Photosynthesis, happens when CO2 enters leaves, but at the same time water exits. About 1000x as many H2O molecules than CO2 is typical. One way that more CO2 helps is by lowering that ratio. If there is not enough water in the soil, or if the 'plumbing' in trees can't get water up to the leaves quickly enough, a lower ratio is a good thing. A new paper discusses this: “Increase in forest water-use efficiency as atmospheric carbon dioxide concentrations rise” Keenan et al. Nature in press, doi:10.1038/nature12291 (most of the abstract) “We find a substantial increase in water-use efficiency in temperate and boreal forests of the Northern Hemisphere over the past two decades. We systematically assess various competing hypotheses to explain this trend, and find that the observed increase is most consistent with a strong CO2 fertilization effect. The results suggest a partial closure of stomata - small pores on the leaf surface that regulate gas exchange - to maintain a near-constant concentration of CO2 inside the leaf even under continually increasing atmospheric CO2 levels. The observed increase in forest water-use efficiency is larger than that predicted by existing theory and 13 terrestrial biosphere models. The increase is associated with trends of increasing ecosystem-level photosynthesis and net carbon uptake, and decreasing evapotranspiration. Our findings suggest a shift in the carbon- and water-based economics of terrestrial vegetation, which may require a reassessment of the role of stomatal control in regulating interactions between forests and climate change, and a re-evaluation of coupled vegetation–climate models.” So we get a bit deeper into the mechanisms involved, and at the end ask for better climate models. Lately, it seems that a lot of people have been taking digs at climate models - not just me! A lot is expected of them, and they need improving to handle these forests things. And the ocean things. Maybe even the soil things. But at least we can be happy about forests and CO2. I have mentioned before that forest mortality is also increasing. What we need is for someone to compare these competing processes. A darn good idea to study, if I do say so myself. The increased mortality observations (from all continents except Antarctica and Greenland, if that counts) have been qualitative so far. That's the problem. The forest growth side has quantitative data from flux towers. Whatever those are, right? Well, I suppose I could tell you...
One of the things that really surprised me about hurricane models was the lack of models that attempted to integrate everything known to affect hurricane behavior. The vast majority started existence using a limited set of variables and incremental modifications are often subordinate to the original priorities. They are definitely useful....as long as they are used knowing their proven limitations. I cannot even get my head around how one validates and refines climate models to work better than hurricane track models. Unfortunately, way too many confuse a lack of high fidelity modeling performance as a proxy for nothing worth modeling. It is essential to start somewhere and iterate intelligently. It is just as essential to progress towards sustainability independent of modeling issues.
From what I have read, the best correlation seems to be between drought resistance and increased co2. I would suspect this varies greatly from species to species. Unfortunately we can not just study changing rain patterns due to climate change versus more drought resistance as that is going on along with man cutting down rain forests and planting orchards, doing farms, or building buildings these all have effects. We can though draw a qualitative conclusion having to do with feeding the world. Changing rainfall conditions and more carbon dioxide doesn't doom agriculture, but may change what plants need to be planted at various locations. Quantitatively we can count stoma then create mathematical models to figure out increased growth, and lack of dying in different rain fall conditions. We have the quantitative folliage increased 11% when carbon dioxide increased 14%, but our models are not very good at figuring out how much of that is changes of rainfall and in man's deforestation and reforestation efforts. Please do.
I think you are asking the wrong question. In the short time frame, 1 year or less, climate models are going to tell you nothing about hurricanes. Trenbleth put forth the hypothesis that more ghg causes a higher frequency of severe hurricanes as a fact as part of one of the IPCC reports based on non-peer reviewed insurance studies that did not even measure severe hurricanes in a historical fashion. This led one of the members of the IPCC who studies huricanes, Chris Landsea resign before it was published. This has caused a great deal of both scientific study to see if climate based huricane models can accurately predict, and punditry from the political blogs that trenbleth or landsea was right. If you can accurately model huricanes with ghg as an input, then you can find data confirming or denying the hypothesis. Here is landsea's opinion, including a lot of the science as it was a couple of years ago. http://www.aoml.noaa.gov/hrd/Landsea/gw_hurricanes/ You can also read trenbleth's opinions, but you will find no data when he talks about it.
My point was primarily focused on the intense difficulties of modeling, using hurricane models as an example. They work good enough to pay attention to in the short term, but can be quite misleading in the long term. (One coworker moved his family based on the 10+ day forecast. Might seem smart, but what he actually did is kept moving to the next ground zero point a couple of time till he finally returned to home exhausted and poorer.) I hopefully was not coming across as trying to tie climate and hurricane models together. I would think climate models would strive to be optimized for the long term and possibly less accurate on very short time scales to really be useful.
lol. He relied too much on the power of prediction, which is poor. We do know Miami is particularly poorly suited. It will be hit again, we just don't know when. The climate models used by the IPCC do a particularly bad job of predicting ocean oscilations, which is where you would want to go for better huricant prediction. Until that is done properely IPCC models will be poor. We can look at a couple of macro items, the rising sea levels, the destruction of barrirer islands, and the large amount of building to predict where the highest insurance losses are, and to estimate how much larger damages from storm surges will be.
2 Hurricanes predicted to hit east coast in Sept. "ATS Formations pending and likely tracks forecast 2013 issued 25 Aug 07.00 utc Between now and End Sept WA expects 2 Hurricane formations and one TS of note (mainly because of its position), as follows (Each forecast has 85% prob of basic success):- WA1 Forming ~29-30th Aug reaching H2 (+/-1) at some point. Prob starting East of Leeward Islands, heads W/NW towards near Bahamas, standard models are likely to suggest landfall on the Islands but WA1 will veer slightly right of their projections and stay in Atlantic and HIT GEORGIA (or Florida/South Carolina) around 6/7th (5-8th) Sept. WA2 Forming ~13-15th Sept reaching H2 (+/-1) at some point, prob less strong that WA1. Prob starting East of Bahamas / Turks & Caicos, heads NW and HITS NORTH (or South) CAROLINA around 17th (16-18th) Sept. WA3 Forming ~27-28th Sept TS not H. Prob starting East of North Carolina, heads NE in Atlantic towards Eire, UK and Europe."
THose are short term predictions of catagory 2 or less. No one is going to move for a cat 2. The question is can models give us a 20 year prediction for cat 3+
Most simplified version> a flux tower has sensors above the plant canopy to measure CO2, temperature, water vapor, and the 3-dimensional wind vector at high sampling rates. I think those rates are now about 10/second, which was cutting-edge technology 20 years ago. Now you just buy the boxes and bolt the gadgets to your tower. If air were visible, we would see twirling vortices of several sizes, from centimeters to about the height of the plant canopy (whatever it may be). Actually if air were visible, we would probably not see anything, but use your imagination. At a given moment, one of those vortices twirls downwards, in proximity to those sensors mentioned above. They detect the amount of CO2 in that small air parcel. At another moment, a different small parcel of air moves upward past the sensors with a different amount of CO2. The computer attached to the sensors keeps track of all those little parcels of air, adds them up, and bam, there is your net flux of CO2 through the horizontal plane defined by the sensor group. It should be net negative (absorbed by the plants) during the day. Positive (net release from respiration) at night. If this is not the case, jiggle the connections or clean insects out of the tubing, or something. This is eddy flux accumulation, the simplest technique. Eddy flux covariance requires matrix math and Greek letters and so forth. The amazing part is that the boxes you bought with the sensors also contain software that does it all for you. You also get heat, water and momentum fluxes in approximately the same way. To the extent that wind blows sideways through the forest (advection), your result is wrong, but the idea is that all those sideways flows balance each other out. Heh heh, it might be true. I think there are now about 200 such towers in the USA and approaching 1000 worldwide. The sites that have been in operation longest provide the strongest evidence that forest physiology is responding to increased CO2. Lacking a tower and all the gadgets, you can still take tree cores in your forest to measure live wood accumulation through time. That is decidedly low-tech, although getting long cores out w/o breaking the tool is an acquired skill. Live wood is only one component of the site net-C balance but it is a big one. We could talk about other components of site net-C balance some other time. Or, we can predict hurricanes about a week in advance: WAVETRAK - North Atlantic - 850mb Vorticity - Latest Available which looks at dynamic vortices on a MUCH larger scale. It's all about vortices, see? Just now you can see Kiko of Baja Calif., which is probably not as scary as it looks. Also, the ragged thing entering the Carib, being broken down by wind shear. Most of the exciting ones start assembling south of the Cape Verde Islands, so keep your focus there.
