Elevated CO2: A Key Driver of Global Greening Observations

By Craig D. Idso of Cato.

"Despite a constant barrage of stories portraying rising atmospheric carbon dioxide (CO₂) as a danger and threat to the planet, more and more scientific evidence is accruing showing that the opposite is true. The latest is in a paper recently published in the journal Scientific Reports, where Lu et al. (2016) investigated the role of atmospheric CO₂ in causing the satellite-observed vegetative greening of the planet that has been observed since their launch in 1978. 

It has long been known that rising CO₂ boosts plant productivity and growth, and it is equally well-established that increased levels of atmospheric CO₂ reduce plant water needs/requirements, thereby improving their water use efficiency. In consequence of these two benefits, Lu et al. hypothesized that rising atmospheric CO₂ is playing a significant role in the observed greening, especially in moisture-limited areas where soil water content is a limiting factor in vegetative growth and function. To test their hypothesis, the three scientists conducted a meta-analysis that included 1705 field measurements from 21 distinct sites from which they evaluated the effects of atmospheric CO₂ enrichment on soil water content in both dryland and non-dryland systems.

According to the authors, the meta-analysis revealed that “increasing atmospheric CO₂ to between 1.2 to 2.0 times the ambient CO₂ level has a positive effect on soil water content” (Panel A, figure below). What is more, the CO₂-induced increase in soil water content was found to be greater in drylands (17%) than non-drylands (9%) (Panel B, figure below). Lu et al. also note their analysis showed “no evidence for any significant effects” of soil texture, vegetation type, land management practices or climate regime on soil water content under elevated CO₂ conditions. Given as much, they conclude that considering the inherent water limitation in drylands, the additional soil water availability brought about by rising atmospheric CO₂ concentrations over the past half-century is “a likely driver of observed increases in vegetation greenness” during this period.



Figure 1. (Panel A) Sensitivity of the soil water response ratio to CO₂ enrichment for the entire data set, calculated as the soil water content under elevated CO₂ divided by the soil water content under ambient CO₂. The closed circles are the observations, with the solid black line providing a linear regression. The red lines represent the 95% confidence intervals of the observations and the dashed grey lines represent the 95% confidence interval of the model. (Panel B) Enhancement of soil water content under elevated CO₂ for dryland versus non-dryland regimes. Adapted from Lu et al. (2016).

Here is yet another study indicating rising atmospheric CO₂ is benefiting the biosphere, rather than harming it.

Reference
Lu, X., Wang, L. and McCabe, M.F. 2016. Elevated CO₂ as a driver of global dryland greening. Scientific Reports 6: 20716, doi:10.1038/srep20716."