This summer, nearly half the world’s oceans are experiencing a marine heatwave, defined as warmer than 90% of previous temperature observations on the same date. There are other possible explanations in addition to climate change and the El Niño/La Niña cycle. New pollution rules have cut airborne sulfur aerosol particles released by commercial ships over parts of the ocean, clearing the air and allowing more sunlight to reach the ocean surface. That in turn might be heating the water along some shipping routes, although the amount is in dispute. In January 2022, an underwater volcano near Tonga blasted 50 million tons of water vapor into the stratosphere. Some researchers believe that vapor might be acting as a planet-warming greenhouse gas and nudging up ocean temperatures. Both theories are still under investigation, and their overall impact is up for debate.
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