Abiotic plastic leaching contributes to ocean acidification

6 pages, 4 figures, 2 tables.-- Data availability: The data are included in the manuscriptOcean acidification and plastic pollution are considered as potential planetary boundary threats for which crossing certain thresholds could be very harmful for the world's societies and ecosystems well-being. Surface oceans have acidified around 0.1 units since the Industrial Revolution, and the amount of plastic reaching the ocean in 2018 was quantified to 13 million metric tonnes. Currently, both ocean threats are worsening with time. Plastic leaching is known to alter the biogeochemistry of the ocean through the release of dissolved organic matter. However, its impact in the inorganic chemistry of the seawater is less studied. Here we show, from laboratory experiments, that abiotic plastic degradation induces a decrease in seawater pH, particularly if the plastic is already aged, as that found in the ocean. The pH decrease is enhanced by solar radiation, and it is probably induced from a combination of the release of organic acids and the production of CO2. It is also related to the amount of leached dissolved organic carbon, with higher acidification as leaching increases. In coastal areas, where plastic debris accumulates in large quantities, plastic leaching could lead to a seawater pH decrease up to 0.5 units. This is comparable to the projected decrease induced in surface oceans by the end of the twenty-first century for the most pessimistic anthropogenic emissions scenariosThis work and C.R.-C.'s involvement were supported by a “Proyecto I+D+I Jóvenes Investigadores-2019” (PID2019-109889RJ-I00) funded by MCIN/AEI 10.13039/501100011033. This work acknowledges the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S) and the Thematic Network of Micro- and Nanoplastics in the Environment (RED2018-102345-T, EnviroPlaNet) funded by MCIN/AEI 10.13039/501100011033Peer reviewe