How greenhouse warming works, and what the IPCC says

Sunlight warms the Earth's surface, and the surface gives off heat as infrared radiation. Greenhouse gases — carbon dioxide, methane, nitrous oxide and others — absorb some of that outgoing heat and re-emit it in all directions, including back towards the surface. This natural greenhouse effect keeps the planet habitable. The problem is the extra effect from added gases: burning fossil fuels, clearing forests and certain farming and industrial practices raise greenhouse-gas concentrations, trap more heat and shift the climate.

The Intergovernmental Panel on Climate Change, the IPCC, does not conduct its own experiments. It assesses thousands of peer-reviewed studies and reports where scientists agree, where they disagree and how confident they are. Its Sixth Assessment Report, completed with the Synthesis Report in March 2023, concluded that human activities have unequivocally caused global warming, with global surface temperature reaching about 1.1 degrees Celsius above 1850–1900 in 2011–2020.

That average hides important differences. Land has warmed more than oceans, the Arctic more than the global average, and extremes have changed faster than averages: hotter heatwaves, heavier rainfall in many places and longer droughts in others. The IPCC also found that every increment of warming intensifies multiple hazards at the same time, so a hotter world is not simply a warmer version of the present one; it is one where heat, flood, fire and crop stress can coincide.

For policy, the IPCC's message is about choices and timing. Deep, rapid and sustained cuts in emissions would slow warming within around two decades, according to the Synthesis Report, while continued emissions make 1.5 degrees harder to hold and increase the need for adaptation. Adaptation — water storage, heat planning, resilient housing — is therefore not an alternative to cutting emissions; both are needed, and the balance between them is a political decision. Students should also distinguish weather, which is short-term, from climate, which is the pattern of weather over decades.