Responding to the CO2 Coalition's "Fact #25" on CO2 and Soil Moisture

CO2 Coalition's "Fact #25" claims that increasing CO2 causes moister soil. We're told that increasing CO2 concentrations means that plant stomata are open for shorter periods, so plants become more efficient (water use efficiency or WUE increases) and don't take as much water from soils. From this they claim that "CO2 fertilization has been linked to decreases in global fire, drought and heat waves."

It's true that studies have shown that in some ecosystems, elevated CO2 has been linked to better water use efficiency (WUE) and improved soil moisture. But CO2 Coalition is being a bit selective in the information they present, even in the two examples they provide. In their first example, they cite research from CSIRO that showed that increasing CO2 has lead to a boost in green foliage in desert areas, but there are also secondary effects: "On the face of it, elevated CO2 boosting the foliage in dry country is good news and could assist forestry and agriculture in such areas; however there will be secondary effects that are likely to influence water availability, the carbon cycle, fire regimes and biodiversity, for example... Ongoing research is required if we are to fully comprehend the potential extent and severity of such secondary effects." The study[1] itself (which CO2 Coalition doesn't reference), concludes, 

The results reported here for warm, arid regions do not simply translate to other environments where alternative resource limitations (e.g., light, nutrients, temperature) might dominate, although the underlying theory remains valid.... The remaining challenges are to develop a more general understanding of how the increase in Ca is shared between Al and El in environments that are not warm and arid and to develop capacity to quantify the multiple potential flow-on effects of fertilization in these environments, such as widespread changes in surface albedo, an increase in fire fuel loads for a given P, and possible reductions in stream flows due to enhanced rooting systems

In other words, increased WUE is a real consequence of elevated CO2, but it's the not only way this human-caused perturbation of the carbon and hydrological cycles may affect ecosystems. There are negative consequences as well.

CO2 Coalition's second example comes from a study[2] of semi-arid shortgrass steppes of Colorado. Again they observe an increase in WUE, but the observe that this is not favorable for all plant species in the area.

Results suggest that a future, elevated CO2 environment may result not only in increased plant productivity due to improved WUE, but also lead to increased water drainage and deep soil moisture storage in this semi-arid grassland ecosystem. This, along with the ability of the major grass species to maintain a favorable water status under elevated CO2, should result in the SGS being less susceptible to prolonged periods of drought."[2] This does not benefit all plant species equally, though. It would eventually "favor deeper-rooted over shallow-rooted species."

And it's not as if further research on the impact of AGW on soil moisture hasn't been done. CO2 Coalition has only cherry picked a couple studies showing the impact of CO2 fertilization on plant transpiration and soil moisture. They do not consider at all the effects of the warming that results from increased CO2. Warmer air holds more water, so global warming is going to have a significant impact on relative humidity. Higher temperatures can result in lower relative humidity and higher vapor pressure deficit (VPD), which dries out soils and restricts transpiration rates in trees. This diminishes their ability to reduce their heat exposure by evapotranspirative cooling, putting them at risk during droughts, wildfires and predation by insects like bark beetles and fungi.

One recent study[3] shows that increasing CO2 does in fact drive "greening" through the CO2 fertilization effect but increasing CO2 also drives warming through the greenhouse effect, and that warming has a strong impact on soil moisture through extreme events such as droughts. The warming and greening effects of CO2 currently favor land being a carbon sink but sometime in the second half of the 21st century, land may well become a carbon source, as the large negative effects of warming outweigh the positive impact of CO2 greening. Once this "peak" is reached, warming may well accelerate, since we will lose a major carbon sink removing CO2 from the atmosphere. The lead author of the study, Julia Green, explains:

Plants lose water when they perform photosynthesis. Therefore, many species stop when water supplies are depleted, meaning they aren’t taking in carbon anymore. The loss of water during photosynthesis is called “transpiration” and it has a cooling effect on the surrounding environment.... As a result, when it is dry and transpiration stops, it can result in warming temperatures. Warming temperatures increase rates of respiration – the loss of carbon from soil and vegetation.”

In fairness, the study results are based on RCP8.5, which is no longer a realistic "business as usual scenario," so the peak may occur later than expected in this study. But what is clear is that CO2 Coalition is ignoring a great deal of science here. Increasing CO2 overall has a negative impact on soil moisture, and is a limiting factor on how much greening can occur. Eventually, we can expect the CO2 fertilization effect to be overpowered by global warming. In fact, there is already some evidence that this is happening, as greening has dampened over the last 20 years or so.[4]

Yet another study[5] found that increasing CO2 reduces stomatal conductance, which aggravates heat stress for plants that can reduce global crop yields and nutrient content. They explain that the reduction of stomatal conductance reduces

the rate of passage of materials through stomata, by reducing both the number of stomata per unit of leaf area and the stomatal apertures. Decreases in stomatal conductance lead to decreases in transpiration, reducing the loss of water from leaves, which is beneficial to plant water conservation. In the case of relatively high CO2 levels, fewer stomata and smaller apertures reduce the heat evapotranspirative cooling capacity of leaves, which aggravates heat stress under water shortage.

CO2 Coalition's "fact" shows now awareness of these lines of evidence, and they offer no response. There's a significant body of literature showing not only positive "greening" effects of CO2 but the negative warming effects of CO2 on plants and crop yields.

References:

[1] Donohue, R. J., M. L. Roderick, T. R. McVicar, and G. D. Farquhar (2013), Impact of CO2 fertilization on maximum foliage cover across the globe's warm, arid environments, Geophys. Res. Lett., 40, 3031–3035, doi:10.1002/grl.50563.

[2] Nelson, J.A., Morgan, J.A., LeCain, D.R. et al. Elevated CO2 increases soil moisture and enhances plant water relations in a long-term field study in semi-arid shortgrass steppe of Colorado. Plant and Soil 259, 169–179 (2004). https://doi.org/10.1023/B:PLSO.0000020957.83641.62

Elevated CO2 increases soil moisture and enhances plant water relations in a long-term field study in semi-arid shortgrass steppe of Colorado

[3] Green, J.K., Seneviratne, S.I., Berg, A.M. et al. Large influence of soil moisture on long-term terrestrial carbon uptake. Nature 565, 476–479 (2019). https://doi.org/10.1038/s41586-018-0848-x

[4] Yuan, W. et al. (2019) Increased atmospheric vapor pressure deficit reduces global vegetation growth, Science Advances, https://advances.sciencemag.org/content/5/8/eaax1396

[5] Ma X, Bai L. Elevated CO2 and Reactive Oxygen Species in Stomatal Closure. Plants (Basel). 2021 Feb 23;10(2):410. doi: 10.3390/plants10020410. PMID: 33672284; PMCID: PMC7926597.

[6] Christoph Bachofen et al, Stand structure of Central European forests matters more than climate for transpiration sensitivity to VPD, Journal of Applied Ecology (2023). DOI: 10.1111/1365-2664.14383