Climate theory predicts that as air warms it will hold more water vapour, which will cause rainfall to be more extreme.
This prediction is borne out by observation. Westra, Alexander and Zwiers of Adelaide University studied water precipitation records of 8000+ good quality weather stations covering more than a century. They published in the Journal of Climate 2013. Two thirds of the stations, especially those near the tropics, showed an increase in extremes of rainfall - either more or less than the average expected. This increase was proportional to air temperature, so that a 1C increase in air temperature was associated with a 7% increase in rainfall extremes.
The value of this study lies in its simplicity and its extent. Rain gauges are the simplest of all weather measurements. And 8000 stations going back 109 years are difficult to argue with.
Flood patterns and river flows are difficult to estimate accurately. Rain gauges are not.
Pall et al. generated several thousand climate model simulations of UK autumn 2000 weather, comparing actual conditions, and cooler conditions with the greenhouse gas component removed. Results were fed into a precipitation-runoff model that is used to simulate severe daily river runoff events in England and Wales (proxy indicators of flood events). In nine out of ten cases the model results indicate that twentieth-century anthropogenic greenhouse gas emissions increased the risk of floods occurring in England and Wales in autumn 2000 by more than 20%, and in two out of three cases by more than 90%. Impressive, but likely to be rejected by contrarians, as it uses computer models, to which they are allergic. Never mind.
We have so far experienced 0.8C of warming. George Osborne believes that 2.0C of warming is something that can be shrugged off as trivial. He is behaving as irresponsibly as he did as an idiot in the Bullingdon Club.
[Update 11/7/15] Study shows that record-breaking rainfalls and droughts are increasing with warming temperatures.