Is there observational evidence for greenhouse cooling at thermospheric altitudes?

Thomas Ulich and Esa Turunen

Geophysical Observatory, FIN-99600 Sodankylä, Finland


Extensive model calculations show that increasing amount of greenhouse gas concentrations in the air cause cooling of the entire middle and upper atmosphere. Based on a scenario of doubling the CO2 and CH4 concentrations, most authors predict a cooling of the order of 10 K in the mesosphere. Observational evidence of cooling is scarce, but tentatively existing. Latest temperature measurements by lidar and from satellites show cooling trends. However these cover only short time intervals, in the order of one or two solar cycles. Evidence in the form of increasing occurence of the noctilucent clouds is suggested. Such observations are difficult to quantify reliably in a long term data set. Lowering of the reflection heights of low frequency radio waves has been presented as evidence of cooling. At thermospheric altitudes Risbeth (1990) predicted cooling by 50 K to cause lowering of the altitude of maximum electron density in the F2 region by 20 km. Observational evidence for such lowering was presented by Bremer, who investigated almost 40 years of data from ionospheric vertical soundings at one single location in mid-latitudes. We examine a larger set of ionospheric data from different stations. While clear cooling is evident at many sites, some ionosonde records show exactly the opposite effect. The interpretation of these observations is not yet clear.