They partly attribute the observed warming, and preceding cooling trends to ocean circulation changes induced by global greenhouse gas emissions and aerosols predominantly generated in the Northern Hemisphere from human activity.

The research, by scientists from CSIRO and the University of NSW, was published in Scientific Reports.

Mr Tim Cowan, lead author of the study, says his group was initially interested in the three decade long cooling below the surface of the Southern Hemisphere subtropical oceans from the 1960s and 1990s. "But what really caught our eye was a rapid warming of these subtropical oceans from the mid-1990s, most noticeably in the Indian Ocean between 300 m to 1000 m depth," said Mr Cowan.

This had the research team asking whether this rapid warming was partly a response to greenhouse gases overcoming the cooling effect of aerosols that peaked globally in the 1980s due to the introduction of clean air legislation across United States and Europe.

To test this, the researchers examined more than 40 state-of-the-art climate simulations that included historical changes to greenhouse gases and aerosols over the twentieth century.

"What we found was that the models do a good job at simulating the late twentieth century cooling and rapid warming in the subtropical southern Atlantic and Pacific Oceans, however they show an around 30-year delay in the warming in the Indian Ocean" said Mr Cowan.

"This delay in the modelled Indian Ocean warming is likely due to the presence of atmospheric aerosols, generated through transport emissions, biomass burning, and industrial smog, together with natural emissions of sea salt and dust - these were also the main cause of the late twentieth century subtropical Indian Ocean below-surface cooling" said Mr Cowan.

The researchers found that models with a delayed peak in Northern Hemisphere aerosol levels after the 1980s had a tendency to simulate a delayed rapid Indian Ocean warming until well after 2020, and that the rate of warming related to how quickly the aerosol levels declined after their peak.

"We know that aerosols in the atmosphere generally cool the Northern Hemisphere by scattering incoming sunlight. This, in turn, increases the movement of heat from the Southern Hemisphere oceans to the Northern Hemisphere oceans via a global oceanic conveyor belt, travelling south from the subtropical Indian Ocean, passing the southern tip of Africa into the south Atlantic and then north along the Gulf Stream" said co-author Dr Wenju Cai.

"Together with a greenhouse gas-induced southward shift the Indian subtropical ocean gyres towards the Antarctic, these processes delay the Indian Ocean warming in the models," Dr Cai said.

"What makes this work fascinating is the fact that human-emitted aerosols have such a large impact on remote ocean temperatures" says Mr Cowan. "For many years aerosols have masked the direct surface warming induced by greenhouse gases in many Northern Hemisphere regions, however in the Southern subtropical Indian Ocean both aerosols and greenhouse gases have historically conspired to produce a net oceanic cooling, and now the reverse of some of these processes is occurring."

Mr Cowan said that despite the observed rapid ocean warming, quantifying exactly how much is due to declining aerosols or increasing greenhouse gases remains difficult, but as human-generated air pollution is all-together phased out, this will undoubtedly reveal the full impact of greenhouse gases.

The research has been supported by the CSIRO Wealth from Oceans National Research Flagship, The Australian Climate Change Science Program and the Australian Research Council Centre of Excellence in Climate System Science.