Oceanic Carbon Cycling Response to Global Temperature Changes - CYCLOCARB

Summary – Introduction – Specific objectives approach and methods – Originality, innovative and interdisciplinary aspect – References – Figures and data

Introduction

Anthropogenic changes in the Earth’s carbon cycle are amongst the most challenging environmental and societal issues for the coming centuries. The increase of atmospheric CO₂ already affects Earth’s climate and the oceanic carbon cycle, leading to rapid global warming (Fig. 1), steep sea level rise, and oceanic acidification, which already affect human societies ¹. The 2015 Paris Agreement sets the goal to limit global warming to ‘well below 2°C above preindustrial levels’, and to ‘pursue efforts’ to limit it to 1.5°C. Such an ambitious target requires foreseeing the evolution of the global carbon cycle. Moreover if some Greenhouse Gas Removal processes are implemented to keep Earth’s climate within the 2°C boundary, climate feedbacks induced by carbon removal also have to be considered.

The oceanic realm stores most of the carbon within the climatic system (>95%), and is directly connected to the largest earth carbon reservoir: marine sediments ², thus deserve a specific attention.

The CYCLOCARB project aims to assess the oceanic carbon reservoir response and feedbacks to global climate changes. It will provide a statistical model of carbon cycling and global temperature changes over the last 150 ka in order to quantify climate sensitivity, relative timing of carbon cycling disruption and temperature changes and their geographical distribution. The project will rely on a comprehensive database for carbon cycling and climate changes indicators of the last climate cycle, on the reconstruction of changes in carbon budget, carbon distribution, and temperature within the ocean-atmosphere system. This approach will provide a new perspective on processes involved in past, modern and future major climatic variations.

State of the art

The marine carbon pool plays a major role in modern and past global carbon cycling and associated climatic changes ¹. For example, nearly 30% of anthropogenic carbon emission had been trapped in the ocean so far ³. A deep understanding of the processes linking oceanic carbon cycle and climate is therefore critical to accurately anticipate the modality and intensity of ongoing global changes. Earth history has already experienced abrupt climate change with identified factors and forcing (variations of atmospheric CO₂, global temperature and sea level, Fig 1). The last glacial cycle shares some characteristics with ongoing climatic changes, thus in some aspect, provides a dampened natural analogue for the anthropogenic forcing, and even for some geoengineering scenarios/experiments ⁴.

The Carbon isotopic composition of foraminifers primarily depends on ambient seawater δ¹³C ⁵, which reflects a variety of processes relevant for the climatic system. A first order control on the isotopic composition of the climatically active carbon pool relies on the equilibrium between carbon sources (carbonate metamorphism, volcanic emission, weathering of carbon-bearing sediments on continental surfaces, and anthropogenic emission), and sinks (sequestration of carbon in ocean, sediment, soils, and terrestrial biomass) to the climatic system. Because carbon stable isotope fractionation occurs during photosynthesis, organic matter δ¹³C is markedly different than the carbon pool from which it was formed (between -20 and -25 ‰) ⁶, and impacts the isotopic composition of carbon reservoirs. For example, the modern massive release of low δ¹³ CO₂ from fossil fuel also affects the carbon isotopic composition of the climatic system⁷ (Fig. 1). The decrease of the δ¹³C of the ocean during the last glacial ⁸, inferred from foraminiferal δ¹³C, has been widely interpreted as reflecting a reduced land biomass at those times, thus an increase in the active carbon pool ⁹ comparable to ongoing changes (Fig. 1). However, despite clues for significant changes in sources ¹⁰ and sinks ^11-13 of carbon to the system, transient disequilibrium, thus variations of carbon inventory within the system is neglected in most of the numerical simulations applied to the last glacial cycle, partly because of a lack of quantitative constraints.

The sinking of ¹³C depleted organic carbon to the deep ocean and its remineralization sequester carbon away from the atmosphere, and reduces the δ¹³C of the deep ocean. It places a first order control on the atmospheric pCO2, as well as on the Δ δ¹³C between the surface and the deep ocean, a process known as the soft tissue pump. Variations of the soft tissue pump are undoubtedly implied in glacial interglacial variations of pCO₂ ^14,15, although its exact contribution and timing at a global scale remain relatively unconstrained. Several hypotheses have been proposed to explain changes in the biological pump over a glacial cycle. Changes in nutrient availability could have driven variations of the intensity of biological production ^16,17, as well as its nature ¹⁸, while changes in the strength of the oceanic circulation ^19-21 or transfer processes¹¹ can also be implied.

Proxies for sea surface temperatures (SST) are derived from organic or inorganic biological remains produced by surface-living marine organisms, and preserved in marine sediments. Paleoceanographic proxies for SSTs are of a critical importance for the evaluation of past climate variability, the actual climate sensitivity regarding to greenhouse gases, and the reliability and the performance of climate simulations. The modeling and proxy communities have tackled this topic over the past 40 years within the framework of several initiatives (CLIMAP²², MARGO²³, PMIP²⁴, COMPARE project initiated at CEREGE, which aim at compiling Mg/Ca and alkenone based SST proxies at low latitude for the last deglaciation ²⁵, fig 1), aiming at comparing the performance and behavior of a wide range of model for several key periods of past climate variability, and the SST proxies. However, due to the diversity of the proxies, the development of extensive global databases for SST proxies lags behind the modelers efforts ²⁶. Moreover, the SST proxy community is rapidly evolving, and new proxies are being developed, while some inconsistencies between proxies have to be reconciled ²⁵ (fig 1).

During the past decades, the scientific community has generated thousands of datasets on the physical and chemical properties of marine surface sediment and sediment cores, aiming at reconstructing the evolution of climatic and oceanic conditions across a wide range of spatial and temporal scales. These records allow for a deeper understanding of the earth system and its evolution through times, and have strongly impacted our perception of ongoing changes. However, the diversity of proxy types and heterogeneous reporting standards have hindered the analysis of globally distributed paleoceanographic time series, despite the tremendous potential utility of large datasets. The importance of standardization/ homogenization in data reports practice and semantic has now been recognized ^27,28 (LinkedEarth NSF initiative). Yet in order to provide a synoptic view of combined effects of temperature and carbon cycling, an effort has to be made to combine different proxies within the same analysis.