Increased CO2 levels doesn't increased magnitute of floods.

From CO2 Science: (link)  
Image: US Geological Survey

Hirsch et al, after Trenberth (1999) and the IPCC ((2007), expected that the increased CO2 in the atmosphere would lead to an increase in magnitude of floods. In their paper published in Hydrological Sciences Journal 57 (link to abstract) they found
In none of the four regions defined in this study is there strong statistical evidence for flood magnitudes increasing with increasing GMCO2. One region, the southwest, showed a statistically significant negative relationship between GMCO2 and flood magnitudes. 
What was learned
The two researchers report that "in none of the four regions defined in this study is there strong statistical evidence for flood magnitudes increasing with increasing GMCO2." In fact, they say that one region, the southwest, showed a statistically significant negative relationship between GMCO2 and flood magnitudes.


What it means
In discussing the meaning of their findings, Hirsch and Ryberg state that "it may be that the greenhouse forcing is not yet sufficiently large to produce changes in flood behavior that rise above the 'noise' in the flood-producing processes." On the other hand, it could mean that the "anticipated hydrological impacts" envisioned by the IPCC and others are simply incorrect.


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