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This is a repository copy of Reduced Arctic sea ice extent during the mid-Pliocene Warm Period concurrent with increased Atlantic-climate regime.

Approximately 30% of the anthropogenic CO 2 emission has been absorbed by the oceans (Friedlingstein et al., 2019;Sabine & Tanhua, 2010). The CO 2 invasion to the oceans since the industrial revolution (∼1850) has caused a decrease of ∼0.1 unit in surface ocean pH, a process commonly known as ocean acidification (OA) (Doney et al., 2009). Numerical model simulations for the end of the century at different scenarios of Abstract Atmospheric CO 2 rise in post-industrial era has resulted in decline in surface ocean pH, commonly known as "ocean acidification (OA)," which has become a threat to marine calcifiers. Instrumental records of ocean pH and its reconstruction utilizing boron isotope (δ 11 B) composition of corals demonstrate a long-term OA trend characterized by large spatio-temporal variability in both Pacific and Atlantic oceans. However, no such record exists to elucidate long-term OA trend of the Indian Ocean. We report the first sub-annually resolved pH record from the Arabian Sea based on δ 11 B measurements on Porites coral from Lakshadweep coral reefs. This pH record is characterized by large variability ranging from 7.93 to 8.65 with no long-term discernable trend. The long-term declining trend expected from the ∼50 ppm increase in atmospheric CO 2 during the coral growth interval appears to be obscured by large surface pH variability in the Arabian Sea. Our investigation reveals that physical oceanographic processes for example, upwelling, downwelling and convective mixing modulated by El Niño-Southern Oscillation (ENSO) largely control surface pH variability and masked expected long-term OA trend resulting from anthropogenic CO 2 rise. Combining the model-based predictions of increase in frequency and amplitude of ENSO events in a future warming scenario and the observed ENSO dependency of surface water pH, we predict more frequent and large pH variability ("pH extremes") in this region. Such pH extremes and their occurrences might be critical for the resilience and adaptability of corals and other calcifiers in Arabian Sea and other similar oceanic settings elsewhere.Plain Language Summary Increase in the atmospheric CO 2 concentration since the industrial revolution (∼1850) has resulted in decrease in ocean pH, known as ocean acidification (OA). Only limited number of pH records are avalable to assess the impacts of OA on marine ecosystems. The available pH records from the Pacific and Atlantic oceans, based on both instrumental observations and coral boron isotope records, demonstrate a long-term declining trend with significant internal variability. However, no such records are available from the Indian Ocean. Here, we provide the first seasonally resolved record of Indian Ocean pH for a 23 year period based on boron isotope study of corals collected from the Lakshadweep, Arabian Sea. pH variability in the Arabian Sea is domiantly controlled by ENSO modulated oceanographic processes such as upwelling and mixing. We did not observe any long-term declining trend corresponding to the ∼50 ppm rise...