New scientific findings indicate that regional clean air policies, while beneficial for public health, could have an unexpected consequence: further weakening the Atlantic Meridional Overturning Circulation (AMOC). This vital system of ocean currents, which plays a significant role in moderating the climate of the UK and Western Europe, is already under threat from the broader impacts of global warming. The research suggests a complex interaction between different human-induced climate factors.
The study, conducted by researchers at the Scripps Institution of Oceanography at UC San Diego, alongside colleagues at the National Center for Atmospheric Research and the University of Miami, explored the intricate relationship between atmospheric aerosols and ocean currents. It found that a reduction in aerosol pollution, particularly from industrial regions in the Northern Hemisphere, could lead to an accelerated weakening of the AMOC. Aerosols, tiny particles suspended in the atmosphere, have a cooling effect by reflecting sunlight back into space, thereby partially offsetting the warming caused by greenhouse gas emissions.
Historically, significant industrial emissions of aerosols in Europe and North America created a 'cooling veil' over these regions. This localised cooling affected ocean temperatures and, in turn, the dynamics of the AMOC. As these regions have successfully implemented policies to reduce air pollution over recent decades – improving air quality and public health – this aerosol-induced cooling effect has diminished. The research indicates that this reduction in cooling could now be allowing the underlying warming trend from greenhouse gases to exert a stronger, more direct influence on the AMOC.
The AMOC is a large system of ocean currents, including the Gulf Stream, that transports warm surface water from the tropics northwards into the North Atlantic. As this water cools and becomes saltier, it sinks and flows southwards at depth, creating a powerful 'conveyor belt'. This process is critical for distributing heat around the globe and is responsible for the relatively mild climate experienced in the UK and much of Europe, compared to other regions at similar latitudes.
Scientists have observed a weakening of the AMOC over the past century, a trend largely attributed to global warming and the melting of the Greenland ice sheet, which introduces freshwater into the North Atlantic and reduces the density of surface water, inhibiting sinking. This new research, which has been peer-reviewed and published in the journal Science Advances, adds another layer of complexity, suggesting that the benefits of clean air policies might inadvertently remove a masking effect that had previously buffered some of the AMOC's decline. It underscores the interconnectedness of various climate change drivers and the challenges in predicting the full range of their interactions.
The findings build upon existing research highlighting the vulnerability of the AMOC. While the immediate implications for the current rate of AMOC weakening are still being modelled, the study provides a more comprehensive understanding of the factors influencing this critical ocean system. It suggests that while cleaning up air pollution is unequivocally good for health and local environments, it necessitates an even greater focus on reducing greenhouse gas emissions to mitigate the broader climate impacts.