The CO2 Fertilisation Effect: What is it?

CO₂ Fertilisation Effect

As stated above, CO₂ can be the constraining factor limiting the photosynthesis process. Thus, in theory, if more CO₂ becomes available – and the other input factors are available in abundance – the photosynthesis process would be scaled up enhancing the amount of atmospheric CO2 bound by plants [2]. The result would be a slower increase in the atmospheric concentration of CO₂, compared to what would be expected solely based on global CO₂ emissions [3]. Additionally, higher atmospheric CO₂ levels have been shown to increase water use efficiency, allowing plants to survive in drier conditions. The result of these two processes is an increased uptake of CO₂ by the biosphere and thus an increase of the land carbon stock [2, 4]. The name of this phenomenon: the CO₂ fertilisation effect [5]. 

 

Limits to an increased uptake of CO₂ by the biosphere

The recent IPCC report states with high confidence that due to the CO₂ fertilisation effect the global land carbon storage is increasing. However, this effect does not offer an easy way out of reducing the amount of CO₂ we put in the atmosphere in the first place, in order to slow down the climate crisis [4].

Firstly, the global picture masks regional differences [6]. CO₂ is only one constraining element of photosynthesis; others being, for example: phosphorus, nitrogen, and water availability [7]. Specifically, water availability, or rather droughts, are a major determinant of the land carbon sink. This is not only because water availability determines photosynthesis but also because droughts enhance forest mortality and the likelihood of wildfires [5]. Similarly, there exists a saturation point at which plants cannot further scale up their photosynthesis levels at the same pace as atmospheric CO₂ levels are increasing [8]. 

Secondly, not all plant species react in the same way to an increase in atmospheric CO₂ levels [6]. In general, there exist two types of plant species: C3 and C4; the numbers referring to the amount of carbon atoms bound in the first photosynthesis product [9]. It is C3 plants, accounting for about 95% of global plant species, that are more heavily constrained by atmospheric CO₂ levels and are thus expected to display a stronger carbon fertilisation effect [10]. 

And finally, temperature is a decisive factor for the efficiency of photosynthesis. In the context of the climate crisis, this means that while the main driver of global warming (CO₂ emissions), leads to an increased uptake of CO₂ by plants, the result (increased temperature), works against this process [3]. Effectively, the question for determining the rate of photosynthesis and thus how much CO₂ will be absorbed then is whether CO₂ or temperature is the dominating factor for any given plant species in a given region [11].

 

Current trends and projections on the CO₂ fertilisation effect

Whether the photosynthesis process is increased due to heightened atmospheric CO₂ levels or weakened due to higher temperatures – and for that matter the general development of the land carbon sink – is a question that hinges on a myriad of interactions that are not yet sufficiently well understood and which science is trying to answer using observations, field experiments, and computer models [11,12]. It seems that currently the fertilisation effect is still at work (between 2000 and 2009 about 14 gigatonnes of CO₂  equivalent were additionally absorbed per year compared to the pre-industrial area (1750) [13]) albeit slowing: the rate at which photosynthesis is enhanced is declining compared to the rate at which CO₂ is put in the atmosphere [14].

However, for the future models predict a shift from a fertilisation towards a warming-dominated period, in which the photosynthesis process will be diminished [11]. In this scenario, plants will no longer act as a sink of CO₂, thus slowing global warming, but will increasingly become a source driving faster temperature increases [11].