Climate Change & the Archives of Deep Time
Concepts, evidence, causes, scope and significance of climate change — read alongside the record Earth has been keeping about itself in ice, rock, coral and wood for the last 800,000 years.
What do we mean by "climate change"?
Before evidence or causes make sense, three ideas need to be pinned down: how climate differs from weather, what the climate system is made of, and how "climate change" is actually defined by the bodies that study and govern it.
Weather vs. Climate
Weather is the state of the atmosphere at a place, right now — temperature, rainfall, wind, pressure, over hours to days.
Climate is the statistical description of weather over a long period — its average and its variability — for a given region. The World Meteorological Organization fixes this period at a 30-year "climate normal" (currently 1991–2020, updated every decade).
Two working definitions
IPCC defines climate change broadly: any change in the state of the climate — identified in its mean or variability — that persists for decades or longer, whatever the cause (natural internal processes, solar or volcanic forcing, or human activity).
UNFCCC defines it more narrowly: change attributed directly or indirectly to human activity that alters atmospheric composition, over and above natural climate variability.
The climate system — five interacting spheres
Climate is not just "air." It emerges from energy and matter exchanged between five components. Click each to see its role.
The evidence: direct records and natural archives
Evidence for climate change comes in two families — direct/instrumental measurements from the last ~170 years, and indirect/proxy evidence that lets us read climate from before thermometers existed (detailed in Layer 05).
The Keeling Curve: the most important line in climate science
Charles David Keeling began continuous CO₂ measurements at Mauna Loa Observatory, Hawaii, in March 1958. It is the longest unbroken instrumental record of atmospheric CO₂ anywhere. Hover the line to read values.
Global temperature — "warming stripes"
Each stripe is a decade's average temperature anomaly relative to 1850–1900. Click a stripe.
Other instrumental indicators
- Sea level: tide gauges + satellite altimetry (since 1993) show ~21–24 cm total rise since 1900, accelerating to ~3.3 mm/year now.
- Arctic sea ice: September minimum extent has declined roughly 12% per decade since the 1979 satellite record began.
- Ocean heat content: oceans have absorbed over 90% of the extra heat trapped by greenhouse gases.
- Phenology: earlier spring flowering, poleward/upward shifts in species ranges, more frequent coral-bleaching events.
Regional case: Himalayan glacier retreat
The Gangotri Glacier (Uttarkashi, Uttarakhand), source of the Bhagirathi–Ganga system, has retreated over 2 km since 1780, averaging about 22 metres/year between 2000 and 2010 according to satellite monitoring by the Wadia Institute of Himalayan Geology and ISRO.
A 2023 ISRO assessment estimated that Himalayan glaciers lost roughly 16.4% of their glaciated area between 1990 and 2019. The Hindu Kush Himalaya as a whole is warming faster than the global average.
Why it matters downstream
The Ganga, Indus and Brahmaputra basins, fed partly by Himalayan snow and ice, support over 600 million people. Glacier retreat first raises river flow (more melt), then threatens lean-season flow as ice reserves shrink — alongside a rising risk of Glacial Lake Outburst Floods (GLOFs), as at South Lhonak, Sikkim (2023).
Causes: natural forcing and human forcing
Climate has always changed for natural reasons. What is new since the Industrial Revolution is a forcing of unmatched speed, layered on top of the natural ones.
Milankovitch (orbital) cycles
Three periodic changes in Earth's orbit and axis alter how sunlight is distributed by latitude and season — the pacemaker of glacial–interglacial cycles (see Layer 05).
Schematic combination, not to scale — for illustrating how three cycles superimpose, not for reading exact insolation values.Solar variability
An 11-year sunspot cycle plus longer "grand minima," e.g. the Maunder Minimum (≈1645–1715), which coincided with part of the Little Ice Age. Satellite-measured solar irradiance changes since 1978 are far too small to explain modern warming.
Volcanic eruptions
Sulfate aerosols reflect sunlight. Mount Pinatubo (1991) cooled the globe by about 0.5°C for roughly a year — episodic, not a long-term trend.
Ocean–atmosphere circulation
ENSO (El Niño/La Niña) and the Atlantic Meridional Overturning Circulation redistribute heat and cause year-to-year variability without setting the long-term trend.
Scope and significance
Climate change is studied across scales — from global assessments down to a single delta island — and by more disciplines than any one department can claim alone.
An interdisciplinary field
Why it matters — significance across sectors
Click a card to expand.
Agriculture
Water Resources
Biodiversity
Coastal Zones
Human Health
Economy
Policy
Exam Relevance
Paleoclimate: reading climate before thermometers
Paleoclimatology reconstructs past climate using indirect ("proxy") evidence, establishing the natural range of variability against which today's change is measured — and testing whether current warming is unusual. It is unusual: the current rate of temperature rise is faster than any period reconstructed for at least the last 2,000 years.
Proxy archives
Ice cores
Trapped air bubbles preserve ancient CO₂ & CH₄; oxygen-isotope ratio (δ¹⁸O) records past temperature. Key records: Vostok (Antarctica, ~420,000 yrs), EPICA Dome C (Antarctica, ~800,000 yrs), GISP2/NGRIP (Greenland, ~120,000 yrs).
Tree rings
Dendrochronology/dendroclimatology: ring width and density track temperature and moisture; cross-dating extends chronologies thousands of years.
Corals
Annual growth bands; Sr/Ca ratios and δ¹⁸O record sea-surface temperature and salinity — useful for reconstructing past ENSO behaviour.
Speleothems
Stalagmite/stalactite δ¹⁸O tracks past rainfall and monsoon strength. Mawmluh Cave, Meghalaya, defines the global golden-spike marker for the current "Meghalayan Age."
Pollen (palynology)
Pollen preserved in sediment/peat reveals past vegetation cover and, by extension, past climate zones.
Ocean & lake sediments
Foraminifera-shell δ¹⁸O tracks ocean temperature and ice volume; annually laminated "varves" give year-by-year resolution.
Boreholes & documents
Temperature-depth profiles in rock preserve surface-temperature history; crop, famine and monastery records extend the recent centuries.
The Ice-Core Reader
Drag to move through 420,000 years of the Vostok ice-core record (Antarctica). Values are simplified/illustrative, following the pattern published by Petit et al. (1999).
Glacial–interglacial pacing
Since the Mid-Pleistocene Transition (~800,000–900,000 years ago), glacial cycles have been paced mainly by the ~100,000-year eccentricity cycle; before that, the ~41,000-year obliquity cycle dominated. Roughly eight full glacial–interglacial cycles are recorded in the EPICA Dome C core.
Named intervals to remember
- Pleistocene (2.6 Ma–11.7 kyr BP): repeated glacial–interglacial cycles.
- Younger Dryas (~12.9–11.7 kyr BP): abrupt return to near-glacial cold during deglaciation.
- Holocene (11.7 kyr BP–present): relatively stable climate that permitted the rise of agriculture.
- 4.2 kyr BP event: a severe multi-century mega-drought, coinciding with the decline of several Bronze Age urban centres; formally marks the start of the "Meghalayan Age," defined at Mawmluh Cave, India.
Timeline
Self-test
20 questions spanning concept, evidence, causes, scope/significance and paleoclimate. Select an answer for each, then submit.
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