Future projections for the tropical Indian Ocean

The tropical Indian Ocean has undergone basin-wide surface warming since the start of the 20th century, with a rate of 0.12°C per decade between 1950 and 2020, which is the fastest among the tropical basins. The warming penetrates the deep ocean, with an increase in the ocean heat content from the surface until 2000m (OHC2000) at a rate of 3.7 zetta-joules per decade during 1960-2016. Here, we summarize our current understanding of the future changes in the Indian Ocean state based on climate model projections. Climate models under the Coupled Model Intercomparison Project Phase 6 project that in response to mid-to-high greenhouse gas emissions the Indian Ocean will very likely experience surface warming of 1.4-3°C between 2020 and 2100, at a rate of 0.17-0.38°C per decade. The OHC2000 is projected to increase at a rate of 16-22 zetta-joules per decade under mid-to-high emission scenarios. Marine heatwaves are projected to increase from 20days per year (during 1970-2000) to 220-250days per year, pushing the tropical Indian Ocean into a basin-wide near-permanent heatwave state by the end of the 21st century. In response to the ocean warming, Earth system models project a significant decline in surface chlorophyll and annual net primary productivity, with the strongest decrease of about 8%-10% in the western Arabian Sea. The Indian Ocean is projected to acidify further, with the surface pH of the tropical Indian Ocean decreasing to a pH below 7.7 by the end of the 21st century, compared to a pH above 8.1 during the early 20th century. Earth system model projections do not agree on the evolution of subsurface oxygen concentrations, calling for an improvement in the representation of biogeochemical processes or improved bias correction techniques. The rapid warming, decline in primary productivity, and acidification will continue to increase the pressure on the marine ecosystem and coral reefs.