Direct answer: Anthropogenic emissions have increased atmospheric CO2 concentrations by transferring carbon from long-term stores into the atmosphere faster than natural carbon sinks can remove it. This strengthens the enhanced greenhouse effect, causing global warming and climate change.
Level: SL and HL: Subtopic 6.2: Climate change: causes and impacts.
The Carbon-Cycle Mechanism
The atmosphere is a carbon store, while processes such as combustion, photosynthesis and ocean absorption are flows. Human activities alter the balance between these flows.
| Anthropogenic activity | How it increases atmospheric CO2 |
|---|---|
| Fossil-fuel combustion | Burning coal, oil and natural gas transfers carbon from long-term geological stores into the atmosphere as CO2. |
| Deforestation | Burning or decomposing cleared vegetation releases stored carbon. Removing trees also reduces photosynthetic carbon uptake. |
| Cement production | Heating carbonate materials releases CO2, while the required energy often comes from fossil fuels. |
| Land-use change | Soil disturbance and vegetation removal can turn ecosystems from carbon sinks into carbon sources. |
Natural sinks, especially oceans and terrestrial ecosystems, absorb some anthropogenic CO2. However, they do not remove all emissions. For example, if human activities emit 40 units of CO2 while natural sinks absorb 20 additional units, the remaining 20 units accumulate in the atmosphere.
Atmospheric concentration therefore rises when inputs exceed outputs. Long-term measurements, including the Keeling Curve from Mauna Loa, show a sustained increase, with anthropogenic emissions accelerating particularly since 1950.
A common misconception is that increased CO2 is caused mainly by natural respiration or volcanoes. These natural flows are part of the existing carbon cycle; the long-term rise results primarily from the additional human transfer of stored carbon and the weakening of biological sinks.
Exam Technique
For an explain question, identify a human activity, describe the carbon transfer, and link it explicitly to atmospheric accumulation. Do not stop at “humans release CO2.” Use systems terminology such as store, flow, source and sink, then connect rising CO2 to the enhanced greenhouse effect.