Can we expect more extreme precipitation on the monthly time scale?

The Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report states that instrumental records show an increase in precipitation by +0.5%-1% decade(-1) in much of the Northern Hemisphere mid- and high latitudes and a decrease of -0.3% decade(-1) over subtropical land areas. It has been postulated that these trends are associated with the enhanced levels of atmospheric CO2. In this context, it is natural to ask how continuing rising levels of CO2 may affect the climate in the future. The past IPCC reports have documented numerous studies where increased greenhouse gas concentrations have been prescribed in global climate model simulations. Now, new simulations with state-of-the-art climate models are becoming available for the next IPCC report [the Fourth Assessment Report (AR4)], and results from a number of these simulations are examined in order to determine whether they indicate a change in extreme precipitation on a monthly basis. The analysis involves a simple record-statistics framework and shows that the upper tails of the probability distribution functions for monthly precipitation are being stretched in the mid- and high latitudes where mean-level precipitation increases have already been reported in the past. In other words, values corresponding to extreme monthly precipitation in the past are, according to these results, becoming more frequent.