Anthropogenic climate change drives non-stationary phytoplankton internal variability

Abstract

Abstract. Earth system models suggest that anthropogenic climate change will influence marine phytoplankton over the coming century with light-limited regions becoming more productive and nutrient-limited regions less productive. Anthropogenic climate change can influence not only the mean state but also the internal variability around the mean state, yet little is known about how internal variability in marine phytoplankton will change with time. Here, we quantify the influence of anthropogenic climate change on internal variability in marine phytoplankton biomass from 1920 to 2100 using the Community Earth System Model 1 Large Ensemble (CESM1-LE). We find a significant decrease in the internal variability of global phytoplankton carbon biomass under a high emission (RCP8.5) scenario and heterogeneous regional trends. Decreasing internal variability in biomass is most apparent in the subpolar North Atlantic and North Pacific. In these high-latitude regions, bottom-up controls (e.g., nutrient supply, temperature) influence changes in biomass internal variability. In the biogeochemically critical regions of the Southern Ocean and the equatorial Pacific, bottom-up controls (e.g., light, nutrients) and top-down controls (e.g., grazer biomass) affect changes in phytoplankton carbon internal variability, respectively. Our results suggest that climate mitigation and adaptation efforts that account for marine phytoplankton changes (e.g., fisheries, marine carbon cycling) should also consider changes in phytoplankton internal variability driven by anthropogenic warming, particularly on regional scales.

Global Warming Increases Interannual and Multidecadal Variability of Subarctic Atlantic Nutrients and Biological Production in the CESM1‐LE

“…However, because global warming also suppresses the sensitivity of chlorophyll to nitrate anomalies by a factor of 2 (Figure 2f), the stdevs of chlorophyll do not increase (Figure 1b) despite intensifying nitrate volatility and high correlations between nitrate and chlorophyll variability. The declining sensitivity of chlorophyll to nutrient anomalies is consistent with tightening top‐down control of the phytoplankton biomass anomalies with global warming (Elsworth et al., 2022). The reduced variance of regionally integrated chlorophyll in the CESM (Figure 1b) thus reflects a balance between the increased variance of bottom‐up drivers (Figures 1e and 1h) and reduced sensitivity to those drivers (Figure 2f) in a warmer world.…”

“…Circumventing these difficulties, this study uses output from a large ensemble of simulations with the Community Earth System Model (CESM1‐LE) (Kay et al., 2015) to quantify how the characteristics of the internal variability of ocean biological production change during the high‐emissions RCP8.5 scenario. As demonstrated by several recent studies of ocean biogeochemistry (Elsworth et al., 2022; Li & Ilyina, 2018; Long et al., 2016; Lovenduski et al., 2016; McKinley et al., 2016; Schlunegger et al., 2019), a single‐model large ensemble is a valuable complement to an ensemble of simulations with different models (as in the Coupled Model Intercomparison Project archive), because the differences between the simulations are attributable to internal Earth system variations without convoluting differences due to model formulation (Deser et al., 2020).…”

“…One might expect that these strong downward trends in the biomass, production, nutrients and their physical drivers might be accompanied by reductions in the corresponding interannual and multidecadal variability (Elsworth et al., 2022). And, indeed, the interannual stdev of the chlorophyll declines by about 30% during the RCP8.5 scenario (Figure 1b) in conjunction with declines in the interannual stdevs of the March MLD and the AMOC (Figures 1k and 1n).…”

Long‐Term Trends in the Distribution of Ocean Chlorophyll

“…In a study focusing on analyzing phytoplankton carbon biomass in an Earth System Model large ensemble, Elsworth et al. (2022) identified decreasing variability of global phytoplankton variance from 1920 to 2100. Our results do not show an overall decreased variability in CHL.…”

“…Compound extreme events, where two or more ocean extremes are happening synergistically (e.g., low oxygen and high temperature) are of particular concern as they can contribute to biological and ecological impacts in different ways (Burger et al., 2022; Gruber et al., 2021; Le Grix et al., 2021). Several studies have considered how the ocean's variance may be responding to climate change, including sea surface temperatures (Alexander et al., 2018), marine carbon dioxide (Landschützer et al., 2018), sea ice (Tareghian & Rasmussen, 2013), sea level (Barbosa, 2008), and phytoplankton biomass (Elsworth et al., 2022). A recent study showed that changes in variance are omnipresent in different aspects of Earth's climate and span physical and ecosystem variables, and tend to be more predominant in variables that are typically not normally distributed such as primary production (Rodgers et al., 2021).…”