1) are primarily controlling climate variability.
In the south-western Pacific, large-scale atmospheric circulation features and the seasonally varying latitudinal position of the tropical convergence zones (Intertropical and South Pacific Convergence Zones (ITCZ and SPCZ) extending from the western Pacific warm pool (WPWP Fig. However, due to spatiotemporal constrains, the currently available coral-based pH reconstructions predominantly focus on the GBR and the marginal South China Sea 12, 13, 14, 15, 16, whereas longer timescale pH changes of the open surface ocean from the Pacific Ocean remain poorly documented 17. Coral-based reconstructions of ocean pH suggest notable recent acidification 12, 13 and decadal variability 12, 14. Additionally, it is crucial to investigate pH changes with links to ocean CO 2 uptake as well as the relationship to sea surface temperature and forcing mechanisms at various timescales.Ī method to document sub-seasonal to annual pH changes is based on the δ 11B signature in scleractinian coral skeleton. To study the natural variability in oceanic CO 2 uptake and carbonate chemistry changes, proxy records in modern marine archives such as corals provide a unique opportunity to go further back in time at a high resolution on the centennial scale. The pH records that span only the most recent decades are far too short to precisely monitor the longer-term evolution of OA and to quantify its impacts on tropical ecosystems.
Presently, accurate measurements of pH in the oceans are near non-existent aside from selected ship track measurements 10 and limited station-based instrumental time series available over the past few decades 11. Moreover, end-of-century emission scenarios predicted by model simulations indicate unprecedented decrease by up to 0.4 pH units 8 compared to the changes of only 0.2 pH units over glacial-interglacial cycles 9. Future projections of coral calcification rates suggest a possible additional decrease by up to 30% in the twenty-first century 7. These critical threats of ocean acidification (OA) on such marine organisms and ecosystems have been documented on the Great Barrier Reef (GBR) in Australia that witnessed a 14% decrease in coral calcification since 1990 6.
One of the major consequences is the decrease of carbonate saturation state that is crucial for calcifying organisms such as scleractinian corals to precipitate their aragonite skeletons 5. Many modelling and experimental studies have indeed shown clear trends of shallow water acidification in lockstep with increasing atmospheric CO 2 4. At present, the oceans take up more than 41% of anthropogenic CO 2 emissions 2, inducing the acidification of the ocean surface with major implications in marine carbonate chemistry and biological ecosystems 3. The concentration of atmospheric CO 2 is increasing at unrelenting rates in response to human activities in fossil fuel combustion and land-use practices 1.
We suggest changing surface winds strength and zonal advection processes as the main drivers responsible for regional pH variability up to 1881 CE, followed by the prominent role of anthropogenic CO 2 in accelerating the process of ocean acidification. High-amplitude oceanic pH changes, likely related to atmospheric CO 2 uptake and seawater dissolved inorganic carbon fluctuations, reveal a coupled relationship to sea surface temperature variations and highlight the marked influence of El Niño/Southern Oscillation and Interdecadal Pacific Oscillation. Here we present evidence of striking secular trends of decreasing pH since the late nineteenth century with pronounced interannual to decadal–interdecadal pH variability in the South Pacific Ocean from 1689 to 2011 CE. Due to limited instrumental measurements and historical pH records in the world’s oceans, seawater pH variability at the decadal and centennial scale remains largely unknown and requires documentation. Increasing atmospheric CO 2 from man-made climate change is reducing surface ocean pH.