Is there observational evidence for greenhouse cooling at
thermospheric altitudes?
Thomas Ulich and Esa Turunen
Geophysical Observatory, FIN-99600 Sodankylä, Finland
Abstract
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.