The current “pause” in global warming that has baffled climate scientists could last at least another decade because of oceanic cooling cycles, according to a new report.
A new report published in the journal Science found that the global warming “slowdown is mainly caused by heat transported to deeper layers in the Atlantic and the Southern oceans, initiated by a recurrent salinity anomaly in the subpolar North Atlantic.”
Heat is trapped more easily because the “waters in the Atlantic current coming up from the tropics are saltier because of evaporation,” notes the BBC. “This sinks more quickly and takes the heat down with it.”
The report, led by Professor Ka-Kit Tung from the University of Washington, also noted the “[c]ooling periods associated with the latter deeper heat-sequestration mechanism historically lasted 20 to 35 years.”
So far, about 30 theories have been put forward about why global temperatures stopped rising in the late 1990s, leading to a near two-decade hiatus in global warming, according to satellite data. Currently, more than half the satellite temperature record — which began in 1979 — shows no warming trend, despite rapidly rising carbon dioxide levels in the atmosphere.
Theories for why the Earth stopped warming in the late 1990s have ranged from declining solar activity to increasing levels of soot in the atmosphere from Chinese coal plants. Some scientists have even argued that there is no pause in global warming and that the planet has actually warmed at a faster rate since the 1990s.
Scientists have also pointed to ocean oscillation cycles, such as El Niño and La Niña patterns in the Pacific ocean. One 2013 study by German scientists found that increasing solar activity and Pacific and Atlantic Ocean oscillation cycles accounted for virtually all the global warming since 1870.
The German study, however, went on to predict a century of global cooling. Tung’s study says that Atlantic Ocean cycles could mean only another decade or so of cooling.
Like Tung, other scientists have also pointed to ocean patterns that effectively trap heat that would otherwise warm the planet. Scientists writing for the RealClimate blog point to data that show the increase in the amount of heat in the oceans amounts to 17 x 1022 Joules over the last 30 years.”
“That is so much energy it is equivalent to exploding a Hiroshima bomb every second in the ocean for 30 years,” they wrote.
“The latter part of the 20th century saw rapid global warming as more heat stayed near the surface,” researchers, led by Tung, wrote in their study. “In the 21st century, surface warming slowed as more heat moved into deeper oceans. In situ and reanalyzed data are used to trace the pathways of ocean heat uptake.”
Tung and his team noted that there were similar pauses in global warming in the past. One was between 1945 and 1975, when the Atlantic and southern ocean currents brought heat deep into the ocean and brought up cooler waters. This led to fears of another ice age, reports the BBC.
From 1976 to the late 1990s, the currents reversed and more warm water stayed on the surface while cooler waters were trapped. Not surprisingly, this trend helped warm the world and spur fears of catastrophic global warming.
“The floats have been very revealing to us,” Tung told the BBC. “I think the consensus at this point is that below 700 meters in the Atlantic and Southern oceans [they are] storing heat and not the Pacific.”
But researchers warned that while ocean currents could stave off warming for another decade, the planet will rapidly warm when the trend reverses itself again.
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