Have you heard the tale about the “missing heat” in the climate system? Well, it turns out it may not have gone missing after all.
Global warming is driven by an imbalance between how much energy the planet takes in from the sun, and how much it lets back out into space in the form of thermal radiation. The planetary energy balance has been off kilter ever since humans started burning massive amounts of fossil fuels, such as coal and oil, for energy and transportation. (Natural factors have altered the energy balance in the past, causing ice ages, for example.)
Today, it’s widely agreed that the planet is warming because we’re taking in more energy than we’re letting out, thanks to growing concentrations of greenhouse gases such as carbon dioxide, which help keep outgoing energy from escaping into space.
In recent years, some researchers have focused on a problem with the planetary energy ledger.
Some measurements have shown that despite taking in more energy than we’re letting out, neither surface temperatures nor ocean temperatures have been warming at a corresponding rate. Notably, researchers Kevin Trenberth and John Fasullo of the National Center for Atmospheric Research (NCAR), have referred to this as “missing heat” or “missing energy.” Trenberth and Fasullo, along with their NCAR colleague Gerald Meehl and others, published a paper last year that sought to explain where the missing heat might be going.
Here’s how the NCAR crew described the missing heat idea in the journal Nature Climate Change: “If there are time periods when globally averaged surface temperatures are not increasing, this excess energy must go elsewhere, either to heat the atmosphere and land, to melt ice or snow, or to be deposited in the subsurface ocean and manifested as changes in ocean temperature and thus heat content.”
The study showed that during short periods when surface warming slowed or stopped altogether, more heat was being deposited into the deep oceans, at depths of 300 meters or more.
Now a new study, published in the journal Nature Geoscience, seems to contradict the missing energy hypothesis. It shows that uncertainties in the observations of upper ocean heat content, first via instruments lowered into the water by ships between 1990 and 2002, and more recently courtesy of a more sophisticated network of about 3,000 Argo buoys, suggest that the missing heat idea is flawed.
“We find that the difference between the heat balance at the top of the atmosphere and upper-ocean heat content change is not statistically significant when accounting for observational uncertainties in ocean measurements,” the study states. In other words, rather than finding the missing heat, the new paper basically says there was no missing heat in the first place.
Much of the uncertainty stems from the switch from ship-based measurements to the modern buoy network, which introduced discontinuities and biases into the data.
The new study finds that the differences between the energy flows at the top of the atmosphere, as measured by satellite-based sensors, and ocean temperature data are within the margin of error. Therefore, the researchers conclude, it’s quite possible that there is no missing heat.
“Differences in variations in ocean heating rate and satellite net [top of the atmosphere energy] flux are well within the uncertainty of the measurements and, therefore, cast doubt on the argument for ‘missing energy’ in the climate system,” the paper states. “We conclude that energy storage is continuing to increase in the sub-surface ocean,” the paper states. The study calls for improving observational capabilities to keep tabs on the energy flows in the climate system.
In a response that was published at David Appell’s Quark Soup blog, Trenberth dismissed the new study’s findings, saying the uncertainties in the ocean heat content data remain too large to draw the conclusion that observational uncertainties are the best explanation for the missing heat. The new study, Trenberth wrote, “seems to have its main point that our earlier paper was wrong. I will certainly disagree with that.”
According to Trenberth, scientists need to significantly reduce the uncertainties in the ocean heat content data before they can suspend the search for the missing heat. “The discrepancies among [upper-ocean heat content data sets] remain huge. We MUST do better,” he told Appell.