  The growing concentrations of the greenhouse gases CO2, CH4 and N2O (GHG) in
the atmosphere are often considered as the dominant cause for the global
warming during the past decades. The reported temperature data however do not
display a simple correlation with the concentration changes since 1880 so that
other reasons are to be considered to contribute notably. An important feature
in this context is the shrinking of the polar ice caps observed in recent
years. We have studied the direct effect of the loss of global sea ice since
1955 on the mean global temperature estimating the corresponding decrease of
the terrestrial albedo. Using a simple 1-dimensional model the global warming
of the surface is computed that is generated by the increase of GHG and the
albedo change. A modest effect by the GHG of 0.08 K is calculated for the
period 1880 to 1955 with a further increase by 0.18K for 1955 to 2015. A larger
contribution of 0.55 +/-0.05 K is estimated for the melting of polar sea ice
(MSI) in the latter period, i.e. it notably exceeds that of the GHG and may be
compared with the observed global temperature rise of 1.0 +/- 0.1 K during the
past 60 years. Our data also suggest a delayed response of the mean global
temperature to the loss of sea ice with a time constant of approximately 20
years. The validity of the theoretical model and the interrelation between
GHG-warming and MSI-effect are discussed.
