Emergence of anthropogenic signals in the ocean carbon cycle

Abstract

Attribution of anthropogenically-forced trends in the climate system requires understanding when and how such signals will emerge from natural variability. We apply time-of-emergence diagnostics to a Large Ensemble of an Earth System Model, providing both a conceptual framework for interpreting the detectability of anthropogenic impacts in the ocean carbon cycle and observational sampling strategies required to achieve detection. We find emergence timescales ranging from under a decade to over a century, a consequence of the time-lag between chemical and radiative impacts of rising atmospheric CO 2 on the ocean. Processes sensitive to carbonatechemical changes emerge rapidly, such as impacts of acidification on the calcium-carbonate pump (10 years for the globally-integrated signal, 9-18 years regionally-integrated), and the invasion flux of anthropogenic CO 2 into the ocean (14 globally, 13-26 regionally). Processes sensitive to the ocean's physical state, such as the soft-tissue pump, which depends on nutrients supplied through circulation, emerge decades later (23 globally, 27-85 regionally).

Potential predictability of marine ecosystem drivers

“…The predictability time horizon of surface pH is mainly determined by a complex interplay between DIC and Alk predictability in the low latitudes and DIC, Alk and temperature predictability in high latitudes. Interestingly, we find longer predictability time horizons for SST than for NPP in the equatorial Pacific, which is in contrast to findings of Séférian et al (2014a). Importantly, this may be indicative of a potential model dependency of the relationship between ecosystem driver predictability.…”

“…Our results suggesting the same global predictability time horizon for all four ecosystem drivers are not inconsistent with time of emergence diagnostics for transient climate warming scenarios where pH (early emergence) and NPP (late emergence) behave oppositely (Frölicher et al, 2016;Rodgers et al, 2015;Schlunegger et al, 2019). Time of emergence is defined as the ratio (large for pH and small for NPP) of the anthropogenic forced change to the background internal variability.…”

“…To date, only a few studies have investigated the predictability of marine ecosystem drivers (Chikamoto et al, 2015;Park et al, 2019;Séférian et al, 2014a). An intriguing finding of these studies is that marine biogeochemical drivers may be more predictable than their physical counterparts.…”

“…An intriguing finding of these studies is that marine biogeochemical drivers may be more predictable than their physical counterparts. Séférian et al (2014a), for example, showed that net primary productivity (NPP) has greater predictability than sea surface temperature (SST) in the eastern equatorial Pacific. They hy-pothesized that SST is strongly influenced by high-frequency surface fluxes, whereas NPP is more directly impacted by thermocline adjustment processes that determine the rate at which nutrients are brought into the ocean's euphotic layer.…”

Is deoxygenation detectable before warming in the thermocline?

“…The answer may have implications for measurement strategies to detect anthropogenic changes in subsurface waters as well as for the impacts of physical and biogeochemical change on marine life. For the surface ocean, earlier studies (Keller et al, 2014;Rodgers et al, 2015;Frölicher et al, 2016;Schlunegger et al, 2019) showed that the anthropogenic signals of pH and pCO 2 emerge earlier than sea surface temperature and O 2 change earlier than productivity. Changes in surface O 2 are tightly coupled to temperature-driven solubility changes and O 2 varies hand in hand with sea surface temperature and the two signals emerge typically concomitantly.…”