David,
Given that the Earth is getting greener, I saw speculation that links this to increased CO2 concentrations. Being a physicist, I don't have an overview over biological literature. Therefore the question, whether there is a well-established cause-effect relation between increasing CO2 levels and increasing tolerance to draught stress.
Thanks,
Bjorn
" Given that the Earth is getting greener " - I have never heard about that but the opposite. What do you mean by 'the Earth is getting greener' (more plants, more algae or more environmental consciousness)?
The graph Kenneth provided is exactly the one I was referring to. According to satellite measurement, the Earth's green plants grow better since a few decades. Some researchers speculated that the only factor that has changed between 1982 and 2015 is the CO2 content of the atmosphere, and derived a mechanism that coupled CO2 content to the plants' ability to regulated their water household. But I don't know whether there is some biological research substantiating these claims. Again, I am a physicist. Hence the question to you, David.
In order to absorb CO2, plants need liquid water which they normally extract from the soil via their roots. Thus an increase in CO2 will have no effect if there is a lack of ground water.
You should also bear in mind that inceased atmospheric CO2 leads to higher surface temperatures, and more evaporation from the oceans leading to increased rainfall. On the other hand, more evaporation from the land means less ground water.
Finally, the map showing increased greening does not show any increase where there is already a lack of ground water e.g. the Sahara. So, it does not appear that increased CO2 does help where there is water stress.
Hello Bjorn and others
There is a considerable literature on the effects of CO2 on water loss of plants and ecosystems. At the organisational level of the plant, it is mainly in artificial or semi-artificial conditions. There is less,but good, information on agricultural ecosystems (crops under" free-air CO2 enrichment" - or FACE), and there are studies on natural ecosystems.
There is a very strong consensus that elevated CO2 decreases the stomatal conductance of leaves, especially in plants with the C3 photosynthetic mechanism, and increases the rate of CO2 assimilation. But the effects also occur in C4 plants. Consequently, water loss (transpiration) from leaves and canopies is decreased, slowing the rate of soil water depletion. Of course, as already stated, if there is extreme water limitation then elevated CO2 will not help, but even a small increase in soil water retention could increase vegetation cover and be detected by imaging techniques. As pointed out by others the smaller water loss raises leaf temperature so the effect of CO2 is complex at the crop/natural ecosystem level and not generally as large as observed on individual plants in semi-artificial environments. Without knowing the background to the global assessment shown, I hesitate to offer an explanation, but slower water loss would maintain vegetation for longer and the effect would be greater in areas with limited water supply. The plants are not more drought resistant but it just takes longer for soil water to be depleted. Also the increased photosynthesis would help to increase vegetation growth and longevity.
This might only be a short-term benefit depending on temperature increase. Warmer conditions accelerates plant development, and has complex effects on reproduction. So ecosystem responses are very difficult to predict. On wheat crops with which I was more familiar than with other systems, doubling CO2 concentration increases dry matter and grain yield by ca. 30% but a 3-4oC temperature rise decreases them by about the same amount. Sugar beet is similar. So elevated CO2 might seem `a good thing' but . as it increases atmospheric and global temperatures it is potentially very dangerous.
So Bjorn in summary, there is a large relevant literature which might provide a mechanism, but we need to think in the longer term before deciding about benefits.
Hope this helps
Regards
David (Lawlor)
Sorry, I forgot to mention the source of the above: “Greening of the Earth and its drivers” by Zaichun Zhu et al., Nature Climate Change 6, 791–795 (2016)
It will be increasingly important, though experimentally tough, to consider interactions between several environmental factors. An example is the interaction of elevated CO2 and drought on plant species of different carbon fixation metabolism Article Growth and yield stimulation under elevated CO2 and drought:...
another example is the interaction with temperature and fertilizer regimes
Article Temperature determines size and direction of effects of elev...
So many interactions...we might have to put our hope in improved modelling programs and AI