Big rain

John Nielsen-Gammon has had a series of posts up about material he has found interesting at the AGU conference last week.

As the the extensive floods in Queensland last year made me consider the impact of floods as a major issue with AGW, I will reproduce Dr J's post from Climate Abyss about possible record heavy rains to come:

Probable maximum precipitation (PMP) is a commonly-used design input value for water projects such as dams for which failure is not an option. It’s estimated, in effect, by assuming that all possible factors contributing to heavy rain (upstart speed, moisture content, duration) come together at the same time and place to produce the flood to end all floods.

Ken Kunkel noted that in a warming climate, one of these is expected to change more than all the others: the moisture content of the air. What about the worst case scenario: the peak moisture content of the air at any given location? Kunkel showed evidence that the peak has indeed increased over time across most of the United States, though there are data quality issues that need to be worked out (historical weather balloon water vapor measurements are pretty dodgy). More importantly, the climate models are consistent in showing increases in the future.

We’re talking about increases of 10% every few decades. This would correspond directly to 10% increases in PMP. And increases much greater than 10% in the cost of new projects. And even greater expenses for retrofitting. That’s unless we decide that we are willing to tolerate a greater risk of man-made catastrophe from dam failure than before.