318
compared to the period 2010-2020. As can be seen by comparing with Fig.
1, the calculated radiation balance agrees quite well with the observational
estimates as obtained from ERBE and from surface observations (e. g.
Hartmann (1993), Ohmura and Gilgen (1993), Hahn et al.(1994)). The
accuracy of the observational data is of the order of 5 Wjm 2 in global annual averages for most quantities. Absorption of both short and long wave
radiation in the atmosphere are still open to discussion and more measurements are required to obtain a better understanding of the 3-dimensional
distribution of radiation forcing. Table 4 shows the changes of the radiation at the top of the atmosphere, in the atmosphere and at the surface of
the Earth, respectively. Due to the ongoing increase in the greenhouse gas
forcing, the model is not in radiative equilibrium. The mean net heating
at the ground during the 2010-2020 is 1.08 Wjm 2 as the result of different
forcing processes as we will discuss below.
5.1
Short wave radiation
There is an increase in the short wave cloud forcing at the top of the atmosphere by 0.65 Wjm 2 due to enhanced cloud albedo. There is also an
increase in the short wave radiative forcing in the atmosphere due to a
higher absorption by water vapour in the atmosphere by 2.15 Wjm 2 . The
surface albedo is slightly reduced, 0.73 Wjm 2 , due to a general reduction
in snow- and ice-cover. The effect is a small reduction in planetary albedo,
an increased warming of the atmosphere and a reduced warming at the
surface. The reduced warming at the surface, 1.81 Wjm 2 , is both caused
by increased absorption in the atmosphere and to increased reflection by
clouds (short wave cloud forcing). The overall changes in short wave radiative processes are so small that it is probably not possible to detect them
by any known observational system.
5.2
Long wave radiation
The enhanced high-cloud emissivity is leading to a slight increase in the
long wave cloud forcing by 0.25 Wjm 2 at the top of the atmosphere. This
is less than the increase cooling due to the increased cloud albedo effect, so
it follows that the overall effect of clouds is to counteract the greenhouse
warming, that is a negative feedback. At the surface the direct and indirect
long wave radiative forcing due to greenhouse gases and water vapour is
equal to 4.57 Wjm 2 , in turn reduced by the increased long wave cloud
compared to the period 2010-2020. As can be seen by comparing with Fig.
1, the calculated radiation balance agrees quite well with the observational
estimates as obtained from ERBE and from surface observations (e. g.
Hartmann (1993), Ohmura and Gilgen (1993), Hahn et al.(1994)). The
accuracy of the observational data is of the order of 5 Wjm 2 in global annual averages for most quantities. Absorption of both short and long wave
radiation in the atmosphere are still open to discussion and more measurements are required to obtain a better understanding of the 3-dimensional
distribution of radiation forcing. Table 4 shows the changes of the radiation at the top of the atmosphere, in the atmosphere and at the surface of
the Earth, respectively. Due to the ongoing increase in the greenhouse gas
forcing, the model is not in radiative equilibrium. The mean net heating
at the ground during the 2010-2020 is 1.08 Wjm 2 as the result of different
forcing processes as we will discuss below.
5.1
Short wave radiation
There is an increase in the short wave cloud forcing at the top of the atmosphere by 0.65 Wjm 2 due to enhanced cloud albedo. There is also an
increase in the short wave radiative forcing in the atmosphere due to a
higher absorption by water vapour in the atmosphere by 2.15 Wjm 2 . The
surface albedo is slightly reduced, 0.73 Wjm 2 , due to a general reduction
in snow- and ice-cover. The effect is a small reduction in planetary albedo,
an increased warming of the atmosphere and a reduced warming at the
surface. The reduced warming at the surface, 1.81 Wjm 2 , is both caused
by increased absorption in the atmosphere and to increased reflection by
clouds (short wave cloud forcing). The overall changes in short wave radiative processes are so small that it is probably not possible to detect them
by any known observational system.
5.2
Long wave radiation
The enhanced high-cloud emissivity is leading to a slight increase in the
long wave cloud forcing by 0.25 Wjm 2 at the top of the atmosphere. This
is less than the increase cooling due to the increased cloud albedo effect, so
it follows that the overall effect of clouds is to counteract the greenhouse
warming, that is a negative feedback. At the surface the direct and indirect
long wave radiative forcing due to greenhouse gases and water vapour is
equal to 4.57 Wjm 2 , in turn reduced by the increased long wave cloud