He won't, because he doesn't recognize that it also benefits the growth of weeds, and that the net effect is that droughts will kill more crops than the CO2 could add to biomass (and we often do not eat the leafs (but rather the seeds/tubers/roots/fruits/etc.). Also, more (non-indigenous) insects will damage a.o. the crops (and increased use of insecticides will pollute the drinking water/aquifers), and the reduced evaporation of plants at elevated CO2 levels will increase temperature further, and also increases erosion and run-offs (which will also change marine biotopes). The increased precipitation from a warming atmosphere will also cause local erosion and runoffs, and huge economic and human damage.
Okay! Lets unpick the nonsense that Bart has expressed in his comments above.
(1) Does he really think that I don't recognize that increased CO2 levels also increase weed growth? Increased CO2 levels encourage the growth of all plants, to varying degrees according to the species, whether a food crop, a tree or a weed. Nature does not discriminate between weeds and other plants. The concept of a weed is a human construct. It's a name given to a species of plant we can't find a use for.
If there's a particular species of 'weed' that thrives really well in elevated levels of CO2, then that's a potential benefit that mankind could exploit, to create mulch, sequester carbon in the soil, help fertilize the deserts, or use for organic farming.
Another basic fundamental of mankind's success and prosperity, is adaption. We survive by adapting to changing conditions. Change itself is another fundamental. We might be able to slow the rate of change, or alter the direction of the change, but nobody can
stop change from occurring.
(2) Bart's idea that
'droughts will kill more crops than the CO2 could add to the biomass' is not supported by the evidence. It's another nonsensical argument. There have been many experiments that have demonstrated that increased CO2 levels result in
the greatest degree of increased growth for plants that are water-stressed.In circumstances where a doubling of CO2 levels, under ideal conditions of sufficient water and essential nutrients, results in a 30% increase in the biomass of a particular species of plant, that same plant when grown in dry or arid conditions, will increase growth by around 60% for a doubling of CO2 levels.
This is because the leaf spores of plants shrink in size as a result of increased CO2 levels, and less evaporation takes place as a consequence, and the plant can thrive with less water. This brings me to the third nonsensical point made by Bart.
(3) He claims,
'the reduced evaporation of plants at elevated CO2 levels will increase temperature further'. What have you been drinking, Bart?
The evaporation
per leaf, or leaf area, is reduced, but the
total number of leaves, and/or the size of the leaves, is increased as a result of increased plant growth, therefore the total amount of evaporation, which produces cooling, is approximately the same, or even greater if we exploit the benefits of increased CO2 and replant forests and irrigate arid regions, and so on. Cor Blimey!
(4) Lastly,
'The increased precipitation from a warming atmosphere will also cause local erosion and runoffs, and huge economic and human damage.'
Bart and I have argued about the significance of comments in the technical report of the latest IPCC report which state that, due to a lack of evidence, there is low confidence that hurricanes, droughts and floods have been increasing since the 1950's, globally.
However, the AR5 does claim with greater confidence that precipitation levels have been increasing, globally, during this period, and I would not disagree with such claims, just as I would not disagree that some slight warming has taken place since the Little Ice Age.
If the climate warms a bit, it is to be expected that more evaporation will take place, more clouds will form, and more precipitation will occur.
However, as populations expand and/or grow in prosperity, more water is required to wash cars, wash solar panels on the roof, wash peoples' bodies, water the garden, grow food, and so on, therefore more dams are built to meet the increased demand for water.
Increased precipitation should help to meet that increased demand, if we organize our affairs sensibly. What's the problem? Certainly not CO2.