Causes of climate change: astronomical and terrestrial

Detail

Two groups of causes

• Climate change means a long-lasting shift in the average weather of a place or of the whole Earth, over decades or longer.
• Class 11 puts its causes into two groups:

Group	Meaning	Main examples	Time scale
Astronomical	Causes that come from outside the Earth, from the Sun or from the Earth's path around it	Changes in solar output (sunspots); Milankovitch cycles	About 11 years (sunspots); tens of thousands of years (orbit)
Terrestrial	Causes that come from the Earth itself	Volcanism; human-caused rise in greenhouse gases; land-use change; aerosols	A few years (volcanoes); decades to centuries (human causes)

• The most important cause today is terrestrial and human-made: the rise in greenhouse gases (covered in Section 4).

Sunspots and solar output

• Sunspots are dark, cooler patches on the Sun's surface. They look dark only because they are cooler than the gas around them.
• Their number rises and falls in a solar cycle of about 11 years.

• Roughly every 11 years, at the top of the cycle, the Sun's magnetic poles flip. The Sun changes from calm to active and stormy [2].

• What some meteorologists claim (Class 11):

• more sunspots → cooler, wetter and stormier weather
• fewer sunspots → warm and dry conditions

• Class 11 says these findings are "not statistically significant". This means the link could be due to chance.

• How small the change is: total solar irradiance (TSI: all the sunlight energy reaching the top of the atmosphere) changes by only about 0.1% over one cycle.

• Solar Cycle 25:
• It began in December 2019.
• On 15 October 2024, NASA, NOAA and the international Solar Cycle Prediction Panel announced that the Sun had reached its solar maximum period [2].
• Scientists expected the maximum phase to last about another year before the Sun starts to decline towards solar minimum. So the peak falls around 2024–25 [2].
• The exact peak month can be fixed only many months later, after a steady fall in activity has been seen [2].

• Sunspot activity in Cycle 25 was slightly higher than forecast [2].

• India's Aditya-L1 (India's first mission to study the Sun):

• It was launched on 2 September 2023.
• Halo-Orbit Insertion happened at about 16:00 IST on 6 January 2024 [3]. A halo orbit is a looping path around the L1 point.
• L1 (Lagrange point 1) is a balance point on the Sun–Earth line, about 1.5 million km from the Earth [3].
• It carries 7 payloads: 4 remote-sensing payloads that look at the photosphere, chromosphere and corona, and 3 in-situ payloads (particle detectors and a magnetometer) that measure the space around L1 [3].

• The payloads were built by ISRO, IIA (Indian Institute of Astrophysics) and IUCAA [3].

• Why the Sun cannot explain recent warming:

• Since 1950, solar output has stayed flat or fallen a little.
• Over the same period, global temperatures rose sharply.
• So the cause of the warming lies elsewhere: it is human greenhouse gases.

Milankovitch cycles

• Milankovitch cycles (Class 11: "Millankovitch oscillations") are slow, regular changes in the Earth's orbit and in the direction of its axis.
• These changes alter insolation (incoming solar radiation: the sunlight energy reaching the Earth) and where it falls.
• They are named after Milutin Milanković, a Serbian scientist who worked on them from about 1920 to 1941.

Cycle	What changes	Range	Period
Eccentricity	Shape of the orbit, from nearly circular to slightly oval	about 0 to 0.06 [4]	about 100,000 and 413,000 years
Obliquity	Axial tilt: the angle of the Earth's axis to its orbit	22.1°–24.5°; now 23.44° and falling	about 41,000 years
Precession	Wobble of the axis, like a spinning top	—	about 26,000 years for the axis; about 19,000–23,000 years when combined with the turning of the orbit ("climatic precession") [4]

• What each cycle does:
• Eccentricity changes the total sunlight the Earth gets over a year [4].

• Obliquity and precession change where (which latitudes) and when (which season) the sunlight falls [4].

• How they cause ice ages:

• Weak summer sunlight at high northern latitudes → winter snow does not melt fully → ice sheets grow.
• Ice reflects more sunlight (ice-albedo feedback; albedo means how much light a surface reflects) → more cooling.
• Colder oceans hold more CO₂ → less greenhouse warming → still more cooling.

• These feedbacks make the small orbital push much stronger.

• Milankovitch cycles are seen as the main pacemaker of the glacial cycles (ice ages) of the last several million years [4].

• Limit: they act over thousands of years, not decades. So they cannot explain the warming since the Industrial Revolution.

Volcanism and climate

• Volcanism is the movement of molten rock (magma) and gases from inside the Earth to the surface.
• How an eruption cools the Earth:
• A large eruption throws aerosols (tiny particles and droplets in the air), mainly sulphur dioxide (SO₂), up into the stratosphere (the layer above about 10–15 km, where there is no rain to wash them out).
• SO₂ joins with water vapour and forms sulphate aerosols [5].
• These droplets reflect sunlight back to space, so less sunlight reaches the surface [5].

• They stay a year or more [5], so the Earth cools for a few years and then recovers.

• Key eruptions:

Volcano	Location	Year	Effect
Tambora	Sumbawa island, Indonesia	1815	Strong cooling
Krakatoa	Sunda Strait, between Java and Sumatra, Indonesia	1883	Global cooling and vivid red sunsets
El Chichón (Class 11: "El Cion")	Chiapas state, southern Mexico	1982	Cooling effect
Mount Pinatubo (Class 11: "Pinatoba")	Luzon island, Philippines (about 15° N)	15 June 1991	Put about 20 million tonnes of SO₂ into the stratosphere [5]; global cooling of about 0.5 °C in 1992 [5]
Hunga Tonga–Hunga Ha'apai	Tonga, South Pacific Ocean	January 2022	Put mainly water vapour into the stratosphere, not SO₂ → a slight warming influence

• Volcanoes and CO₂:
• Volcanoes on land and under the sea release less than 1% of the CO₂ that humans release each year [6].
• The human CO₂ emission of about 35 gigatonnes in 2010 was about 80 to 270 times the yearly volcanic CO₂ [7].
• It would take about 700 Pinatubo-sized eruptions every year to equal human CO₂ emissions [6].
• Volcanic cooling lasts only a short time. Human emissions continue all the time.

Human (anthropogenic) causes

• Anthropogenic means caused by humans.
• The biggest human cause is the rise in greenhouse gases (CO₂, methane, nitrous oxide and others) from burning fossil fuels (see Section 4).
• Other human causes:
• Land-use change: cutting forests releases stored carbon and changes the albedo.

• Aerosols from industry and vehicles: they partly cool the Earth and hide some of the warming.

• The IPCC AR6 (Sixth Assessment Report, Working Group I, 2021) says it is "unequivocal" (beyond doubt) that human influence has warmed the atmosphere, ocean and land.

Prelims Hooks

• Sunspots are cooler (not hotter) patches on the Sun; the cycle is about 11 years. Class 11 calls the sunspot–weather link "not statistically significant".
• Solar Cycle 25: began December 2019; NASA and NOAA announced its solar maximum period on 15 October 2024 [2].
• Aditya-L1: launched 2 September 2023; halo orbit around L1 (about 1.5 million km from Earth) on 6 January 2024; 7 payloads (4 remote-sensing + 3 in-situ) [3].
• Match the Milankovitch cycles: eccentricity – orbit shape – about 100,000 years; obliquity – axial tilt (22.1°–24.5°) – about 41,000 years; precession – wobble – about 26,000 years.
• Earth's tilt is now 23.44° and decreasing.
• Trap: eccentricity changes the total sunlight received; obliquity and precession mainly change its spread over latitudes and seasons [4].
• Trap: volcanic SO₂ cools the Earth; the 2022 Hunga Tonga eruption (water vapour) had a slight warming effect.
• Volcano–country pairs: Pinatubo – Philippines (1991); El Chichón – Mexico (1982); Tambora (1815) and Krakatoa (1883) – Indonesia.
• Volcanoes emit less than 1% of human CO₂ [6].

Mains Points

• Natural causes cannot explain modern warming:
• The Sun's output has been flat or falling since 1950.
• Milankovitch cycles act over millennia.
• Volcanoes cool the Earth only for a few years and emit under 1% of human CO₂ [6].

• This leaves human greenhouse gases as the main cause, as IPCC AR6 also concludes. Use this for GS-I and GS-III answers on attribution.

• Feedbacks turn small pushes into big changes: a small Milankovitch change, made stronger by the ice-albedo and CO₂ feedbacks, ended and began ice ages. This shows the climate system is sensitive, which is why even about 1 °C of human warming matters.

• Volcanic cooling is a natural model for geoengineering: Pinatubo's sulphate cooling of about 0.5 °C [5] is the basis for proposals to inject stratospheric aerosols. The risks include monsoon disruption for India, and the cooling stops if the injection stops.
• India's space science role: Aditya-L1 [3] improves space-weather forecasting and helps protect satellites and power grids, which matters most during solar maximum [2]. It adds Indian data to the global study of how the Sun affects climate.