Improved Understanding of the Role of the Oceans in Global Warming

The sun warms the Earth's surface. Additional greenhouse gases and associated positive feedbacks (like, additional additional greenhouse gasses) increase that effect. So, it gets warmer, and by "it" we mean the "surface" of the Earth. This is usually measured as the temperature near the surface across the land and the surface of the sea (Sea Surface Temperature or SST). But over 90% of the heat added by global warming goes into the ocean.

We know how much heat goes into the ocean (other than SST) two ways. One is direct measurements using equipment that samples water at depth, and the other is by super amazing precise measurements of how big the ocean is (reflected in the altitude of the surface), which increases as it heats up. Direct measurements are sparse and do not go back a long ways, and are very rare in the southern hemisphere compared to the northern hemisphere.

A paper just out in Nature Climate Change looks more closely at the Southern Hemisphere by combining direct measurements, estimates from ocean expansion, and some fancy modeling. The study suggests that the estimate of heat held in the upper 700 meters of the ocean in the Southern Hemisphere since 1970 was several percent too low owing to the lack of good data.

The study looks at the period from 1970 to 2004, prior to the deployment of some (but not yet sufficient) improved measurement technology. Study author Paul Durack notes, “Prior to 2004, research has been very limited by the poor measurement coverage. By using satellite data, along with a large suite of climate model simulations, our results suggest that global upper-ocean warming has been underestimated by 24 to 58 percent. The conclusion that warming has been underestimated agrees with previous studies, however, it’s the first ... estimate [of] how much heat we’ve missed.”

This is a lot of heat. From the paper, "For perspective, these adjustments represent more than double the 1970‐2004 heat storage change for all non‐ocean (terrestrial, cryospheric and atmospheric) heat reservoirs combined..."

What does this mean?

It may mean that there is more heat added to the Earth's surface than we thought there was, which means that any empirical estimates of the effects of global warming would need to be increased. But the real meaning may only be understood when we have a better handle on what happens to heat within the ocean, as shallow waters interact with deeper waters, and how the ocean as a system interacts with the atmosphere. And this heat, at this depth, does interact with the atmosphere. Despite the rather spectacular nature of this finding, its greatest significance is probably that it is a major step towards quantifying what may be the biggest single unknown related to climate change: what is happening in the ocean. It may, though this is subject to revision, increase the higher end of the estimate of "climate sensitivity" which is a measure of how much the surface of the earth will warm given a doubling of pre-industrial levels of atmospheric CO2. In that sense, this is potentially unpleasant news.

I have an FAQ on this research that I'm not sure I can provide a link for, but I'll past the part that addresses the importance of the research, in the view of the authors:

What are the implications of long-term underestimates to ocean warming?
Quantifying how much heat is accumulating in the Earth system is critical to improving our understanding of climate change already underway and to better assess how much more we can expect in decades and centuries to come. Our key result is that the warming of the global ocean in recent decades has been substantially underestimated. These findings will likely lead to a revisit of previous sea-level and climate sensitivity estimates, and to a re-examination of how scientists deal with poorly sampled aspects of the climate system. A key lesson to be learned from our work is that observing the global ocean is critical, and that prior to the recent improvement in global coverage of ocean observations, a substantial and very important part of the global climate system was under-observed. In order to better understand past and future climate changes it is imperative that the global ocean is adequately observed, as it plays a critically important role in the Earth's climate and its change.

I have a feeling there will be a lot of discussion of this over the next few days.

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From NASA Jet propulsion Laboratory:

The cold waters of Earth’s deep ocean have not warmed measurably since 2005, according to a new NASA study, leaving unsolved the mystery of why global warming appears to have slowed in recent years.

So we are in fact getting better data, which shows that there is no missing heat!

By Kevin Finnegan (not verified) on 06 Oct 2014 #permalink

Actually, we are getting better data that shows that there is a considerable amount of "missing" (not at the surface) heat in the ocean.

Kevin seems to be trying to think like a lawyer about science. NASA may well revise their estimate, or the authors of this study may revise theirs if NASA's is better, or it may turn out that NASA's study left out the vast southern hemispheric oceans and so needs to be evaluated in light of that fact.

people can walk and chew gum at the same time. studies in science are not all or nothing, they are parts of a puzzle and on study can be party true or mostly true or merely apply to limited cases or to limited data sets or circumstances. it is up to us to evaluate the evidence and put it all into context.

deniers of all sorts tend to treat studies as all or nothing weapons to be used in a legalistic debate and in order to 'win' a debate. they can do that sometimes but mostly they are pieces in a bigger picture that we are trying to understand. deniers do not want to understand or see the bigger picture, they just want to score points.

If you heated up the ocean from 700 meters to the bottom by a fraction of a percent, it would be a huge amount of energy. The distribution of heat in the deep ocean isn't necessarily even. Cold water goes down, warm up, so in a warming ocean, we would not expect too much eat there.

Meanwhile, the amount of heat in the upper 700 meters is actually rather large.

Kevin Finnegan is directly quoting the first 2 lines of the NASA report. He should have kept reading.

"deep ocean" is below 2000m
"Scientists at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, analyzed satellite and direct ocean temperature data from 2005 to 2013 and found the ocean abyss below 1.24 miles (1,995 meters) has not warmed measurably. "

"Coauthor Felix Landerer of JPL noted that during the same period warming in the top half of the ocean continued unabated, an unequivocal sign that our planet is heating up."…

Felix Landerer is also a co-author of the study that Greg refers to above.

The density change with temperature of seawater warms/cools is not linear. In fact there is a maximum density at a few degrees C. So heat that warms deep water will have less effect of ocean volume than that which heats shallow water. So volume changes in the overall ocean (sea level rise, if you can account for water inputs/outputs (melting landice, changes in land storage etc.)), don't directly tell you how much heat has been absorbed by the oceans.

By Omega Centauri (not verified) on 06 Oct 2014 #permalink

What about all the:
- ice that has heated (this is not measured)
- ice that has melted (this "heat" is by definition "missing", as it doesn't equate to any temperature changes)

By Craig Thomas (not verified) on 06 Oct 2014 #permalink

Re Omega Centauri @ #6, sea water does not show the density maximum like pure water. The dependence on temperature is lower at low temperatures but remains negative to below 0 C so it is possible to derive temperature information from volume data.

Craig Thomas: Good question. But if you calculate the heat needed to melt all the ice that's melting (upwards of a trillion tonnes per year), you find it's small, about 0.01-0.02 W/m2.

By David Appell (not verified) on 10 Oct 2014 #permalink

enSKog: Do you know if the thermal expansion of (sea)water has much dependence on pressure?

By David Appell (not verified) on 10 Oct 2014 #permalink