Climate Change and the Missing Heat

by Bill Chameides | February 3rd, 2012
posted by Erica Rowell (Editor)

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Glory be. A substantive debate about climate science.

While pseudo debates about scientific facts and relative certainties are all the rage in certain media outlets, it is refreshing to encounter a real, live, heated (but still courteous) debate among climate scientists.

Such a debate is currently underway on the topic of the “missing heat.” You may recall the infamous quote from Kevin Trenberth from the “climategate” e-mails that was taken out of context: “The fact is that we can’t account for the lack of warming at the moment and it is a travesty that we can’t.” The travesty to which Trenberth was referring was not, let me be perfectly clear, that global warming had ceased as some have opined, but that heat was building up in the Earth system and we could not find it, hence the missing heat.

Accounting, Climate Style

Every year the Earth’s overall energy balance sheet, with respect to energy in and energy out, is slightly out of whack. Because of the action of greenhouse gases like carbon dioxide (CO2), the Earth absorbs slightly more energy in the form of sunlight than it radiates back out to space in the form of infrared (also called terrestrial) radiation. The amount of the imbalance is of the order of a half to one watt per square meter, and if we believe the first law of thermodynamics — and we most definitely do — that bit of extra energy has to show up somewhere within the Earth system.

But where? To find out, you need a sort of climate accountant, someone who adds up the energy balance sheets for all the assets (or heat reservoirs in the Earth’s climate system) and sees if the total of these is changing over time in a manner that matches the imbalance between the income (energy from the Sun) and the expenditures (infrared radiation going out to space).

Just such a careful accounting was carried by Jim Hansen of NASA’s Goddard Institute for Space Studies (GISS) and colleagues and reported in a paper they published in the journal Science in 2005. The authors were able to balance the books. The amount of extra heat being absorbed was in pretty good balance with the amount of extra energy that was accumulating in the Earth system.

Some of that extra energy shows up as an increase in the atmosphere‘s heat content — the manifestation of that being the increase in atmospheric temperature, a k a global warming. But the amount of heat the atmosphere absorbs each year is a tiny fraction of the total amount of extra heat the Earth absorbs. By far, most of the energy, perhaps 90 percent, is absorbed by the ocean. (This and the fact that the ocean has such a huge heat capacity puts a lot of thermal inertia into the climate system and is why there’s a ~ 30 year time lag between the release of a greenhouse gas and the climate’s equilibration.)

It’s important to note that Hansen et al’s energy balance sheet was based not only on a climate-model simulation; it was largely based on observations as well. For example the ocean’s increasing heat content was derived from long-term measurements of ocean temperatures.

So What’s Missing? Heat.

But just when it seemed like we had it all balanced up, things began to get confusing. Starting in about 2003, and more or less coincident with the establishment of a more extensive ocean monitoring network, the ocean data began to show very little buildup of heat in the surface ocean. How could that be, scientists asked. The heat imbalance between solar and terrestrial radiation was still at work so heat was still being added to the earth system. If it wasn’t going into the ocean, where was it going? Where is the missing heat?

Trenberth concluded that, given the current monitoring networks, it’s not possible to quantify the size of the missing heat or even if it really existed, let alone where it’s located. The reason: the current monitoring networks lack the requisite accuracy. It was, in his now infamous words, a “travesty” that we were not able to resolve this issue because of the inadequacy of our monitoring networks.

Does Anyone Know Where the Missing Heat Is?

Over the past few years, scientists have advanced various hypotheses about the missing heat. For example, Gerald Meehl of the National Center for Atmospheric Research and colleagues suggested that there has been a (most likely transient) enhancement in the transport of heat from the surface to the deep ocean, and the missing heat is in the deep ocean. This could explain the apparent stasis in temperature increases in the atmosphere and surface ocean — what would normally go into increased atmospheric and surface ocean temperatures has recently been going into the deep ocean.

In a paper published late last year in the journal Atmospheric Chemistry and Physics, Hansen, of GISS, and colleagues, revisited their study of the Earth’s budget and concluded that the budget can still be balanced without invoking missing heat — they put a bit more energy into the ocean and melting ice, while arguing that the Earth has been absorbing a little less energy because of the role of cooling particles in the atmosphere. (Look for more on this paper next week.)

What Heat Missing?

And now along comes another study, published last week in the journal Nature Geoscience by Norman Loeb of NASA’s Langley Research Center and colleagues who argue that the whole notion of missing heat is questionable. Given the inherent uncertainties in our ability to estimate the ocean heat content and energy imbalance from solar and terrestrial radiation, one cannot say with any confidence that the rate of increase in the ocean’s heat content has slowed or that there is any imbalance in the energy budget.

Following the publication of the Loeb paper, Trenberth posted a comment. He took issue with some aspects of Loeb et al’s estimate of the uncertainties. But more to the point argued that Loeb et al’s basic conclusion is essentially the same as his: the accuracy of our measurement networks is inadequate to resolve the Earth’s heat budget. One can’t definitively establish the existence of missing heat but cannot rule it our either.

So, is the missing heat really missing? Could it be hiding in the deep ocean? We don’t know yet. However, there is one thing we can predict with pretty good certainty. With the planned cutbacks in U.S. research funding, the answers will almost certainly not be resolved by improved monitoring networks supported by the United States or, thus, by better data.

Is that a travesty? Depends, I suppose, on how much the prospect of missing heat bothers you. If you’re a climate scientist, I suspect it might cause you to lose some sleep. If you’re someone who’s just wanting to decide what to do about climate change, I’d advise you to stay out of the missing-heat weeds and consider the bigger picture — like what’s at stake.

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