Declining Aerosols to Blame for Rapid Subtropical Ocean Warming

The subtropical oceans in the Southern Hemisphere are warming rapidly as the climate changes. Now, scientists may have pinpointed why the water's temperature is rising so quickly. It turns out that, surprisingly, declining aerosols could be linked to the current issue.

The scientists were first interested in the three decade-long cooling below these subtropical oceans from the 1960s to the 1990s. Yet as they were studying this phenomenon, they were struck by the recent rapid warming in the oceans. More specifically, they saw a phenomenal increase in temperatures in the Indian Ocean.

In order to examine this particular event a bit further, the researchers looked at more than 40 climate simulations that included historical changes to greenhouse gases and aerosols over the twentieth century. This allowed them to see if atmospheric conditions might have affected the temperature increase.

"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 Tim Cowan, lead author of the new study, in a news release. "This delay in the modeled 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."

In fact, the researchers found that the 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. The rate of warming also related to how quickly the aerosol levels declined after their peak.

Aerosols can cool the Earth, scattering incoming sunlight as it enters our atmosphere. This, in turn, can increase the movement of heat from the Southern Hemisphere oceans to the Northern Hemisphere oceans via sea currents. Combine this with a greenhouse gas-induced southward shift of the Indian subtropical ocean gyres toward the Antarctic, and the processes delay the Indian Ocean warming in models.

"What makes this work fascinating is the fact that human-emitted aerosols have such a large impact on remote ocean temperatures," said Cowan in a news release.

The findings reveal a little bit more about how the ocean was warmed so rapidly. As aerosols declined and as greenhouse gases increased, temperatures have increased in the waters. That said, it's still difficult to quantify how much this warming is due to the aerosols and how much is due to the greenhouse gases. However, the research does show that it is, in part, due to declining aerosols.

The findings are published in the journal Scientific Reports.