Maarg Manthan · Topic 15.7

Greenhouse Effect and Non-Greenhouse Gases and Aerosols

Environment & Ecology › Climate Change Basics and Causes · Topic 15.7

The greenhouse effect is the warming of the Earth's surface and lower atmosphere because certain gases absorb the infrared (heat) radiation given off by the surface and re-emit part of it downward. Sunlight passes through the atmosphere largely unimpeded and warms the ground; the ground radiates infrared, which greenhouse gases trap. The natural greenhouse effect keeps the average surface temperature near 15 degrees Celsius instead of about minus 18, making life possible. The main greenhouse gases are water vapour (the largest natural contributor, which acts as a feedback), carbon dioxide, methane, nitrous oxide, ozone, and human-made halocarbons, hydrofluorocarbons, perfluorocarbons, sulphur hexafluoride and nitrogen trifluoride. The Kyoto Protocol covers CO2, CH4, N2O, HFCs, PFCs and SF6, with NF3 added later. Nitrogen, oxygen and argon, which make up about 99 percent of dry air, are not greenhouse gases because they do not absorb infrared. Aerosols are tiny particles in the air: sulphate aerosols reflect sunlight and cool the climate; black carbon absorbs sunlight and warms it; dust, organic aerosols and sea salt also matter; aerosols also affect clouds. Reducing air pollution cuts aerosols and can unmask warming.

5 min read · UPSC Prelims · Environment & Ecology · Climate Change Basics and Causes

At a Glance

Natural effect Raises Earth to about 15 C
Biggest natural GHG Water vapour
Kyoto basket CO2, CH4, N2O, HFCs, PFCs, SF6
Non-GHGs N2, O2, Ar
Aerosols Cooling and warming types
Exam link UPSC Prelims, Environment & Ecology: Climate Change Basics and Causes

Where Does It Fit?

This topic explains the physical mechanism behind warming, after the causes and sector links. It leads to the measures of radiative forcing, footprints and global warming potential. Previous topic: Sustainable Agriculture Techniques. Next topic: Radiative Forcing and Global Warming.

How Does the Greenhouse Effect Work and Which Gases Matter?

Gas or agent	Role and notes
Water vapour	Largest natural greenhouse contributor; its amount rises with warming, so it amplifies change
Carbon dioxide	Main human-caused greenhouse gas; stays for hundreds of years; above 420 ppm
Methane	Short-lived but potent; from wetlands, livestock, rice, fossil fuels
Nitrous oxide	Long-lived and potent; from fertilisers and combustion
Ozone	Greenhouse gas in the troposphere; protective in the stratosphere
Halocarbons and F-gases	CFCs, HFCs, PFCs, SF6 and NF3; very high warming potential and long lives
Non-greenhouse gases	Nitrogen, oxygen and argon do not absorb infrared

What Do Aerosols Do?

Sulphate aerosols: from burning coal and volcanoes; reflect sunlight and cool
Black carbon: soot from incomplete combustion; absorbs sunlight, warms the air and darkens snow
Organic carbon and dust: mixed effects
Direct effect: scatter or absorb sunlight
Indirect effect: change cloud droplet size and lifetime
Net effect: human aerosols have a net cooling that masks part of the warming
Global dimming: drop in surface sunlight in polluted regions
Policy: cutting black carbon helps both health and climate; cutting sulphate removes cooling

What Else Should You Remember?

Natural greenhouse effect: average Earth temperature of about 15 C compared with minus 18 C without it
Absorption bands: greenhouse gases absorb in infrared bands, not in visible light
Incoming and outgoing energy: Earth absorbs sunlight and sends out infrared in balance
Energy imbalance: extra greenhouse gases cause the Earth to absorb more than it emits
Venus: a planet with a runaway greenhouse effect and a very dense CO2 atmosphere
Mars: thin atmosphere gives a weak greenhouse effect
Water vapour feedback: warm air holds about 7 percent more moisture per degree
Residence times: CO2 lasts centuries, methane about a decade, N2O about 100 years
Kyoto gases: CO2, CH4, N2O, HFCs, PFCs, SF6, and NF3 from the Doha Amendment
Montreal gases: CFCs are greenhouse gases but are controlled by the Montreal Protocol
Tropospheric ozone: a short-lived greenhouse gas and air pollutant
Brown clouds: Atmospheric Brown Clouds are haze layers of aerosols over South Asia
Aerosol cooling: the cooling effect hid part of greenhouse warming in the twentieth century
Volcanic aerosols: cause temporary cooling after eruptions
Marine cloud brightening: proposed geoengineering using sea salt aerosols
Stratospheric aerosol injection: proposed geoengineering to reflect sunlight, with risks
Indian aerosols: high loads over the Indo-Gangetic plain affect monsoon rainfall
Black carbon cookstoves: clean cooking cuts black carbon and indoor pollution
Short-lived climate pollutants: methane, black carbon, HFCs and tropospheric ozone; cutting them has quick benefits
Climate and Clean Air Coalition: launched in 2012 to cut short-lived climate pollutants
Link to previous: see Sustainable Agriculture Techniques
Link to next: see Radiative Forcing and Global Warming
Link to ozone: see Smog, Ozone Depletion and Acid Rain
Caution: concentrations and lifetimes are updated by each IPCC report, so check the latest
Exam hint: separate greenhouse gases, non-greenhouse gases and aerosols, and cooling from warming aerosols
Kyoto Protocol: adopted in 1997 and in force 2005, it set binding emission targets for developed countries covering six gases, later with NF3 added in the Doha Amendment
Infrared radiation: the Earth emits heat as infrared radiation, and greenhouse gases absorb part of it before it escapes to space
Energy balance: about 340 watts per square metre of sunlight arrives on average, and part is reflected by clouds, ice and aerosols
Cloud role: clouds both cool by reflecting sunlight and warm by trapping heat, and their net feedback is uncertain
Human share of CO2: the extra CO2 since 1850 has increased the heat trapping and caused the energy imbalance seen in the oceans

Why Does It Matter for the Exam?

Largest natural GHG: Water vapour
Cooling aerosol: Sulphate
Warming aerosol: Black carbon
Current affairs link: Short-lived climate pollutants
Common errors: calling nitrogen a greenhouse gas

As an analytical point, the greenhouse effect is natural and essential, but the added gases are driving the imbalance. Because some pollutants cool and others warm, cleaning the air must be planned alongside deep cuts in CO2.

Why Is This Topic Important?

Trap Infrared
Gases CO2, CH4
Particles Cool or warm

What Came Next?

The next topic covers radiative forcing and global warming. Next topic: Radiative Forcing and Global Warming.

Key People and Terms

Greenhouse effect Heat trapping by gases
Aerosol Airborne particle
Black carbon Soot
F-gases Fluorinated gases
SLCP Short-lived climate pollutant
Infrared Heat radiation

Exam Corner

Points to Remember

Natural effect gives about 15 C.
Largest natural GHG: water vapour.
N2, O2, Ar are not GHGs.
Sulphate cools.
Black carbon warms.

Do Not Confuse With

Greenhouse effect and enhanced greenhouse effect Natural versus human-added
Sulphate and black carbon Cooling versus warming aerosol
Ozone layer and greenhouse ozone Stratospheric shield versus tropospheric gas

Memory Hook

Sunlight in, heat out, gases hold the heat; sulphate shades, soot warms.

Mains Angle

Questions ask for gases and aerosols. These points help.

Mechanism Infrared trapping
Gases Kyoto basket
Aerosols Cool and warm

A Question You May Face

An original practice question, not a past paper question.

“Explain the greenhouse effect, the role of greenhouse gases and the climate effects of aerosols.”

How to Answer

Mechanism Infrared.
Gases Basket.
Aerosols Types.
Policy SLCPs.

GS Relevance

Prelims: gases, aerosols, Kyoto basket. GS Paper 1 and 3: atmosphere and climate.

Frequently Asked Questions

What is the greenhouse effect?

The greenhouse effect is the warming of the Earth's surface and lower atmosphere because gases such as water vapour, carbon dioxide and methane absorb the infrared radiation emitted by the surface and send part of it back down. The natural effect keeps the average temperature near 15 degrees Celsius, rather than about minus 18, so it makes life possible.

Which gases are greenhouse gases?

The main greenhouse gases are water vapour, carbon dioxide, methane, nitrous oxide and ozone, plus human-made halocarbons and fluorinated gases such as CFCs, HFCs, PFCs, sulphur hexafluoride and nitrogen trifluoride. The Kyoto Protocol covers carbon dioxide, methane, nitrous oxide, HFCs, PFCs and sulphur hexafluoride, with nitrogen trifluoride added later.

Why are nitrogen and oxygen not greenhouse gases?

Nitrogen and oxygen, with argon, make up about 99 percent of dry air, but their molecules are simple two-atom or single-atom forms that do not absorb infrared radiation in the way that three-atom or more complex molecules such as water vapour and carbon dioxide do. They therefore let heat radiation pass through.

How do aerosols affect climate?

Aerosols are tiny particles suspended in the air. Sulphate aerosols reflect sunlight and cool the surface, while black carbon absorbs sunlight and warms the air and snow. Aerosols also change cloud properties. Overall, human aerosols have had a net cooling effect that has partly masked greenhouse warming, but they harm health.

What are short-lived climate pollutants?

Short-lived climate pollutants are agents such as methane, black carbon, tropospheric ozone and some HFCs that stay in the atmosphere for days to about a decade but strongly affect climate. Cutting them reduces warming quickly and improves air quality and health, which is why initiatives like the Climate and Clean Air Coalition target them.

PYQ Practice — Statement Analysis

1 Water vapour is the largest natural greenhouse contributor.

True

And it acts as a feedback.

2 Nitrogen is a greenhouse gas.

False

It does not absorb infrared.

3 Black carbon warms the atmosphere.