Global warming — causes, evidence and consequences

Global warming is the recent rapid increase in the Earth's average surface temperature, caused mainly by human activity. The bigger picture — long-term shifts in weather patterns and biodiversity — is called climate change.

Greenhouse effect (the science)

Some gases in the atmosphere absorb infrared radiation that the Earth re-emits, trapping heat near the surface. The main greenhouse gases are:

• Carbon dioxide (CO₂) — released by combustion, deforestation.
• Methane (CH₄) — released by livestock (cattle), rice paddies, landfill, wetlands.
• Water vapour — also a greenhouse gas, but its level mostly responds to others.

The greenhouse effect is natural and necessary — without it, Earth would be ~33 °C colder. The problem is that human activity has increased the concentration of these gases, intensifying the effect.

Evidence for global warming

• Global average surface temperature has risen by about 1 °C since pre-industrial times.
• Atmospheric CO₂ concentration has risen from ~280 ppm (1850) to >420 ppm today (Keeling curve).
• Polar ice caps and glaciers shrinking — measurable from satellite imagery.
• Sea levels rising — about 20 cm in the last 100 years.
• Shifts in species distribution — animals and plants moving polewards.
• Coral bleaching events in warm seas.

Major human contributions to global warming

1. Burning fossil fuels (coal, oil, gas) for electricity, heating and transport — large CO₂ source.
2. Deforestation — fewer trees absorb CO₂; burning the trees releases it.
3. Agriculture — cattle release methane; rice paddies release methane; fertiliser breakdown.
4. Landfill waste — anaerobic decay releases methane.

Consequences for biodiversity and ecosystems

• Loss of habitat — polar bears as ice melts; coral as oceans warm/acidify.
• Range changes — many species shift towards the poles or up mountains; those at the limits run out of space (e.g. high-altitude alpine flora).
• Mismatched timing — pollinators emerging at different times to flowering, disrupting food chains.
• Extreme weather — storms, droughts, floods cause local extinctions.
• Sea level rise — threatens coastal habitats (mangroves) and human settlements.
• Ocean acidification (from absorbed CO₂) — harms shell-forming organisms.

Why some uncertainty? (the science of evidence)

Climate is naturally variable — temperatures fluctuate from year to year. But the long-term trend over decades is clear, and:

• Multiple independent lines of evidence agree.
• Computer models matching observed warming require human CO₂ emissions to fit the data.
• Almost all (~97 %) of climate scientists agree.

The exam doesn't expect you to debate climate change — only to describe the evidence and effects.

What can be done? (lead-in to B7.7)

• Reduce burning fossil fuels: switch to renewables (solar, wind, hydro), nuclear.
• Improve energy efficiency.
• Reduce deforestation; reforest where possible.
• Reduce meat consumption / improve livestock practices to cut methane.
• Capture methane from landfill and biogas plants.
• Carbon capture and storage (engineering solution).

Links

Built on B7.3 (carbon cycle), B7.5 (deforestation, peat). Connects to B7.7 (maintaining biodiversity) and B7.9 (food security).