Image of a molecule of water (H2O), a carbon dioxide molecule (CO2) and a methane molecule (CH4). Greenhouse gases allow the inward passage of high energy rays from the sun but they absorb the lower energy infrared radiation that is then emitted from the warmed Earth. This is analogous to the way a greenhouse (or glasshouse) warms up (the glass allows in the suns rays but then traps the heat inside). These gases help to keep the Earth at a reasonable temperature but the concern is that the concentrations of some of them in the atmosphere (notably CO2 and CH4) is increasing and that this will lead to an increase in the Earth's temperature, with consequent climatic changes. Predicting what will happen requires complex computer modelling but the scientific consensus is that we are warming up.