What are the demands on an observational program to detect trends in upper tropospheric water vapor anticipated in the 21st century?

An ensemble of runs made by a regional climate model
was used to estimate the expected change in humidity in the atmosphere between 1950 and 2100. Output from the model runs at a midlatitude European site is used as a proxy for a perfect climate record of specific humidity at the 300 hPa level from which trends were derived. The trend analysis demonstrates that it takes 30 years for a trend to show up in the perfect climate record. The model output was degraded to simulate observations made with the best radiosonde equipment for recording water vapor. Results indicate that it would be necessary to conduct observations for at least five decades until a trend expected by current models could be confirmed by observations to within 20%.