The only thing we can be sure of about the future is that it will be absolutely fantastic. So if what I say now seems to you to be very reasonable, then I have failed completely. Only if what I tell you appears absolutely unbelievable, have we any chance of visualizing the future as it really will happen.

- Arthur C. Clarke (h/t Brin)

Monday, September 24, 2007

CO2 -> plants -> hydrology

This is a few weeks old, but I haven't seen much mention in the blogosphere. Perhaps I missed it.

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:

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.

"Very cross" indeed. Hail Britannia!

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.


Michael Tobis said...

Just got this nice comment in email (Thanks!):


I tried to comment on your blog but I couldn't get the mark up to work. The EA is the UK's Environment Agency. I am not sure of their job description, but amongst other things they look after water quality and regulate waste disposal. They produce an excellent interactive map which highlights all the environmental risks in the area. Zoom in on the Thames to the east of London. The area on the south banks of the Thames is called the Thames Gateway and is where the government want to build a large number of the new houses they keep promising.


I also looked for the 20 times reference. This is possibly the source - the Foresight Report, written in 2004 - that suggests that damages from floods could increase from £1 billion now to £25 billion "in the worst case scenario" though I have not read the report to find out when they think the worst case scenario could occur.

Foresight Report

Thanks for the In it for the Gold blog and all your comments on Real Climate. They are a real inspiration to fight the misinformation and start making changes in ones lifestyle.


Anonymous said...


Yes the Foresight report is a resonably depressing read, however, their findings are based on 'without mitigation' assumptions so one would hope that we may be able to keep things down a bit.

From a different perspective, in the UK we operate a 'plan based' planning system (Regional, Local and site specific all integrating seamlessly) which, under the new Planning Policy Statement 25 now requires that a Flood Risk Assessment (FRA), is completed for all planned development. Although site specific FRAs are still pretty straightforward (especially if your new garage isn't on the floodplain), to prepare the more Strategic FRAs involves a fair bit of modelling (done by consultants not the EA). Currently the climate change allowance in most SFRAs is for a projected 20% increase in flow (ontop of current 1% annual exceedence probability) to cover until 2100.
However, I'm aware of a recently new development in Sheffield that was built to an AEP + 30% standard had water lapping at the door tread in the June flooding whilst others built to the lower SoP got flooded (appreciating of course that that event was of lower prob than AEP). It's got people wondering.



Erin said...

A possible reason why this hasn't gotten a ton of press is because of the data we have involving how long plants respond to CO2 levels. In the FACE (free atmospheric CO2 experiment) experiments funded by NSF throughout the US (and the world) as well as in smaller chamber experiments of plants grown in pots, the response of most plants to increased CO2 levels is relatively short lived.

This is why we can immediately dismiss anyone who tries the "CO2 is just plant fertilizer" response to concerns about greenhouse gas emissions. I'd be interested in seeing some long term data on plant responses of the type described by Betts et al. before I get too concerned that what they observed may be a long term effect.

Michael Tobis said...


I'm not a biologist, but I think you are conflating a couple of effects. This isn't about CO2 fertilization, which is certainly limited by availability of other resources (among them climate stability).

It's about drought resistance. Plants are always in a tradeoff between losing water through transpiration and gaining CO2. If there is more CO2 about, they don't need to expose themselves to as much water loss. This has two benefits from the point of view of those of us in semiarid zones: 1) the plants are hardier to drought and 2) they absorb less water and release less to the atmosphere, thus increasing runoff and hence water availability for other purposes.

The first point is good news all around, I'd think, (some ecologist will presumably find an invasive species that porves me wrong, but until then anyway...) but the second is bad news for flood sensitive regions; the plants will not help you as much as they would have.

All true at biomass equilibrium.

Dano said...

Well, if my EnvHort degree still rattles around in my brain correctly, the lower soil moisture due to closed plant stomata will foster more weedy plant invasion and lower atm HOHvap transport, which will likely foster droughty conditions (something Dano brought up years ago as a problem). Increasing drought trend in the American West will certainly wreak short-term havoc with forest health, which may mean less snowpack and attendant reservoir storage problems.

Erin needs to point out that it is growth and metabolism response that FACE has issues with, not stomatal closing which, IIRC, persists.

Anyway, we have our indicators. Now can we find the political will to manage to them or use them to manage to goals?



Michael Tobis said...

Dano, I don't get it. Closed stomata means less withdrawal of moisture from the root system which means moister soils, which is what Betts et al are warning about.

As for the snowpack, there's not much call for that in these parts, but yes, of course that will contribute to water stress in the other Colorado River (not the one in Texas...)

Adam said...

Apologies for the personal digression, but being a Sheffield resident this statement has raised curiosity:

"However, I'm aware of a recently new development in Sheffield that was built to an AEP + 30% standard had water lapping at the door tread in the June flooding"

Which development was that Hugh? Obviously if it was a private development just the area would be interesting.

Also, how much of the Meadowhall area was done to the 20% FRA? The road there seems to (flash) flood reasonably frequently.

Dano said...


The lower soil moisture comes from lower overall rainfall & more episodic precipitation (lower overall rainfall because of the reduced HOHvap transport). Afforestation programs seek, for one reason, to reinstate the HOHvap transport that plants provide and remember the BOR around the turn of the 20th C tried to green up the American West by afforestation.

And in this sort of regime pcpn infiltration into the soil is harder because it is more difficult to saturate soil when it is dry (IIRC Van Der Waals forces). Moist soil absorbs pcpn more easily than dry soil - Colorado cities tell their folk to run their irrigation cycles twice to efficiently water - the first (short) cycle moistens the soil and the second cycle gets absorbed.



Hugh said...

Hi Adam

Sorry but I'm not aware of the building's location in the city (I have great respect for my friend's concerns over commercial confidentiality so I never ask too much... even though planning applications and SFRAs are public documents the consultants still get very spooked when site specifics are spoken about). All I know is that it was a substantial commercial premises set amongst other newish commercial premises.

As far as Meadowhall goes, I'm afraid I don't really know the city (I research community resilience to low-probability coastal flooding) but to have a requirement for the AEP + 20% any development would only have been planned during the last couple of years as the EA Flood Map, Catchment Flood Management Plans, PPG25 (now PPS25) and, most recently, SFRAs came into effect.
For info the Sheffield SFRA was only completed by Jacobs last December and now acts as 'the' document the LPA will use to clarify which part of the floodplain any prospective development will sit on

Sorry I can't be more help.


PS. In relation to the Foresight report mentioned in my earlier response I'm conscious that I used the phrase 'non-mitigation assumptions'. Just to clarify I used 'mitigation' in the 'hazards' sense of the word rather than the climatology sense i.e. to include all structural and non-structural measures (levees, land use planning etc.) rather than just emissions reduction. I suppose you could sustitute 'non-adaptation assumptions'

Thnx for allowing the deviation Michael

Adam said...

Firstly, thanks Hugh & Michael for allowing my indulgence.

Secondly, that's still very interesting information Hugh and I think I know the area if not the actual building. The more general stuff is also all new and interesting as well.