The concentration of tropospheric carbon dioxide at Mauna Loa, Hawaii, has increased by about 13.5% between 1958 and 1994. Based on the assumption of doubled greenhouse gases, numerical models predict the stratopause to cool by up to 15 K. The mesosphere and thermosphere are expected to cool by 10 K and up to 90 K, respectively. In 1992, K. Serafimov and M. Serafimova (J. Atmos. Terr. Phys., 54, 847-850, 1992) suggested that measurements of radio wave absorption would be the most sensitive indicators of possible anthropogenic influences on the mesosphere and lower thermosphere.

Radio wave absorption basically depends upon electron density and electron-neutral collision frequency, which, in turn, is a function of neutral air temperature and neutral particle density. The effect of a cooling on the collision frequency is not obvious, since a temperature decrease implies a pressure increase.

The absorption of radio waves is obtained on routine base at many sites around the world by so-called riometers (relative ionospheric opacity -meter), which measure the intensity of the cosmic radio noise that has passed through the ionosphere. Since the galactic radio flux is constant over long periods of time, the day-to-day changes of apparent intensity are due to variations in ionospheric attenuation. The amount of absorption is obtained by comparing the measured power of the cosmic radio noise with the power expected for absence of absorption.

He we use the detailed 55-ion Sodankylä Ion Chemistry (SIC) Model to investigate the effect of a 10 K cooling of the entire mesosphere on radio noise absorption at 30 MHz. The altitude profile of the absorption is complicated, but the net effect is a decrease in absorption. Furthermore we study the temporal behaviour of the cosmic radio noise absorption measured by the Finnish Riometer Chain, comprising of 9 riometers at 7 Finnish stations between 60° N and 70° N.