Background
The authors write that "about 17,500 years ago, atmospheric CO2 concentration started to rise from 180 ppm, levelled off at 280 ppm around 15,000 years ago, and broadly remained at 280 ppm until the Industrial Revolution," whereupon it began to rise again, recently reaching the 400 ppm mark, with many people expecting it to rise to 700 ppm by the end of the current century.

What was done
Wondering "whether the transition from current to higher CO2 can be thought of as a continuation of the past trajectory of low to current CO2 levels," Temme et al. "performed a meta-analysis of low CO2 growth experiments on 34 studies with 54 species," quantifying "how plant traits vary at reduced CO2 levels."

What was learned
The four Dutch researchers report that "on average across all species," a 50% reduction in current atmospheric CO2 reduced net photosynthesis by 38%, intrinsic water use efficiency by 48% and total plant dry biomass by 47%.

What it means
Temme et al. conclude that in terms of carbon gain and whole-plant growth rate, responses to low CO2 are, in fact, somewhat more extreme than responses to high CO2, the latter of which gradually diminish in magnitude as the air's CO2 concentration rises higher and higher.