Oceanographers may have solved one of the biggest sea mysteries in years: why the upper ocean didn’t warm between 2003 and 2010, even as heat-trapping greenhouse gases accumulated in the air above.
Natural climate fluctuations have combined to keep the upper ocean cool, scientists report in an upcoming issue of Geophysical Research Letters. Much of the sun’s trapped energy has been released back into space, the modeling study suggests, while another large chunk is tucked away deep beneath the oceans.
“This is not rare. This is the normal variable behavior of climate,” says lead author Caroline Katsman, an oceanographer at the Royal Netherlands Meteorological Institute in De Bilt. “It’s not strange to go for eight years in a row with no heat gain.”
Temperatures in the upper 700 meters of the ocean rose over the last two decades of the 20th century before flattening out in 2003. Because climate scientists think the upper ocean must ultimately absorb most of the extra energy trapped by greenhouse gases, this lack of warming is particularly puzzling.
Katsman and her colleague Geert Jan van Oldenborgh used a computer model to probe natural influences on ocean temperatures, such as the El Niño climate pattern (which releases heat from the oceans back into space) and deep circulation patterns in the North Atlantic (which can bury heat at greater depths). The model suggested that these two factors happened to combine starting in 2003, keeping excess heat out of the upper ocean.
“What the study basically says is that it is no surprise that we might get a few years where the upper ocean is warming faster or slower than what is expected based on the greenhouse gas warming,” says Josh Willis, an oceanographer at the Jet Propulsion Laboratory in Pasadena, Calif. “This is important because we really are only just beginning to understand how big the natural variations in upper ocean temperature could be.”
Still, the new study relies on just one computer model, cautions Matthew Palmer, an oceanographer at the Met Office Hadley Centre in Exeter, England. Palmer and his colleagues have recently used a different model to show how the ocean can shift heat toward its depths over the course of about a decade. That work, also scheduled to appear in Geophysical Research Letters, supports the idea that much of the oceans’ extra heat is simply buried. So just looking at surface temperatures isn’t always a reliable way to detect global warming, Palmer says.
More insight should soon come from Argo, the global array of ocean buoys that measures temperature and other variables to a depth of 2,000 meters. Argo has already seen hints of warming below 1,000 meters, says Katsman, but many more years of data are needed before any such trend could be confirmed. “If we had better ocean observations, we could be much more confident about this,” says Palmer.
Within the next few years, El Niño and Atlantic circulation patterns will probably shift and allow the upper ocean to begin warming up again, says Katsman.