Ice cores: the strongest evidence

• Ice core means a long cylinder of ice drilled out of an ice sheet. Each yearly snow layer traps tiny air bubbles, which are real samples of the old atmosphere.
• Vostok (a Russian station in East Antarctica) gave the first long record, covering about four glacial cycles.
• The EPICA Dome C core (East Antarctica) carries the CO₂ record back to 800,000 years, adding two more full glacial cycles [5][6].
• CO₂ is measured directly from the trapped air. Temperature is worked out from deuterium (heavy hydrogen), shown as the change from the 1950 average [6].
• CO₂ and Antarctic temperature rose and fell together through eight glacial cycles [5].

• Natural CO₂ mostly stayed between 180 and 280 ppmv (parts per million by volume). It fell below that range twice, between 800,000 and 650,000 years ago [5].

• Result: pre-industrial CO₂ never went above about 300 ppm in 800,000 years. Today's higher levels therefore point to human influence.

Timeline: warm ages, ice ages and the Holocene

• Early warm Earth: about 500–300 million years ago, in the Cambrian, Ordovician and Silurian periods.
• Glacial period means a cold phase when ice sheets spread towards the equator and down mountains. Interglacial period means a warmer phase between two glacials, when the ice retreats. We live in an interglacial now.
• Pleistocene epoch (about 2.58 million to 11,700 years ago): glacials and interglacials came and went many times.
• Last Glacial Maximum (LGM) means the time when ice last reached its greatest extent.
• NCERT dates it to about 18,000 years ago. Current estimates say about 26,000–19,000 years ago, with the peak near 21,000 years ago.
• Land ice was at its maximum from at least 22,000 to 19,000 years ago. The LGM ended at about 19,000 ± 250 years ago, when ice volume fell by about 10% within a few hundred years [9].
• Global sea level was about 130 m lower than today [9].

• About 22,000 years ago, the ice sheets grew fast and lowered sea level by about 17 m in only 500 years [10].

• Holocene means the present warm interglacial epoch. It began 11,700 years ago [4] (NCERT: 10,000 years ago).

• In June 2018, the IUGS (International Union of Geological Sciences) and the International Commission on Stratigraphy split the Holocene into three stages [4]:

Stage	Years before present	Boundary marker (GSSP)
Greenlandian	11,700 – 8,200	Greenland ice core
Northgrippian	8,200 – 4,200	Greenland ice core (NGRIP)
Meghalayan	4,200 – present	Stalagmite, Mawmluh Cave, Meghalaya

• GSSP (Global Boundary Stratotype Section and Point) is the single "golden spike" in rock or another deposit that officially marks where a time unit begins.

Where it occurs

• Antarctica: the Vostok and EPICA Dome C ice cores (East Antarctica) [5][6].
• Greenland: ice cores that mark the Greenlandian and Northgrippian stages.
• Mawmluh Cave, East Khasi Hills, Meghalaya (near Sohra/Cherrapunji):
• Its stalagmite is the GSSP for the Meghalayan Stage [2][3].
• Oxygen isotope study of its layers gives a detailed record of Holocene climate and of the past monsoon [2].
• The stage begins with a megadrought about 4,200 years ago (the 4.2 kiloyear event), seen in North-East India and elsewhere [2].

• The "Meghalayan Age Caves" are on India's UNESCO World Heritage Tentative List [2].

• Glacial landforms: the Himalaya and Alps, and high latitudes such as Scandinavia and Canada.

• India's continental shelf: much of it was dry land at the LGM, when sea level was about 130 m lower [9].
• Rajasthan (Thar Desert):
• Wet and cool around 8,000 BC.
• Wetter than today in 3,000–1,700 BC. Rivers and lakes supported farming.
• Centre of the Harappan civilisation in 2,000–1,700 BC.

• Drier after about 1,700 BC, which led to today's desert.

• North India: repeated summer and winter droughts between 4,200 and 3,970 years ago [12].

Don't confuse with

• Weather: weather is short-term (one hot summer, one failed monsoon). Climate change is a long-term change in average weather.
• Interglacial vs glacial: in a glacial, ice sheets spread. In an interglacial, the ice retreats. The Holocene is an interglacial, not a post-ice-age "end" of glaciation.
• Holocene vs Anthropocene:
• The Holocene is the official epoch we live in (Meghalayan Age).

• The Anthropocene was proposed with a start in 1952, marked by a plutonium spike from nuclear tests [8]. It was rejected 12–4, with 2 abstentions, on 4 March 2024 [8], and the IUGS upheld the rejection on 20 March 2024 [7]. It is only an informal term.

• Vostok vs EPICA Dome C: Vostok gave the first long record (about four glacial cycles). EPICA Dome C is longer, about 800,000 years [5][6].

Prelims Hooks

• The Meghalayan Age (4,200 years ago to present) is marked by a stalagmite in Mawmluh Cave, Meghalaya. It is the only Holocene stage whose marker is in India [2][3].
• Holocene stages from oldest to youngest: Greenlandian → Northgrippian → Meghalayan, formalised in 2018 [4].
• Trap: the Anthropocene is NOT an official epoch. It was rejected 12–4 on 4 March 2024, and the IUGS upheld this on 20 March 2024 [8][7].
• Proxy–record pairs: tree rings → wet/dry years; glacial lake sediments → warm/cold periods; ice-core air bubbles → past CO₂; stalagmites → monsoon strength; corals → sea-surface temperature.
• LGM: about 26,000–19,000 years ago, with the peak near 21,000 (NCERT: 18,000). Sea level was about 130 m lower [9].
• The Earth was warm 500–300 million years ago, in the Cambrian, Ordovician and Silurian periods.

Mains Points

• Natural variability vs today's warming (GS-I/GS-III):
• Proxies show that climate always changed, but slowly, over thousands of years.
• Ice cores show natural CO₂ stayed at about 180–280 ppmv for 800,000 years [5].

• Today's rise is faster and goes beyond that range. This shows that human emissions are now the main driver.

• Climate and civilisation (GS-I):

• About 4,200 years ago the monsoon weakened, so river flows and rainfall fell. Farming that depended on the monsoon came under stress, and Harappan cities slowly emptied as people moved east.
• This was a debated cause. A 2024 study says the 4.2 ka event was not unusual among Holocene climate swings [11]. Shifting rivers (Ghaggar-Hakra), falling trade and floods are other explanations.

• Lesson: monsoon failure can reshape societies. This is a warning for India's monsoon-dependent farming today.

• Geoheritage and the Anthropocene idea (GS-I/GS-III):

• Mawmluh Cave gives India a global stratigraphic marker. Its place on the UNESCO Tentative List supports protecting the caves from limestone mining in Meghalaya [2].
• The Anthropocene has no formal status, but it is still useful in policy. It shows that human impact (greenhouse gases, land-use change, biodiversity loss) works on a planetary scale.

Related concepts

• Climate proxies
• Glacial and interglacial periods
• Pleistocene epoch
• Holocene
• Past climate of Rajasthan

Read more

• Climate has always changed: proxies, ice ages and the Holocene