Abstract
The impact of anthropogenic climate change on marine net primary production (NPP) is a reason for concern because changing NPP will have widespread consequences for marine ecosystems and their associated services. Projections by the current generation of Earth System Models have suggested decreases in global NPP in response to future climate change, albeit with very large uncertainties. Here, we make use of two versions of the Institut Pierre Simon Laplace Climate Model (IPSL-CM) that simulate divergent NPP responses to similar high-emission scenarios in the 21st century and identify nitrogen fixation as the main driver of these divergent NPP responses. Differences in the way N-fixation is parameterized in the marine biogeochemical component PISCES of the IPSL-CMs lead to N-fixation rates that are either stable or double over the course of the 21st century, resulting in decreasing or increasing global NPP, respectively. An evaluation of these 2 model versions does not help constrain future NPP projection uncertainties. However, the use of a more comprehensive version of PISCES, with variable nitrogen-to-phosphorus ratios as well as a revised parameterization of the temperature sensitivity of N-fixation, suggests only moderate changes of global-averaged N-fixation in the 21st century. This leads to decreasing global NPP, in line with the model-mean changes of a recent multi-model intercomparison. Lastly, despite contrasting trends in NPP, all our model versions simulate similar and significant reductions in planktonic biomass. This suggests that projected plankton biomass may be a much more robust indicator than NPP of the potential impact of anthropogenic climate change on marine ecosystems across model.
Highlights
Net Primary Production (NPP) by marine phytoplankton is responsible for nearly 50% of global carbon fixation (Field et al, 1998), and is the basis of almost all marine food chains, controlling the availability of energy and food for upper trophic 30 levels
4 Conclusions and Recommendations Two versions of the IPSL Earth System Models (ESMs) (IPSL-CM5A and IPSL-CM6A) are shown to project diverging global net primary production (NPP) trends in the 21st century under a similar high-emission scenario, because of the specificities of the diazotrophy parameterization employed in the different versions of the marine biogeochemical component PISCES
The use of additionnal PISCES versions confirms the role of diazotrophy parameterizations in driving divergent NPP responses in all subtropical gyres, with increased
Summary
Net Primary Production (NPP) by marine phytoplankton is responsible for nearly 50% of global carbon fixation (Field et al, 1998), and is the basis of almost all marine food chains, controlling the availability of energy and food for upper trophic 30 levels. 50 Despite being used extensively, including in international assessment reports such as the Special Report on the Ocean and Cryosphere in a Changing Climate (IPCC SROCC, Pörtner et al, 2019) and the Global assessment report on biodiversity and ecosystem services (IPBES, Diaz et al, 2019), these projections of future marine NPP are subject to large uncertainties, as demonstrated by inter-model differences (Frölicher et al, 2016) This is especially the case at the regional level, as shown in the Arctic Ocean (Vancoppenolle et al, 2015), in the Southern Ocean (Leung et al, 2015), and in the tropical oceans 55 (Kwiatkowski et al, 2017; Tagliabue et al, 2020; Tagliabue et al, 2021). Even within one specific model, poorly constrained assumptions around key biological components can drive substantial uncertainty in the projected changes in NPP across the 60 tropical Pacific (Tagliabue et al, 2020)
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