Plants love CO2, right?
More CO2 makes plants grow more, right?
It follows that a global rise in atmospheric CO2 will produce healthier plants globally, right?
Wrong! Wrong! Wrong!
Try this simple experiment. Place a paper towel on a work surface. Add water a drop at a time. At first the paper will absorb every drop of water. Eventually, though, there must come a time when the paper is so loaded with water that any excess is not absorbed. This illustrates a fundamental principle of all systems:
any excess input to a system above the system's handling or processing capacity will not be processed.
Put another way, for any system, whether in engineering, biology, socio-economics or any other dynamic system whatsoever:
when any input quantity exceeds the ability of the system to forward or to process that input then the system will fail to operate as designed, intended or evolved.
Input overload can lead to either a simple failure to process the excess or a failure of the entire system.
The recent U.K. general election affords a classic example of a human system failing from this cause. The voter turnout was so much greater than expected that, when polling stations were closed at 10 pm, many voters were turned away. This has probably contributed to the U.K. having a hung parliament - a partial systems failure. When a failure to process votes leads to anarchy, then that is a total systems failure where 'hung parliament' is in danger of taking on a whole new meaning.
Plants and excess CO2
A photosynthesizing organism is a tiny factory. CO2 and H2O go in, energy is consumed, sugars are stored, H2O and O2 come out. The processing limits of every such organism as a factory are set by the number of processing units, the amount of CO2 and H2O, and the amount of sunlight available. These amounts vary in ratio in different climatic zones.
Green plants have adapted to current climatic zones. A plant may have adapted to excess humidity, excess heat or excess CO2. Adaptation is a process which occurs over geological-scale time periods.
If a plant has adapted to process CO2 within a particular range of values, then it will have no mechanism for processing CO2 above that range. Some plants will simply not make use of any excess CO2, exhibiting a simple failure mode. Other plants will exhibit a mode typical of all living things: as the amount of a nutrient increases beyond an upper limit the uptake of that nutrient begins to fall. Ultimately, at the point where uptake is insufficient to support life, the organism dies.
The only way to reliably and predictably increase plant uptake of CO2 is to increase the number of plants. A limit is reached again where the plants in an area are absorbing all available insolation. Adding more plants beyond that point produces a crowding effect whereby the biomass sequesters ever less carbon as ever more plants are added.
Every system has limits. Living organisms are biological systems and so they must have limits.
A photosynthesizing organism pushed beyond its biochemical processing limits will not suddenly mutate into a new species.
Plants thrive on CO2 up to a limit. Beyond that limit, excess CO2 may, depending on the type of plant:
have no effect;
reduce CO2 uptake;
kill the plant.
Further discussion of this topic:
Is The 'plants Love CO2' Theory Valid?
Related / further reading:
Rising CO2 Keeps Trees From Cooling Earth