Projected decreases in future marine export production: the role of the carbon flux through the upper ocean ecosystem

Abstract

Abstract. Accurate projections of marine particle export production (EP) are crucial for predicting the response of the marine carbon cycle to climate change, yet models show a wide range in both global EP and their responses to climate change. This is, in part, due to EP being the net result of a series of processes, starting with net primary production (NPP) in the sunlit upper ocean, followed by the formation of particulate organic matter and the subsequent sinking and remineralisation of these particles, with each of these processes responding differently to changes in environmental conditions. Here, we compare future projections in EP over the 21st century, generated by four marine ecosystem models under the high emission scenario Representative Concentration Pathways (RCP) 8.5 of the Intergovernmental Panel on Climate Change (IPCC), and determine the processes driving these changes. The models simulate small to modest decreases in global EP between −1 and −12 %. Models differ greatly with regard to the drivers causing these changes. Among them, the formation of particles is the most uncertain process with models not agreeing on either magnitude or the direction of change. The removal of the sinking particles by remineralisation is simulated to increase in the low and intermediate latitudes in three models, driven by either warminginduced increases in remineralisation or slower particle sinking, and show insignificant changes in the remaining model. Changes in ecosystem structure, particularly the relative role of diatoms matters as well, as diatoms produce larger and denser particles that sink faster and are partly protected from remineralisation. Also this controlling factor is afflicted with high uncertainties, particularly since the models differ already substantially with regard to both the initial (presentday) distribution of diatoms (between 11-94 % in the Southern Ocean) and the diatom contribution to particle formation (0.6-3.8 times higher than their contribution to biomass). As a consequence, changes in diatom concentration are a strong driver for EP changes in some models but of low significance in others. Observational and experimental constraints on ecosystem structure and how the fixed carbon is routed through the ecosystem to produce export production are urgently needed in order to improve current generation ecosystem models and their ability to project future changes.

Evolution of Future Black Sea Fish Stocks under Changing Environmental and Climatic Conditions

“…The observed decrease in NPP in the biochemical model is in agreement with other modeling studies using future climate change scenarios such as the comparison of four different global models that predict lower net primary production rates for future (2012-2100) climate under IPCC's emission scenario RCP8.5 (Laufkötter et al, 2015(Laufkötter et al, , 2016. Several other studies of the global ocean projected global marine net primary production to decrease in response to future climate change (Bopp et al, 2001(Bopp et al, , 2013Boyd and Doney, 2002;Steinacher et al, 2010;Marinov et al, 2013;Cabré et al, 2014).…”

Ideas and perspectives: climate-relevant marine biologically driven mechanisms in Earth system models

“…For example, the organic carbon pump will likely be altered by changes in stratification (Steinacher et al, 2009), the ocean's molecular viscosity (Taucher et al, 2014), and plankton community composition (see, e.g., Laufkötter et al, 2016). The alkalinity pump may be affected by changes in freshwater input or Table 2.…”

Southern Ocean Phytoplankton Blooms Observed by Biogeochemical Floats

“…The biological carbon pump, which involves the production of organic carbon via photosynthesis and export to depth, is argued to be responsible for maintaining ∼90% of the vertical gradient of dissolved inorganic carbon (Sarmiento, ) and ∼10% of the total carbon flux to the deep ocean (Siegenthaler & Sarmiento, ). While there is no evidence for any recent global‐scale changes in the biological carbon pump (McKinley et al, ), its contribution to carbon sequestration may change in a future warming climate (Hauck et al, ; Henson et al, ; Laufkötter et al, ; Moore et al, ). Consequently, phytoplankton and biological activity are important contributors to both natural and anthropogenic carbon sequestration via export production and maintaining the dissolved inorganic carbon gradient (Gruber et al, ).…”