  Observed changes such as increasing global temperatures and the
intensification of the global water cycle in the 20th century are also robust
results of coupled general circulation models. In spite of this success
model-to-model variability and biases that are small in first order climate
responses however, have implications for climate predictability especially when
multi-model means are used. We show that most climate simulations of 20th and
21st century A2 scenario performed with IPCC-AR4 models have deficiencies in
simulating the global atmospheric moisture balance. Large biases of only a few
models affect the multi-model mean global moisture budget and an imbalanced
flux of -0.14 Sv exists whereas the multi-model median imbalance is only -0.02
Sv. For most models, the detected imbalances furthermore change over time. As a
consequence, in 13 of the 18 IPCC-AR4 models examined, global annual mean
precipitation exceeds global evaporation, indicating that there should be a
"leaking" of moisture from the atmosphere whereas for the remaining 5 models a
"flooding" is implied. The source / sink of moisture can also be regarded as
atmospheric latent heating / cooling and hence as positive / negative
perturbations of the atmospheric energy budget or non-radiative forcings in the
range of -1 to +6 W/m2 (median is +0.1 W/m2). The global atmospheric moisture
transport from oceans to land areas, which impacts the terrestrial water cycle
also increases by about 5% with a model-to-model range from 1 to 13% in the
21st century. The variability is partially due to the different implementations
of aerosol forcings in the models. The assessment of the pole wards shifts of
dry zones in climate simulations show that the multi-model means of the two
subsets of leaking and flooding models produce spatial shifts similar in size
as expected from 21st century global warming simulations.
