The contemporary ocean provides a climate service by absorbing excess heat and carbon from the atmosphere, thereby slowing the pace of rising global temperatures. This service, however, comes with penalty — namely ocean acidification and ocean warming, which alter the ocean carbon cycle and impact marine ecosystems. These changes in the ocean are imminent, however potentially disguised by the natural variability of the climate system. “We sought to address a key scientific question: when, why and how will these important changes become observable in the global ocean?” says lead author, Sarah Schlunegger of Princeton University.
The paper focuses on the detectability or “Time of Emergence” of such anthropogenic changes in the ocean carbon cycle. Time of Emergence reveals when such changes could emerge above natural variability, allowing these changes to be potentially detected by the global observing system and indicating timescales for which impacts of climate change on marine ecosystems may be realized.
In order to reveal the influence of natural variability on future climate states, a specific experimental design called a Large Ensemble is conducted with a climate model. The Large Ensemble simulates many different potential future climate states that could result from a combination of anthropogenic climate change and random chance. These experiments were performed with GFDL’s Earth System Model, a climate model which has an interactive carbon cycle, such that changes in the climate and carbon cycle can be considered in tandem.
The paper finds that processes tied directly to the rise in atmospheric CO2 exhibit relatively short emergence timescales of a few (1-3) decades. These include acidification, its impacts on the cycling of carbon, and changes in the rate at which the ocean sequesters CO2 from the atmosphere. In contrast, processes tied indirectly to the rise in atmospheric CO2 through its gradual modification of climate and subsequently ocean circulation take many (3-11+) decades to emerge. These include changes in upper-ocean mixing, nutrient supply and the biological cycling of carbon.
The long timescales of emergence indicate that observing platforms and programs need to be maintained for many decades to come in order to effectively monitor the changes occurring in the ocean. The paper uses the simulations to show that the detectability of some changes in the ocean would benefit from an altered observational sampling strategy. These include looking deeper into the ocean for changes in chlorophyll, and capturing changes in both summer and winter (rather than just the annual mean) for the ocean-atmosphere exchange of CO2.
The results also indicate that many types of observational efforts are critical for our understanding of our changing planet and our ability to detect change, including time-series or permanent locations of continuous measurement, as well regionally resolved sampling programs, and globally resolved remote sensing platforms.
Emergence of Anthropogenic Changes in the Ocean Carbon Cycle, Sarah Schlunegger, Keith B. Rodgers, Jorge L. Sarmiento , Thomas L. Frölicher, John P. Dunne, Masao Ishii and Richard Slater, Nature Climate Change (2019) doi: 10.1038/s41558-019-0553-2
Corresponding author: Sarah Schlunegger, Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, NJ, USA (firstname.lastname@example.org)
The project was funded by a NASA Earth Science Division grant and results from international collaboration, including researchers from the Center for Climate Physics in South Korea, the University of Bern in Switzerland, NOAA Geophysical Fluid Dynamics Laboratory in the USA and the Meteorological Research Institute in Japan.
Abstract (directly from manuscript)
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 CO2 on the ocean. Processes sensitive to carbonate-chemical 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 CO2 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).