The Intergovernmental Panel of Climate Change (IPCC) confirmed concentrations of atmospheric greenhouse gases and radiative forcing to increase as a result of human activities. Nevertheless, changes in radiative forcing related to increasing greenhouse gas concentrations could not be experimentally detected at Earth's surface so far. Here we show that atmospheric longwave downward radiation significantly increased (+5.2(2.2) Wm⁻²) partly due to increased cloud amount (+1.0(2.8) Wm⁻²) over eight years of measurements at eight radiation stations distributed over the central Alps. Model calculations show the cloud-free longwave flux increase (+4.2(1.9) Wm⁻²) to be in due proportion with temperature (+0.82(0.41) °C) and absolute humidity (+0.21(0.10) g m⁻³) increases, but three times larger than expected from anthropogenic greenhouse gases. However, after subtracting for two thirds of temperature and humidity rises, the increase of cloud-free longwave downward radiation (+1.8(0.8) Wm⁻²) remains statistically significant and demonstrates radiative forcing due to an enhanced greenhouse effect.