The CO2 Fertilization Effect refers to the fact that increased atmospheric CO2 levels can encourage plant growth. This is a well known effect, and is exploited in farms which grow crops in greenhouses with artificially elevated CO2 levels.
The effect has a dramatic impact in computer modelling of climate change - because when CO2 levels rise, plants grow faster, thereby absorbing more carbon from the atmosphere. This provides a 'negative feedback' effect, slowing down climate change. Some estimates of the CFE suggest that it may account for up to 60% of the current terrestrial carbon sink.
A 2020 international study found not only that the size of the gloabl CFE may have been overstimated, but also that the effect currently appears to be diminishing each year (over the last 40 years or so) - for unknown reasons.
The enhanced vegetation productivity driven by increased concentrations of carbon dioxide (CO2) [i.e., the CO2 fertilization effect (CFE)] sustains an important negative feedback on climate warming, but the temporal dynamics of CFE remain unclear.
The implications are that current climate models may not be correct - and that climate change may accelerate faster than the models have been suggesting.
Ultimately, these results indicate that terrestrial photosynthesis may not increase as much as models project, possibly reducing the potential of land-based climate mitigation, further accelerating global warming and exacerbating the efforts required for meeting climate targets.
See Recent global decline of CO2 fertilization effects on vegetation photosynthesis Science 11 Dec 2020:, Vol. 370, Issue 6522, pp. 1295-1300