Global warming and climate change: evidence, causes, greenhouse gases, physical and human factors

Atmospheric Processes and Global Climate Change

Learning Objective

Explain the evidence for recent global warming, distinguish natural from anthropogenic causes, describe the role of greenhouse gases and the energy‑budget processes that drive climate, and evaluate the physical and human factors that modify climate change.

2.1 Energy Budgets

2.1.1 Components of the Earth’s Energy Budget

Natural greenhouse effect: The 150 W m⁻² of absorbed long‑wave radiation represents the natural greenhouse effect – the process by which atmospheric gases trap part of the Earth’s infrared emission, keeping the surface ≈ 33 °C warmer than it would be without them.

2.1.2 Latitudinal Pattern of Radiation

• Equatorial regions receive the greatest solar energy (≈ 420 W m⁻²) → net energy surplus.
• High‑latitude regions receive far less (≈ 150 W m⁻²) → net energy deficit.
• Excess energy is transferred pole‑ward by atmospheric and oceanic circulations (see 2.1.3).

2.1.3 Global Transfers of Energy

Atmospheric circulation – three‑cell model

• Hadley cells (0°–30°): warm air rises at the equator, moves pole‑ward aloft, descends in the subtropics (trade‑wind belts).
• Ferrel cells (30°–60°): mid‑latitude westerlies; air rises at the poleward edge of the Hadley cell and sinks at the equatorward edge.
• Polar cells (60°–90°): cold air descends at the poles, rises at ≈ 60° latitude, completing the meridional loop.
• Jet streams: fast upper‑tropospheric winds that mark the boundaries between cells – the subtropical jet (≈ 30°) and the polar jet (≈ 60°). They transport heat east‑west and influence storm tracks.

Oceanic circulation

• Surface currents (e.g., Gulf Stream, Kuroshio, Brazil Current) – driven by wind stress, Coriolis effect and continental boundaries; move warm water pole‑ward.
• Thermohaline (deep) circulation – dense, cold, salty water sinks in high‑latitude regions and returns equator‑ward at depth, forming the global “conveyor belt”.
• Ocean currents carry ≈ 90 % of the meridional heat transport, making them the dominant pathway for excess tropical heat.

2.1.4 Seasonal & Diurnal Variations

• Seasonal variation – Earth’s 23.5° axial tilt changes the distribution of insolation, creating summer‑winter cycles. The seasonal contrast drives monsoon systems (e.g., South‑Asian summer monsoon) and modifies the strength of atmospheric cells.
• Diurnal variation – Day‑night heating and cooling. Land surfaces (low heat capacity) experience larger temperature swings than oceans, producing a greater diurnal temperature range over continents.
• Both variations affect cloud formation, jet‑stream position and the magnitude of the greenhouse effect.

2.2 Evidence of Global Warming

• Global mean surface‑air temperature rise of ≈ +1.1 °C since the pre‑industrial period.
• Increasing ocean heat content and sea‑surface temperatures.
• Widespread glacier retreat, loss of Arctic sea‑ice extent, and thinning of the Antarctic ice sheet.
• Global mean sea‑level rise of ≈ 20 cm since 1900 (thermal expansion + ice melt).
• Shifts in phenology – earlier flowering, leaf‑out, and animal migrations.
• Instrumental records (thermometers, satellites) corroborated by proxy data:
  • Tree‑ring widths (temperature & precipitation).
  • Ice cores (past CO₂, CH₄ concentrations and temperature proxies).
  • Sediment cores and pollen records (vegetation changes).

2.3 Causes of Climate Change

2.3.1 Natural Factors

1. Solar variability – 0.1 % change in total solar irradiance over the 11‑year sun‑spot cycle.
2. Volcanic eruptions – Stratospheric sulphate aerosols reflect solar radiation, producing short‑term cooling (e.g., Mt. Pinatubo 1991, ~0.5 °C cooling for 2–3 years).
3. Orbital variations (Milankovitch cycles) – Eccentricity, axial tilt and precession modify the distribution of insolation over 10⁴–10⁵ year timescales, driving ice‑age cycles.
4. Natural greenhouse‑gas fluctuations – Methane from wetlands, permafrost, and natural fires.

2.3.2 Anthropogenic (Human) Factors

1. Combustion of fossil fuels (coal, oil, gas) → CO₂ emissions.
2. Deforestation and land‑use change → reduced carbon uptake, altered albedo and surface roughness.
3. Agricultural practices → CH₄ from ruminants & rice paddies; N₂O from synthetic fertilisers.
4. Industrial processes → fluorinated gases (CFCs, HFCs, PFCs, SF₆) with very high GWP.
5. Waste management → landfill methane emissions.
6. Urbanisation → heat‑island effect, higher energy demand and modified moisture fluxes.

2.4 Greenhouse Gases (GHGs)

GHGs absorb outgoing long‑wave infrared radiation, producing a net warming known as the enhanced greenhouse effect** when concentrations increase above pre‑industrial levels.

Radiative forcing from a change in CO₂ concentration can be approximated by:

ΔF = 5.35 ln(C / C₀)

where C = present concentration and C₀ = pre‑industrial concentration.

¹ GWP values are relative to CO₂ over a 100‑year time horizon (IPCC AR6, 2021).

2.5 Physical Factors Influencing Climate

• Solar radiation – primary energy source; long‑term variations (e.g., solar cycles) affect the whole system.
• Albedo – surface reflectivity; loss of ice and snow lowers albedo, increasing absorbed solar energy.
• Atmospheric circulation – Hadley, Ferrel and Polar cells, jet streams and monsoon systems redistribute heat.
• Ocean currents – surface gyres and the thermohaline conveyor move the majority of excess tropical heat pole‑ward.
• Volcanic aerosols – sulphate particles in the stratosphere reflect sunlight, causing temporary cooling.
• Orbital parameters (Milankovitch cycles) – eccentricity, axial tilt and precession modulate insolation patterns over tens to hundreds of thousands of years.

2.6 Human Factors Amplifying Climate Change

1. Increased fossil‑fuel consumption → higher CO₂ and CH₄ emissions.
2. Land‑use change (deforestation, urban expansion) → reduced carbon sinks, lower albedo and heat‑island effects.
3. Agricultural intensification → CH₄ from ruminants/rice, N₂O from fertilisers.
4. Industrial production of synthetic gases with very high GWP.
5. Infrastructure development → changes in surface roughness, moisture fluxes and energy demand.

2.7 Impacts of Global Warming

• More frequent and intense heatwaves.
• Altered precipitation patterns – increased drought risk in some regions and heightened flood risk in others.
• Sea‑level rise threatening low‑lying coastal communities and island nations.
• Ocean acidification (↑ H⁺ from dissolved CO₂) harming coral reefs and shell‑forming organisms.
• Shifts in species distributions, phenology and higher risk of biodiversity loss.
• Economic and health impacts – reduced agricultural productivity, spread of vector‑borne diseases, and greater energy demand for cooling.

2.8 Mitigation and Adaptation Strategies

Effective responses combine mitigation (reducing GHG emissions) with adaptation (preparing for unavoidable changes).

2.8.1 Mitigation

• Transition to renewable energy (solar, wind, hydro, tidal, geothermal).
• Improve energy efficiency in industry, transport and buildings.
• Reforestation, afforestation and sustainable forest management to enhance carbon sinks.
• Carbon capture and storage (CCS) and emerging negative‑emission technologies (e.g., bio‑energy with CCS).
• Policy instruments:
  • Carbon taxes – price the carbon content of fuels.
  • Emissions trading schemes (ETS) – cap‑and‑trade systems.
  • Subsidies and feed‑in tariffs for low‑carbon technologies.

2.8.2 Adaptation

• Coastal protection – sea walls, managed retreat, mangrove restoration.
• Water‑resource management – efficient irrigation, rain‑water harvesting, drought‑resistant crops.
• Climate‑resilient agriculture – altered planting dates, crop diversification, agroforestry.
• Urban planning – green roofs, reflective surfaces, expanded public transport to reduce heat‑island effects.
• Early‑warning systems for extreme weather events and disaster‑risk reduction strategies.

2.9 Suggested Diagrams for Examination

• Global energy‑budget diagram (showing S_in, reflected short‑wave, OLR, and greenhouse‑gas absorption).
• Latitudinal radiation diagram (equatorial surplus → polar deficit).
• Three‑cell atmospheric circulation with labelled trade‑winds, westerlies, polar easterlies and both jet streams.
• World map of major surface ocean currents and a schematic of the thermohaline conveyor belt.
• Greenhouse‑effect schematic (short‑wave in, surface absorption, long‑wave out, GHG absorption and re‑emission).

2.10 Summary Checklist for Examination (AO1–AO3)

1. Identify and explain at least three pieces of evidence for recent global warming (instrumental and proxy).
2. Describe the four components of the Earth’s energy budget and sketch the global energy‑budget diagram.
3. Explain how atmospheric cells, jet streams and major ocean currents transfer excess tropical energy to higher latitudes.
4. Distinguish natural from anthropogenic causes of climate change, including solar variability, volcanic aerosols, Milankovitch cycles, and human emissions of CO₂, CH₄, N₂O and fluorinated gases.
5. List the major greenhouse gases, their main sources, atmospheric lifetimes and 100‑yr GWP values.
6. Discuss the physical factors that influence climate (solar radiation, albedo, circulation, ocean currents, volcanic aerosols, orbital changes).
7. Explain how human activities amplify the natural greenhouse effect (enhanced radiative forcing).
8. Outline key impacts of climate change on physical systems (temperature, precipitation, sea level, oceans) and on human systems (health, agriculture, economies).
9. Evaluate mitigation and adaptation options, giving concrete examples and mentioning relevant policy tools (carbon tax, ETS, subsidies).