Climate change has wrought major changes to ocean stability faster than previously thought, according to a recent study, raising alarms over its role as a global thermostat and the marine life it supports.
The research published in Nature looked at 50 years of data and followed the way in which surface water “decouples” from the deeper ocean. Climate change has disrupted ocean mixing, a process that helps store away most of the world’s excess heat and a significant proportion of CO2. Water on the surface is warmer – and therefore less dense – than the water below, a contrast that is intensified by climate change. Global warming is also causing massive amounts of fresh water to flush into the seas from melting ice sheets and glaciers, lowering the salinity of the upper layer and further reducing its density. This increasing contrast between the density of the ocean layers makes mixing harder, so oxygen, heat and carbon are all less able to penetrate to the deep seas.
The study’s lead author, Jean-Baptiste Sallée (pictured) said that while scientists knew that these processes were underway the rate of change is much quicker than previously thought, “more than six times quicker”, in fact. The report used global temperature and salinity observations obtained between 1970 and 2018 with a focus on the summer months. It said that the barrier layer separating the ocean surface and the deep layers had strengthened world-wide – at a much larger rate than previously thought.
Researchers also found that, contrary to their expectations, winds strengthened by climate change had also acted to deepen the ocean surface layer by five to 10 metres per decade over the last half century. A significant number of marine animals live in this surface layer, with a food web that is reliant on phytoplankton. But as the winds increase, the phytoplankton are churned deeper, away from the light that helps them grow, potentially disrupting the broader food web. These are “not small changes that only some experts care about”, Sallée said. “They represent a fundamental change in the underlying structure of our oceans. Way more pronounced than what we thought until now.”
Scientists are increasingly troubled by the warming of oceans, warning that it can lead to more extreme weather, such as more frequent, and more powerful hurricanes.
Global Warming Is ‘Fundamentally’ Changing The Structure of Our World’s Oceans, Science Alert, 2021-03-25