It was a massive heist that received little attention. Several hundred trillion joules of energy were disappearing every second. Investigators suspected the deep ocean was involved, but couldn’t find any leads. There’s no need to panic, though— a fresh look at the evidence shows that the energy may never have been missing in the first place.
Most people know that greenhouse gases trap heat near the Earth, warming the planet. We fixate on records set by temperatures of the near-surface atmosphere to track the warming caused by anthropogenic greenhouse gas emissions. But the atmosphere is only part of the picture. There are other reservoirs that take up heat energy as well—most notably, the ocean. In fact, about 90 percent of the energy added by the increase in greenhouse gases has gone into the ocean.
The 2000s saw lots of La Niñas, the cold phase of the El Niño/Southern Oscillation that lowers surface temperatures. If those temperatures are your only measure of global heat content, enough La Niñas may get you thinking that there's been a slowdown in the warming trend our planet has been experiencing. You get a much different picture when you look at the Earth as a whole, though. During La Niña years, the Earth actually gains more energy than it would otherwise. Conversely, El Niño years make surface temperatures warmer but slows the rise in total energy.
That’s mainly the result of changes in cloudiness, precipitation, and storm tracks that come along with La Niña or El Niño conditions. For example, clearer skies in the tropical Pacific (La Niña) can allow more solar radiation through, whereas increased evaporation (El Niño) moves heat from the ocean to the atmosphere while boosting cloudiness.
We now have satellite networks that measure the incoming solar radiation and the outgoing infrared radiation, so we can track the changes in the planet's heat content pretty well. If the incoming solar radiation is greater than the outgoing infrared, energy was added to the system. If that energy goes into the atmosphere, we can track it using a vast network of weather stations (and satellites, as well) that enable calculations of global near-surface atmospheric temperature.
The ocean is a tougher nut to crack. We used to rely on ship-based temperature profiles for the surface ocean, but the Argo program changed that in 2003. This array of 3,000 instrumented floats measures temperature (among other things) in the upper 2 kilometers of the ocean. That's a lot more detail, but creates a significant shift in the sorts of data we have.
In 2010, Kevin Trenberth and John Fasullo (of the National Center for Atmospheric Research) published an article in Science describing a discrepancy in our accounting of Earth’s energy budget. While the satellite tracking of incoming solar radiation and outgoing infrared radiation between 2004 and 2008 clearly showed that the net addition of energy was increasing, measurements of ocean heat content showed a decline. It was sort of like seeing that your checking account balance had gone down by $1000, but finding that you had only written checks totaling $300. You know the money is gone, but where did it go?
The missing energy didn’t show up in any of the other energy budget terms we can track. That may not be as exciting as a seemingly faster-than-light neutrino, but it was a very important disparity. Trenberth and Fasullo suggested that the energy could be moving into the deep ocean, which we currently can’t monitor. Lots of modeling had previously shown that, in a warming climate, the upper ocean heat content will occasionally decline as energy moves into the deep ocean. In a 2011 paper, they also mentioned an alternate possibility: the uncertainty in our ocean heat content measurements is simply very large, which would make the discrepancy an experimental error (and therefore much less interesting).
A large source of potential error was that the switch from the ship-based measurements of ocean heat content to the Argo array entailed all kinds of difficult-to-quantify uncertainties. (New instruments that were operated differently, uneven distribution and changing density of measurement points as floats were gradually deployed, etc.) A new paper in Nature Geoscience makes headway by re-examining the ocean heat content data and accounting for that complex uncertainty.
The group’s ocean heat content record differs slightly from other analyses (just as global surface temperature series from NASA and NOAA don’t come out exactly the same), but the pivotal bit is that the uncertainty during the Argo transition period really was quite large. In fact, the difference between the ocean heat content and net total energy data is not statistically significant—it’s well within the uncertainty. That suggests that the missing energy might not be so missing.
At least one thing remains clear in all the datasets—the Earth is steadily gaining energy. Between 2001 and 2010, the amount of energy reaching the Earth has exceeded the amount leaving by an average of about 0.5 watts per square meter.
Still, it’s critically important that our energy accounting improve, and that’s a formidable task. It would be encouraging to declare the case of the missing energy “solved,” but it’s not so encouraging that our measurements are too uncertain to settle the matter. As the authors conclude, “the large inconsistencies between independent observations of Earth’s energy flows points to the need for improved understanding of the error sources and of the strengths and weaknesses of the different analysis methods, as well as further development and maintenance of measurement systems to track more accurately Earth’s energy imbalance on annual timescales."
Nature Geoscience, 2012. DOI: 10.1038/NGEO1375 (About DOIs).
Listing image by Photograph by Olivier Dugornay
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