Assessment of time of emergence of anthropogenic deoxygenation and warming: insights from a CESM simulation from 850 to 2100 CE

Abstract

<p><strong>Abstract.</strong> Marine deoxygenation and anthropogenic ocean warming are observed and projected to aggravate under continued greenhouse gas emissions. These changes potentially adversely affect the functioning and services of marine ecosystems. A key question is whether marine ecosystems are already or will soon be exposed to environmental conditions not experienced during the last millennium. We find that anthropogenic deoxygenation and warming in the thermocline have today already left the bounds of natural variability in respectively 60 % and 90 % of total ocean area in a forced simulation with the Community Earth System Model (CESM) over the period 850 to 2100. Natural variability is assessed from last millennium (850–1800) results considering forcing from explosive volcanic eruptions, solar irradiance, and greenhouse gases in addition to internal, chaotic variability. Control simulations are typically used to estimate variability. However, natural variability in oxygen (O<sub>2</sub>) and temperature (T) are systematically larger than internal variability (e.g. the latter amounts to 20 % for T and 60 % for O<sub>2</sub> in the thermocline), rendering such estimates of natural variability to be biased low. Results suggest that anthropogenic change in apparent oxygen utilisation (AOU) and in O<sub>2</sub> solubility (O<sub>2,sol</sub>) are earlier detectable by measurements than in O<sub>2</sub> in the tropical thermocline, where biological and solubility-driven O<sub>2</sub> changes counteract each other. Both natural variability and change in AOU are predominantly driven by variations in circulation with a smaller role for productivity. Ventilation becomes more vigorous in the tropical thermocline by the end of the 21st century, whereas ideal age in deep waters increases by more than 200 years until 2100. Different methodological choices are compared and the time for an anthropogenic signal to emergence (ToE) is earlier in many thermocline regions when using variability from a short period, the control, or estimates from the industrial period instead variability from the last millennium. Our results highlight that published methods lead to deviations in ToE estimates, calling for a careful quantification of variability and that realised anthropogenic change exceeds natural variations in many regions.</p>

Biogeochemical Timescales of Climate Change Onset and Recovery in the North Atlantic Interior Under Rapid Atmospheric CO2 Forcing

“…Our results are consistent with Hameau et al. (2019) and Tjiputra et al. (2018a), who also suggest relatively higher ToD timescales for DO in shallower domains of the NAtl (<600 m), except for the regions between the center of the subtropical gyre and the western side of the basin.…”

“…A recent study by Hameau et al. (2019) using a different model (CESM) focused on changes within the thermocline (200–600 m) under RCP 8.5 (Representative Concentration Pathways) future scenario also suggested delayed ToD for Temperature around ∼20°N by the end of the century within the thermocline domain, which is consistent with our results. Similarly, Hameau et al.…”

“…Similarly, Hameau et al. (2019) also state that this pattern is characteristic of a climate change buffer zone, influenced either by high natural variability or inhibited anthropogenic response or even a combination of both.…”

A Positive Cooling Feedback for the Neoproterozoic Snowball Earth Initiation Due To Weakening of Ocean Ventilation

“…Both IAGE and VAGE trace the time elapsed since the last time a water mass was at the sea surface except that the latter considers the effect of sea ice (J. Zhang, 2016). IAGE is tracked by most ocean models and has been used to study the ocean stratification and relevant implications (e.g., ocean deoxygenation) under warm climate with limited sea ice, such as the present‐day and future warmer climate (Gnanadesikan et al., 2007; Hameau et al., 2019). While VAGE is not always available and most suitably used when studying the ocean ventilation during the glacials (Gu et al., 2020, 2021; J. Zhang, 2016).…”

Emerging Global Ocean Deoxygenation Across the 21st Century

“…Time of emergence (ToE) is used to identify the time when the signal of change in a climate variable emerges from its internal variability in the absence of external forcings (Hawkins & Sutton, 2012; King et al., 2015). Previous studies have focused on the global ToE of oxygen deoxygenation at a fixed depth range of generally 0–1,000 m (Frolicher et al., 2016; Hameau et al., 2019; Henson et al., 2017; Long et al., 2016). For example, deoxygenation around the thermocline, where the potential density surface is 26.5 kg m −3 , was projected to emerge from internal variability in the 2030s based on Community Earth System Model (CESM) large ensemble simulations under the representative concentration pathways 8.5 (RCP8.5) scenario (Long et al., 2016).…”