Human activities add greenhouse gases or reduce their removal. This strengthens the greenhouse effect and raises global temperatures.
Population growth can increase demand for energy, transport, housing and food. Industrialisation expands production that often uses large amounts of energy. Emissions also depend on consumption per person, technology, energy sources and policy. People and countries therefore contribute unequally.
Burning coal, oil and gas releases carbon from long-term geological stores as carbon dioxide. Power generation, transport and industry are major sources. Methane comes from fossil-fuel leaks, livestock digestion, rice cultivation and decomposing organic waste. Agricultural soils and nitrogen fertilisers also release nitrous oxide.
Forest clearance has two effects. Burning vegetation and disturbing carbon-rich soils release stored carbon. Fewer growing trees then remove carbon dioxide through photosynthesis. Regrowth can absorb carbon again, but it may take a long time and may not replace all that was lost.
Measurements show long-term warming despite short-term fluctuations. The IPCC's 2021 assessment found that each of the four decades ending in 2020 was warmer than the one before it in the record since 1850. Its 2023 synthesis put the 2011-2020 global average about 1.1 degrees C above 1850-1900. These figures refer to those periods and the global average; local changes differ.
Step by step
Release greenhouse gases
Fuel burning, industry, farming and waste add gases to the atmosphere.
Reduce removal
Forest clearance releases stored carbon and leaves fewer trees to absorb carbon dioxide.
Strengthen warming
Higher greenhouse-gas concentrations enhance the greenhouse effect.
Worked example: A two-part forest explanation
Burning cleared vegetation releases carbon dioxide, and the smaller forest removes less carbon dioxide through growth. These changes can raise atmospheric concentrations, strengthening the greenhouse effect. Counting only smoke misses the lost sink.
Watch out for this
The country with the largest population must emit the most per person.
Total and per-person emissions answer different questions. Energy sources, consumption and production systems matter as well as population.
Check your understanding
Which answer best explains the climate effect of forest clearance?
- Only the colour of the land changes.
- Stored carbon may be released and future carbon uptake may decrease.
- Every new field instantly removes the same carbon as the old forest.