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Ocean warming will likely continue for centuries

According to an international team of climate scientists, even if societies curb greenhouse gas emissions and manage to reach a “net zero” scenario by mid-century – as many countries pledged in the Paris Agreement in 2015 – slow emerging changes such as deep ocean warming and sea-level rise will continue long afterward due to a phenomenon called “ocean thermal inertia.” Thus, climate actions need to be established at multiple time scales, taking into account developments over larger, centuries-long periods. 

The global ocean, which covers about 70 percent of the Earth’s surface, is slower both to absorb and release heat than land. Moreover, its enormous mass and heat capacity makes it much more capable of storing heat than land or even air, turning it into the most important controlling component of the Earth’s climate. 

This “ocean thermal inertia” offers both good and bad news with respect to global warming. While our planet would heat up much faster without oceans, the oceans will keep on warming for a long time, even if we manage to stop emissions completely by mid-century: like a speeding train taking time to slow down once the brakes are hit, the climate system will continue to change for a considerable time afterwards, as heat is transported downwards into deeper ocean waters.

“This process means that while surface warming may stabilize at about 1.5-2°C when global emissions reach net-zero emissions, sub-surface ocean warming will continue for at least hundreds of years, yet we normally only talk about climate action on the scale of a few decades to the end of the century at the most,” explained study lead author John Abraham, a professor of Mechanical Engineering at the University of St. Thomas in Minnesota. “That needs to change.” 

“Changes to the ocean will also continue to impact extreme weather over these longer periods, as well as sea-level rise,” added study co-author Lijing Cheng, an ocean and climate scientist at the Institute of Atmospheric Physics from the Chinese Academy of Sciences. “And infiltration of sea water into fresh water supplies can affect coastal food supplies, aquifers, and local economies. Other impacts that are connected to ocean warming and so need to be considered for the very long term include more damaging storm surges, coastal erosion, marine heatwaves, ocean acidification, and marine oxygen depletion.”

Moreover, key decision-makers, city planners, and vulnerable communities should also take into account the possibility of “high impact, low probability” events, such as an abrupt slowdown of the Atlantic Meridional Overturning Circulation (AMOC), passing a tipping point for losing an ice sheet, massive methane emissions from the seafloor or thawing permafrost, or abrupt shifts in ocean ecosystems, including major extinction events.

The study is published in the journal Atmospheric and Oceanic Science Letters.

By Andrei Ionescu, Staff Writer

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