Climate science and the greenhouse effect

The first principle

A planet stays at a steady temperature only when the energy it radiates to space equals the sunlight it absorbs. Sunlight arrives as short wavelengths that pass through the air, but the warm ground sends energy back as long wave infrared, and a few gases absorb that infrared and send part of it back down. Adding more of these gases makes the planet lose heat less easily, so it must warm until it radiates enough again, and the rest of climate science follows from that imbalance.

The core

The energy balance, worked out

• The Sun's input. At Earth's distance the Sun delivers about S₀ = 1,361 W m⁻², but Earth intercepts it on a disc of area πR² while it radiates from a whole sphere of area 4πR², so the average input over the surface is S₀/4 ≈ 340 W m⁻². About 30 per cent is reflected by clouds, ice and air (the albedo, α ≈ 0.3), which leaves S₀(1 − α)/4 ≈ 238 W m⁻² absorbed.
• Earth without greenhouse gases. A body radiates σT⁴ by the Stefan–Boltzmann law, with σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴. Setting σT⁴ = 238 gives T⁴ ≈ 4.2 × 10⁹ K⁴, so T ≈ 255 K, or about −18 °C. The real average is about 288 K (15 °C), and the gap of about 33 K is the natural greenhouse effect.
• Why the two kinds of radiation differ. Wien's law, λₘₐₓ = 2,898 µm K / T, puts the Sun's peak (about 5,800 K) near 0.5 µm, which is visible light, and Earth's peak (288 K) near 10 µm, which is thermal infrared. The atmosphere is largely transparent to the first and partly opaque to the second.
• Why only some gases trap heat. A molecule absorbs infrared only if its vibration changes its electric dipole moment. Nitrogen (N₂) and oxygen (O₂), which make up 99 per cent of dry air, are symmetric pairs of identical atoms and cannot do this, while carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), water vapour and the fluorinated gases can. Carbon dioxide's bending vibration absorbs strongly near 15 µm, close to Earth's emission peak.

Radiative forcing and climate sensitivity

• Radiative forcing is the change in the energy balance, in W m⁻², caused by a driver such as extra carbon dioxide, before the climate responds. For carbon dioxide it grows with the logarithm of concentration, ΔF ≈ 5.35 ln(C/C₀) W m⁻² (Myhre and others, 1998), because the centre of the 15 µm band is already saturated and further gas absorbs mainly at its edges. A doubling therefore adds about 5.35 × ln 2 ≈ 3.7 W m⁻² by this formula, and the Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) assesses about 3.9 W m⁻² once rapid adjustments are included.
• Equilibrium climate sensitivity is the warming after carbon dioxide doubles and the climate settles. AR6 gives a best estimate of 3 °C, with a likely range of 2.5 °C to 4 °C.
• The present forcing. Carbon dioxide averaged 423.9 parts per million (ppm) in 2024, about 152 per cent of the pre industrial level of about 278 ppm, after a record rise of 3.5 ppm in one year (World Meteorological Organization, WMO, 2025). With the formula above, carbon dioxide alone now forces about 5.35 × ln(424/278) ≈ 2.3 W m⁻².

Gases and the global warming potential

• Global warming potential (GWP) compares gases: it is the radiative forcing from a pulse of one kilogram of a gas, added up over a time horizon, divided by the same for one kilogram of carbon dioxide. Over 100 years methane scores about 27 to 30, but over 20 years about 80, because it lasts only about 12 years in the air, so cutting methane cools fastest. Under the Paris rulebook of 2018, countries report in carbon dioxide equivalent using the 100 year values of the Fifth Assessment Report.
• Carbon dioxide is different in kind. It has no single lifetime: oceans and plants take up much of a pulse within decades, but between 15 and 40 per cent is still in the air after a thousand years, so its warming is effectively permanent and grows with the total ever emitted.

Dimensions

• Atmosphere. Warming is uneven in a telling way: the lower atmosphere warms while the stratosphere cools, which is what more greenhouse gas predicts and what a brighter Sun would not. WMO found 2024 the warmest year on record, about 1.55 °C above 1850 to 1900, though the Paris goal refers to a twenty year average rather than one year.
• Ocean. The ocean has taken up more than 90 per cent of the extra heat since 1970, and dissolved carbon dioxide forms carbonic acid that has lowered surface pH by about 0.1, which is about 30 per cent more hydrogen ions because the scale is logarithmic; this ocean acidification makes it harder for corals and shellfish to build calcium carbonate.
• Sea level. Global mean sea level rose about 0.20 m between 1901 and 2018, and the rate rose from 1.3 mm a year (1901 to 1971) to 3.7 mm a year (2006 to 2018). Thermal expansion gave about half of the rise since 1971 (AR6).
• Cryosphere and feedbacks. Melting snow and ice expose darker surfaces that absorb more sunlight, warm air holds about 7 per cent more water vapour per degree by the Clausius–Clapeyron relation, and thawing permafrost releases carbon dioxide and methane; all three are positive feedbacks, while the extra radiation from a warmer surface (the Planck response) keeps the climate stable.
• Equity. Warming tracks cumulative emissions, so who used the atmosphere first bears on who should cut first, the scientific base of the principle of common but differentiated responsibilities.

In practice

The carbon budget

• Warming tracks cumulative carbon dioxide. AR6 found a near linear relation, the transient climate response to cumulative emissions, of about 0.45 °C for every 1,000 billion tonnes (Gt) of carbon dioxide, with a likely range of 0.27 °C to 0.63 °C. A temperature limit is therefore a limit on total emissions, called the remaining carbon budget.
• The AR6 numbers. From the start of 2020 the remaining budget was about 500 Gt for an even chance of 1.5 °C and 400 Gt for a two in three chance, and about 1,350 Gt for an even chance of 2 °C and 1,150 Gt for a two in three chance.
• The update. The Indicators of Global Climate Change study of 2025, led by Piers Forster, put human caused warming at 1.36 °C in 2024 and the remaining 1.5 °C budget at about 130 Gt from the start of 2025, a little over three years of present emissions of about 40 Gt a year.

Detection and attribution

• Detection shows a change is outside natural variability; attribution finds its causes by matching fingerprints. Stratospheric cooling and the falling share of carbon 13 and carbon 14 in air (because fossil carbon is depleted in both) all point to fossil fuels rather than the Sun or volcanoes, and AR6 called human influence on warming unequivocal.
• Event attribution compares how likely an extreme is in today's climate with a modelled climate without human emissions. The World Weather Attribution group found the India and Pakistan heatwave of March and April 2022 about 30 times more likely because of warming.

Tipping points

• A tipping point is a threshold beyond which a system reorganises, often abruptly and sometimes irreversibly, such as the Greenland and West Antarctic ice sheets, the Amazon forest and the Atlantic Meridional Overturning Circulation (AMOC). AR6 judged the AMOC very likely to weaken this century but, with medium confidence, not to collapse before 2100, and the Global Tipping Points Report of October 2025 concluded that warm water coral reefs have already passed their thermal tipping point.

India's science

• National assessment. The Ministry of Earth Sciences (MoES) published India's first national climate assessment in 2020, which found a rise of about 0.7 °C in average temperature over 1901 to 2018, and the Indian Institute of Tropical Meteorology (IITM), Pune, built India's first Earth system model, IITM-ESM, which contributed to the model runs behind AR6.
• India's inventory. India's Fourth Biennial Update Report (December 2024) put national emissions in 2020 at 2,959 million tonnes of carbon dioxide equivalent without land use and 2,437 million tonnes after forests and land absorb their share, with energy giving about three quarters.

Issues and way forward

• The budget is almost spent. At about 40 Gt a year the 1.5 °C budget will be used within a few years, so overshoot followed by carbon removal is now part of most pathways, and only deep cuts this decade limit its size.
• Uncertainty is not ignorance. Clouds and aerosols still widen the sensitivity range, because aerosols from coal and diesel cool by reflecting sunlight and their cleanup unmasks warming.
• Short lived pollutants matter. Methane and black carbon warm strongly but briefly, so cutting them buys time, though it cannot replace cutting carbon dioxide, whose effect accumulates.

Common confusions

• Greenhouse effect and global warming. The greenhouse effect is natural and keeps Earth habitable at 15 °C; global warming is the extra warming from the extra gas humans add.
• Ozone hole and global warming. The ozone hole is stratospheric ozone destroyed by chlorine and bromine, which lets in more ultraviolet light; it is a separate problem, though chlorofluorocarbons and their substitutes are also greenhouse gases.
• Radiative forcing and GWP. Forcing measures how much an amount of gas present now changes the energy balance; GWP compares one kilogram of a gas with one kilogram of carbon dioxide over a set horizon.
• Equilibrium sensitivity and the budget relation. Sensitivity is warming per doubling after the climate settles; the transient response to cumulative emissions is warming per tonne emitted, and it is the one used for carbon budgets.

For the exam

• Keyword bank: energy balance, Stefan–Boltzmann law, infrared absorption, radiative forcing, equilibrium climate sensitivity, global warming potential, carbon dioxide equivalent, remaining carbon budget, cumulative emissions, positive feedback, ice albedo feedback, tipping point, detection and attribution, ocean acidification, overshoot.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction with data: Carbon dioxide reached 423.9 ppm in 2024, about 152 per cent of its pre industrial level, after the largest yearly rise on record (WMO Greenhouse Gas Bulletin, 2025).
  • Introduction with a report: The IPCC's Sixth Assessment Report found it unequivocal that human influence has warmed the atmosphere, ocean and land, with warming of about 1.1 °C over 2011 to 2020 against 1850 to 1900.
  • Conclusion: Because warming tracks cumulative emissions, every tonne avoided counts permanently, so the case for early action is physics before it is policy.
  • Conclusion: With the 1.5 °C budget nearly spent, cutting methane fast, cutting carbon dioxide to net zero and adapting to the warming already locked in must now run together.