Time of emergence of multiple ocean ecosystem stressors in a large ensemble with an Earth System Model

Abstract:

Marine organisms and ocean ecosystems are increasingly stressed by human-induced changes. Since many of these stressors – including rising ocean temperatures, ocean acidification, ocean deoxygenation, and perturbations to ocean biological productivity – co-occur in time and space, marine organisms and ocean ecosystems are increasingly being subjected to the simultaneous impact of multiple stressors. The cumulative impact of multiple stressors on ocean ecosystems may lead to damages far more severe than those from individual threats alone.

Detecting trends in the individual and combined stressors, however, is challenged by natural variability of the climate system. Here we use a 30-member ensemble simulation from GFDL's Earth System Model ESM2M under a historical/RCP8.5 concentration pathway over the period 1950 to 2100 to evaluate the time of emergence (ToE) at which the signal of changes in the individual and combined ecosystem stressors emerges from the noise of natural climate variability. The natural variability of the ensemble suite is used to quantify the variance in the system against which the ensemble mean signal is to be detected.

We show that the ToE in ocean acidification occurs much earlier than for other stressors, namely at the end of the 20^th century in most regions of the global ocean. In contrast, for primary productivity the signal does not emerge from the noise over most of the global ocean before the end of the 21^st century. The ToE pattern for sea surface temperature with early emergence simulated in high latitudes has reversed spatial patterns relative to the deoxygenation ToE pattern in the top 100 to 600 meters. However, for sea surface temperature, the trend emerges earlier (late 20^th/early 21^st century) for the majority of the tropical oceans, compared mid-21^st century for ocean deoxygenation. The combined ToE for the different multiple stressors and the implications for adaption and mitigation policies will be discussed.