by Julia Gaus
One of the central concepts in biology is photosynthesis: plants using sunlight to transform water and CO2 into chemically bound energy and oxygen . This process enables plants to synthesize the necessary chemical building blocks for their growth and provides a source of oxygen and food for animals and humans. But while sunlight is available in abundance, water, atmospheric CO2 and other necessary input factors are limited thus determining the scale at which the photosynthesis process can occur . So, what happens as human activity causes an increase in atmospheric CO2 levels? In other words, how is the land carbon sink – atmospheric CO2 being bound by plants – altered by a greater availability of CO2 for photosynthesis?
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 . 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 . 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 .
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 .
Firstly, the global picture masks regional differences . CO2 is only one constraining element of photosynthesis; others being, for example: phosphorus, nitrogen, and water availability . 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 . 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 .
Secondly, not all plant species react in the same way to an increase in atmospheric CO2 levels . 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 . 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 .
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 . 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 .
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) ) albeit slowing: the rate at which photosynthesis is enhanced is declining compared to the rate at which CO2 is put in the atmosphere .
However, for the future models predict a shift from a fertilisation towards a warming-dominated period, in which the photosynthesis process will be diminished . 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 .
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