A B S T R A C T Variations in surface water hydrographic properties and dissolved inorganic carbon (DIC) were evaluated in the coastal Bay of Bengal using observations carried out during MarchÁApril 1991 and 2011, including 8 yr monthly time-series observations during 2005 and 2013. The coastal Bay of Bengal is characterised by relatively fresher, more basic and lower pCO 2 in 1991 compared to 2011. The rates of decrease in pH, increase in DIC and pCO 2 per decade were consistent with global trends in the southwestern (SW) coastal Bay of Bengal, whereas rates in the northwestern (NW) coastal Bay of Bengal were observed to be 3Á5 times higher. The associated recent increase in sulphate and nitrogen aerosol loadings over NW Bay of Bengal from the Indo-Gangetic Plain and Southeast Asia during winter and spring may be mainly responsible for the increased acidity in recent years. Thus, this region, which was previously considered to be a significant sink for atmospheric CO 2 , now seems to have become a source of CO 2 to the atmosphere.