The enhanced greenhouse effect has increased global mean temperatures and changed wider climate patterns, including precipitation, extreme weather, ice cover and sea level. In IB ESS subtopic 6.2, this is core content for both SL and HL.
The mechanism
The natural greenhouse effect keeps Earth warm enough for life. Greenhouse gases absorb and re-emit some of the infrared radiation released by Earth's warmed surface, reducing the rate at which energy escapes to space.
Human activities, especially fossil-fuel combustion and deforestation, have increased atmospheric concentrations of greenhouse gases such as carbon dioxide and methane. This enhanced greenhouse effect creates an energy imbalance: more infrared radiation is retained, so the atmosphere and oceans warm until the Earth system approaches a new, hotter equilibrium.
The common misconception is that the greenhouse effect itself is harmful. It is natural and essential; anthropogenic enhancement of it causes current global warming and climate change.
| Climate-system change | Consequence |
|---|---|
| Higher atmospheric and ocean temperatures | More frequent or intense heat extremes and changing evaporation rates |
| Changed atmospheric and ocean circulation | Shifts in precipitation, including greater drought or flood risk in some regions |
| Melting land ice and thermal expansion | Rising sea levels and increased risk to coastal communities |
| Warmer oceans | Coral bleaching, altered marine food webs and biome shifts |
| Reduced snow and sea-ice cover | Lower albedo, creating a positive feedback loop that amplifies warming |
For example, repeated mass coral-bleaching events on Australia's Great Barrier Reef demonstrate how increased ocean temperatures can reduce ecosystem resilience. Climate impacts are unevenly distributed: at HL, students should connect this to environmental justice, because societies that contributed least to emissions are often highly vulnerable.
Exam technique
For an explain question, link the full causal chain: anthropogenic emissions increase greenhouse gas concentrations, more infrared radiation is absorbed and re-emitted, global temperature rises, and climate patterns change. Do not describe climate change as temperature increase alone; include wider systemic impacts and a named example.