A significant study by Betts et al. of the Hadley Centre in the UK examines the observational record for biotic-CO2 feedback on hydrology. Plant physiology is substantially affected by the increases in CO2, directly, without any reference to climate change. Betts confirms that the net effect of the increased availability of CO2 is to reduce water uptake by plants and increase total runoff.
Assessments of the effect of increasing carbon dioxide concentrations on the hydrological cycle that only consider radiative forcing will therefore tend to underestimate future increases in runoff and overestimate decreases. This suggests that freshwater resources may be less limited than previously assumed under scenarios of future global warming, although there is still an increased risk of drought. Moreover, our results highlight that the practice of assessing the climate-forcing potential of all greenhouse gases in terms of their radiative forcing potential relative to carbon dioxide does not accurately reflect the relative effects of different greenhouse gases on freshwater resources.The BBC article that pointed me there concludes that this result reduces the likelihood of droughts (especially, I'd think, in biologically productive regions), and increases the likelihood of flooding. Similar articles appear elsewhere in the UK press. No sign of it over here. Here's how the Telegraph spins it:
"Very cross" indeed. Hail Britannia!
Dr Betts said that the effect was a double edged sword: "It means that increases in drought due to climate change could be less severe as plants lose less water.
"On the other hand, if the land is saturated more often you might expect that intense rainfall events are more likely to cause flooding."
He said that until now scientific models had only looked at the effect of gases such as carbon dioxide, methane and nitrous oxide on global warming.
If one wanted to look at their full effect on flooding and drought, the effects on plants had to be considered too.
Dr Betts said he had communicated his results to the EA, who have been working on a Government study which said the flooding risk to rivers could increase up to 20 times by the 2080s.
This estimate would now have to be revised upwards.
What had the EA said to that? "They were very cross," he said.
Anyone have an idea out there what this "20-fold" business is about? Is the Telegraph out of control again? (And what's the "EA" anyway?)
As for the big picture, it looks to me like another example of difficulties communicating across disciplinary boundaries. It's hard enough to have climatologists and hydrologists communicating effectively, without having to bring plant physiologists into the loop. In the present case, what climatologists consider climatology has very little to do with the global change dynamics at issue.
Now climatologists don't always have to be the intermediary and in this case we shouldn't be. In fact biologists commonly consider themselves a customer of hydrology and so do hydrologists. In this case the coupling works the other way around and perhaps the phenomenon was missed for a while.
The push from the early 90's to create an overarching discipline of "earth system science" that ought to be looking for and systematizing these sorts of couplings seems to have run out of steam from what I can tell. The troubles at NASA can't be helping.