By Bill Chameides
NASA climate scientist Jim Hansen and colleagues weigh in on the missing heat issue.
In 2009 Kevin Trenberth, of the National Center for Atmospheric Research, raised the issue of missing heat in a paper [pdf] titled "An Imperative for Climate Change Planning: Tracking Earth's Global Energy." In principle the excess heat trapped in the Earth as a result of greenhouse gases must show up somewhere on the Earth as heat; and, given its large heat capacity, we expect most of that heat to end up in the ocean.
Starting in about 2003, though, a slowdown in heat buildup was observed in the ocean while greenhouse gases continued to build up in the atmosphere, trapping more heat. Six years later, Trenberth asked [pdf] in the journal Current Opinion in Environmental Sustainability, where was the missing heat? In his now famous hacked e-mail message, Trenberth lamented that our observation systems' inability to resolve this issue was "a travesty."
Missing Heat Investigated in the Journals
As I described in a post last week, Norman Loeb of NASA's Langley Research Center and colleagues argued recently in the journal Nature Geoscience that, given the uncertainties in current observation networks, it's not clear that heat buildup in the ocean slowed down or, ergo, that any heat has gone missing at all.
Today, let's look at another paper on the subject, one that appeared in the journal Atmospheric Chemistry and Physics late last year and that reached a different conclusion. This paper by Jim Hansen of NASA's Goddard Institute for Space Studies and colleagues is a very lengthy and meaty paper that covers a plethora of issues related to the climate and the Earth's energy budget. Despite its technical weightiness, it contains some wonderfully clear, accessible statements for non-climate-science types about how the greenhouse effect works and where some of the fundamental uncertainties lie in our understanding of the climate's response to greenhouse gases. If nothing else, the introduction [pdf] is worth a read.
Tracking the Earth's Energy Flow During Two Periods: 1993-2008 and 2005-2010
My focus will be on just one aspect of the Hansen et al paper -- their take on the Earth's energy budget during two periods of time: 1993-2008 and 2005-2010. which we'll call Period 1 and Period 2.
(These two periods reflect two improvements in the accuracy of ocean measurements: Period 1 marks the start of satellite measurements of sea level; Period 2 begins with a nearly fully deployed Argo float system with good spatial coverage of the oceans. More on Argo [pdf].)
The major factor in assessing the Earth's energy budget is the ocean -- that's where some 90 percent of the excess energy absorbed by the planet due to greenhouse gases ends up going. In other words, to a very rough approximation, whatever energy is added to the Earth's system should show up in the ocean as extra heat.
Hansen and colleagues estimate from ocean data that the oceans absorbed about 0.8 watts per square meter in Period 1, but only 0.58 watts per square meter in Period 2. Loeb et al found a similar decrease in the oceanic heat uptake over time, but calculated a large uncertainty and dismissed the difference as being insignificant. However, Hansen et al estimate a smaller error uncertainty than that of Loeb et al, and argue that the difference is significant.
So Hansen et al come down on the side of Trenberth in the sense that they conclude the ocean is absorbing less heat now than before (about 0.2 watts per square meter less).
The Sun's Role
But where Hansen et al diverge from Trenberth is on the issue of whether there is any missing heat. Hansen et al say no. Here's their reasoning.
It turns out that the Sun has been going though its own peregrinations over this period as well. If you remember, between 2008 and 2010 we went through one of the most intense solar minima in recent history, during which time the Sun sent us a little bit less energy in Period 2 relative to Period 1.
How much less? Funny you should ask: On the basis of satellite measurements of solar radiation, Hansen et al. estimate that the energy we've been getting from the Sun dropped by 0.14 watts per square meter between 2000 and 2009. This drop-off in the Sun's output plus an increase in cooling due to aerosols is roughly the same as the difference in the heat buildup in the ocean over the two periods. And voilà, the budget is in balance -- there is no missing heat because the Sun never sent it our way.
Hansen et al make a special point of noting the fact that the Earth continued to accumulate excess heat even during a period when we had a less active Sun delivering less energy:
"The strong positive energy imbalance during the solar minimum ... constitute a smoking gun, a fundamental verification that human-made climate forcing is the dominant forcing driving global climate change. Positive net forcing even during solar minimum assures that global warming will be continuing in the decades to come."
It would appear that the Sun isn't the only driver on this planetary bus.