Full text: Atmospheric general circulation models (AGCMs) equipped with water isotope diagnostics present a major step forward in the understanding of the global cycle of the stable water isotopes, 18O and Deuterium. Several studies published until now focused on a detailed comparison between simulated isotope signals in meteoric water and the IAEA/GNIP network in order to gain further insight into the water cycle as numerically represented by the AGCMs. Another set of studies focused on the application of AGCMs on paleo time scales (from inter-annual to glacial/interglacial cycles). In our contribution here, we discuss the global balance of the water isotopes under varying boundary conditions. We use the ECHAM4 general circulation model which was run under boundary conditions corresponding to different time slices throughout the Holocene until the last glacial (pre-industrial, 6 Kyr BP, 11 Kyr BP, 11 Kyr BP, 14 Kyr BP, 16 Kyr BP, 3 different runs for 21 Kyr BP, 175 Kyr BP). A further simulation was performed corresponding to the estimated boundary conditions for a possible future doubling of the atmospheric CO2 concentration. The global balance of the water isotopes is controlled by the isotopic signal emitted in the tropics and subtropics. In these regions, principally sea surface temperatures and the relative humidity in the planetary boundary layer affect the isotopic composition of evaporated vapour. We therefore discuss the influence of this principal water vapour source on extra-tropical precipitation and its isotopic composition. We specifically focus on the possibility of a compensation effect between low and high latitudes in the global balance of the water isotopes. This approach is evaluated by analysing the global water isotope budget for the last 50 years on one hand as simulated by a long-term integration of the ECHAM4 model forced with observed SSTs for the same time period and, on the other hand, as observed by the IAEA/GNIP network.