The greenhouse effect is absolutely essential for life on Earth. Without the natural greenhouse effect, the temperature on the planet’s surface would be -18 °C, as opposed to our present-day average of 15 °C. The reason for these cosier temperatures is that the solar radiation received by Earth is not all released back into space – some of it is trapped by the atmosphere, like in a greenhouse. Shortwave solar radiation penetrates the atmosphere relatively unimpeded; at the surface of the planet it is transformed into longwave heat radiation and reflected back out towards space. This longwave radiation, however, cannot penetrate the atmosphere as easily as shortwave radiation. That means it is partly reflected back to Earth, heating up its surface. This is the effect of trace gases such as water vapour, carbon dioxide and methane in the atmosphere, which let shortwave solar radiation pass fairly easily but keep back the longwave radiation coming from Earth’s surface. Human activity has influenced the greenhouse effect in a number of different ways, causing a rise in our planet’s temperature – the “anthropogenic greenhouse effect”.
Saskia Blank of the Ideas 2020 team answered this question.