Greenhouse Gases

Greenhouse Gases

• Since the Industrial Revolution, human activities like burning fossil fuels, deforestation, and agriculture have altered the atmosphere’s composition

• These actions release GHG that trap heat, keeping the troposphere warmer than it would be naturally.

• This has caused a radiative imbalance—more energy is retained than released— pushing the climate system out of equilibrium.

• Amplified by feedbacks, this imbalance drives accelerated global warming observed since the 1970s, reaching unprecedented temperatures in thousand of years

• Greenhouse gases make Earth habitable

Global warming

• It is an increase in average global temperature that can lead to climate change

• The increase in Earth’s average surface temperature due to the buildup of greenhouse gases (e.g. CO2, methane)

• Key facts

  • Earth’s temperature has risen by ~1.1 degree C

  • Most warming has occurred in the past 40 years

• Primary cause: Human activities, especially burning fossil fuels

GreenHouse Effect:

• The greenhouse effect occurs when gases in the Earth’s atmosphere trap heat emitted by the planet, preventing it from escaping back into space

• Human Contribution:

  • Burning fossil fuels (for energy, factories, cars, buses) generates greenhouse gas emissions

  • These gases act like a blanket wrapped around the Earth, trapping the sun’s heat and raising temperatures

Greenhouse Gases: Types

• Not the Sun or Orbits:

  • Orbital cycles and solar energy influence climate but do not explain global warming

• Main Driver:

  • Increased greenhouse gases in the atmosphere are the only mechanism consistent with observed warming

• Key greenhouse gases

  • Water vapor (H2O)

  • Carbon dioxide (CO2)

  • Methane (CH4)

  • Nitrous oxide (N2O)

• Mechanism:

  • These gases absorb infrared radiation from Earth’s surface

  • Their molecular bonds vibrate like springs, converting radiation into heat(kinetic energy)

  • Re-radiate heat in all directions— some escapes, some returns to earth, amplifying warming

• Main greenhouse gases: CO2 and CH4

• CO2 comes from using gasoline for driving a car or coal for heating a building

• Clearing land and cutting down forests can also release CO2

• Agriculture, oil, and gas operations are major sources of CH4 emissions

• Energy, industry, transport, buildings, agriculture, and land use are among the main factors causing greenhouse gases

Greenhouse Gas Rise

• Since the Industrial Revolution activities like fossil fuel burning, deforestation, and agriculture have released long-lived GHG

• Key GHGs:

  • CO2, CH4, N2O, CFC-12, CFC-11

  • Together they contribute ~96% of direct radiative forcing since 1750

  • Remaining 4% from other halogenated gases (e.g. HCFC-22, HFC-134a)

• GHG concentrations have surged over the past 35 years, disrupting the Earth’s energy balance.

• This imbalance, amplified by climate feedbacks, drives accelerated global warming, with modern temperatures exceeding those of the past thousands of years

Water Vapor

• Most abundant Greenhouse Gas:

  • Accounts for 36%-70% of the total GHG effect

  • Up to 3% by mass in warm, tropical near-surface air

  • Can be 10,000x more abundant than other GHG

• Not a Climate Driver:

  • Short residence time— only hours to days before precipitation

  • Requires other long-lived GHG to maintain atmospheric warmth

• Positive feedback Loop:

  • Warming from CO2, CH4, etc. allows more water vapor to stay in the atmosphere

  • Water vapor amplifies warming, reinforcing the greenhouse effect

Carbon Dioxide

• Natural CO2 Role:

  • Emitted by volcanoes & geological processes; essential for photosynthesis and climate stability

  • Acts as a GHG that prevents Earth from freezing

• Ocean-Atmosphere CO2 Exchange:

  • CO2 dissolves in cool seawater; ocean acts as both sink and source

  • Warming—> CO2 release—> more warming;

  • Cooling—> CO2 uptake —> more cooling

• Anthropogenic Disruption:

  • Pre-industrial CO2 ~280 ppm; equilibrium state

  • Modern emissions—> oceans now absorb more CO2 than they emit

CO2: Current Levels & Trends:

• 2022: 15 gt carbon emitted —> 45% in atmosphere, 30% in oceans, 25% in biosphere

• June 2023: 422 ppm CO2—> ~48% increase from pre-industrial levels

• Keeling Curve: Long-term CO2 rise, documented since 1958, shows steady and accelerating growth

• Long Residence Time:

  • 50% remains after 30 years, 30% gone by 2300, but 20% persists for millennia

  • Long-term climate legacy from today’s emissions

Methane

• Potency & lifespan

  • CH4 is ~20x stronger than CO2 over 100 years in warming the atmosphere

  • Shorter lifespan:~10-12 years before oxidizing to CO2 +H2O

• Natural Sources: produced naturally by anaerobic bacteria in

  • Wetlands, termites, ocean floor, permafrost

  • Stored as methane hydrates in deep oceans & Arctic

• Human Sources

  • Paddy fields, coal mining, gas production, landfills, livestock

  • >60% of global CH4 emissions are anthropogenic

• Trends & Risks

  • CH4 levels have increased >150% since 1750 (~1.88 ppm)

  • Rising again due to melting permafrost & hydrate release

  • Sea-level rise —> more wetlands—> more CH4 (positive feedback)

Nitrous Oxide

• Potency & Persistence

  • 310x stronger than CO2 per kg (100-year scale)

  • Very long lifespan: over 150 years in the atmosphere

  • Present at 0.334 ppm, but highly impactful

• Natural Sources

  • Bacterial activity in soils (tropical & temperate) and oceans

• Human Sources (~40% of total)

  • Agriculture (especially fertilizers), livestock, biomass burning, and industry

• Trends & Concerns

  • Atmospheric N2O is increasing by .26% per year

  • Current level is 18% higher than preindustrial (~270 ppb)

Ozone

• Location & Role

  • Found in both stratosphere and lower troposphere

  • In troposphere, acts as an anthropogenic GHG

• Sources

  • By-product of photochemical reactions involving nitrogen oxides and CO

  • Emissions come from industry, internal combustions engines, and biomass burning

• Concentration & Lifetime

  • Tropospheric concentration: .03-.06 ppm

  • Short residence time: 25 days

• Warming Contribution

  • Accounts for 3%-7% of observed global warming

  • 4th most important GHG

• Historical Increase

  • Increased by ~36% since the Industrial Revolution

  • Nearly doubled since 1800

• Montreal Protocol (1987)

  • International treaty to limit emissions of ozone-depleting substances (ODS)

  • Driven by global public pressure and scientific evidence of ozone depletion

• Success & Recovery

  • Ozone depletion reduced to ~4% below 1964-1980 average

  • Ozone layer is gradually recovering due to global compliance

Halocarbons

• Human-Made Chemicals in the Atmosphere

  • Released over the past 20 years from industrial processes

  • Includes:

    • Chlorofluorocarbons (CFCs)

    • Perfluorocarbons (PFCs)

    • Hydrofluorocarbons (HFCs)

    • Sulfur hexafluoride (SF6)

    • 34+ other trace chemicals identified by the IPCC

• Potency vs Concentration

  • Present in parts per trillion (ppt)— extreme concentrations

  • But 2,000-3,000 times more effective than CO2 in trapping heat

  • Some are up to 23,900 times more potent than CO2

• Climate Impact

  • Despite low concentrations, these gases contribute to global warming

  • Cumulative effect matters, especially with long atmospheric lifespans

Climate Sensitivity to GHG

• What is Climate Sensitivity?

  • Measures how much Earth’s surface temperature rises with increased GHG, especially CO2

  • Two main types:

    • TCR ( Transient Climate Response): short-term warming before full adjustment

      • Range 1.2 degrees C to 2.5 degrees C

    • ECR (Equilibrium Climate Response): Long-term warming after full system balance

      • Range 1.8 degrees C to 5.6 degrees C

$\delta$ Ts= $\lambda$ $\delta$ F

• Where:

  • $\delta$ Ts= change in surface temperature

  • $\lambda$ = climate sensitivity (~0.8 K/Wm²)

  • $\delta$ F= radiative forcing (W/m²)

  • Example: for $\delta$ F = 2.0 W/m² —> $\delta$ Ts= 1.6 degrees C

• Positive Feedbacks (water vapor, ice melt) amplify warming

• Feedbacks cause uncertainty in long- term projections

• Climate sensitivity determines how much Earth warms with rising CO2

• Understanding it is crucial for climate policy and risk assessment

Heating the Atmosphere

• Solar Radiation reaches Earth spread over a wide area, with peak intensity in UV, visible, and Infrared wavelengths.

• Earth’s spherical shape and rotation reduce average incoming solar energy to ~342 W/m²

• Earth absorbs sunlight and emits infrared radiation back to space

• Greenhouse gases (CO2, CH4, H2O) absorb this infrared radiation, trapping heat and warming the atmosphere

• This process explains why earth is warmer with an atmosphere and supports the hypothesis that GHG are the primary drivers of recent climate change

Solar Energy Distribution on Earth

• If Earth were flat and directly facing the sun, it would receive ~1,370 W/m² at the top of the atmosphere— like 14 household 100-watt bulbs over a table

• Because Earth is a sphere, solar energy is spread across the sunlit hemisphere, reducing it to ~680W/m²

• This explains why solar energy is diluted as it reaches and spreads out over Earth’s surface

Uneven Solar Energy Distribution

• The Sun’s energy is not evenly distributed across Earth’s surface

• Due to Earth’s spherical shape, sunlight strikes the equator directly, but hits the poles at an angle, spreading energy over a larger area

• Earth’s tilt causes seasonal darkness at each pole during winter, further reducing solar input there

• These differences drive climate patterns, contributing to temperature gradients between the equator and poles

Earth’s radiation Balance

• As Earth absorbs solar energy, it also emits infrared (IR) radiation back into space from all surfaces, day and night

• This terrestrial radiation is in the infrared spectrum and is about 1 million times less intense than solar radiation

• The amount of energy Earth emits varies by season, latitude, and longitude

• Over time, Earth reaches a radiative equilibrium— a balance between incoming solar and outgoing infrared radiation— measured at the top of the troposphere

Energy Imbalance and Climate Dynamics

• Uneven solar heating within the troposphere creates a global energy imbalance

• Equator warms more than poles, leading to a strong equator-to-pole temperature gradient

• This gradient drives atmospheric and ocean circulation, shaping Earth’s climate

• Polar ice reflects much of the sun’s energy (high albedo), reinforcing cooling at the poles and amplifying the temperature difference