The greenhouse mechanism, observations and attribution studies agree that human influence is the main cause of contemporary global warming.
Earth receives energy mainly as shortwave solar radiation. The surface absorbs energy and emits longwave infrared radiation. Greenhouse gases absorb and emit infrared at particular wavelengths. Their natural presence keeps the surface warmer than it would be without this atmospheric effect.
Adding long-lived greenhouse gases initially reduces the energy escaping to space at the same temperature. The surface and lower atmosphere warm until energy flows adjust. This enhanced greenhouse effect concerns outgoing longwave radiation, not a layer that simply reflects all incoming sunlight back down.
Attribution asks which influences explain observed change. Scientists combine measured greenhouse-gas increases, radiation physics, surface and ocean warming, and patterns through the atmosphere. Climate-model experiments compare responses to natural influences alone with responses that include human influences. No single hot day proves the cause of a long-term trend.
The IPCC, the Intergovernmental Panel on Climate Change, synthesises these independent lines of evidence. Its 2023 assessment concludes that human activities caused global warming and reports about 1.1 degrees C of warming in 2011-2020 relative to 1850-1900. That is a dated decade-average comparison, not the latest annual temperature.
Natural variability affects individual years and regions. Solar changes and volcanic aerosols also affect climate, but natural factors alone do not reproduce the observed sustained warming. Human-produced aerosols have partly offset greenhouse warming; this cooling influence does not remove the net human warming signal.
Step by step
Identify a physical cause
More greenhouse gases reduce outgoing energy at the initial temperature.
Compare evidence
Check observations, spatial patterns and model experiments.
Judge competing explanations
Test whether natural influences alone account for the sustained change.
Worked example: A fair attribution test
A short warm spell could result from weather variability. A multi-decade warming trend across land and ocean, combined with increasing greenhouse gases and their known radiative effects, provides much stronger evidence. Attribution depends on consistency across several lines of evidence, not just correlation in one graph.
Watch out for this
Past natural warming proves contemporary warming is natural.
The cause must be tested for the period in question. Different influences can produce warming at different times.
Check your understanding
Which comparison best supports attribution rather than merely detecting warming?
- Testing observed patterns against responses to natural-only and natural-plus-human influences.
- Showing that one city had a hot afternoon.
- Comparing global temperatures before and after industrialisation without testing other causes.