What is the role of the ocean’s abyss in global warming?¹

I’ve already posted on a study published in Nature Climate change that shows that the amount of extra global warming related heat in the Southern Oceans is greater than previously thought. There is another paper in the same journal by Llovel et al, “Deep-ocean contribution to sea level and energy budget not detectable over the past decade.” This paper verifies previous research that the oceans absorb a lot of the excess heat, but looks specifically at the ocean below 2,000 meters, which the paper referrs to in places as “deep” but that we should probably call “abyssal.”¹ The paper concludes that the abyss is not warming. This is bad news, because if it was warming the total effects of global warming on the surface would be potentially less, or at least, stretched out over a longer period of time. But, it is not unexpected news. We already suspected that the abyssal ocean does not absorb much of the surface heat, while the shallower ocean absorbs quite a bit.

Research done prior to 2012 (e.g. Hansen et al 2011) parceled out the energy imbalance the Earth experiences from anthropogenic global warming. The extra heat caused by AGW from 2004 to 2010 was divided among the upper ocean (71%), the deeep ocean (5%), with the rest going various other places (only 4% over land). The new paper suggests that the abyssal ocean takes up closer to zero heat.

There are three complexities you need to be aware of to interpret this finding. First is the complexity in the climate system, second is the complexity of the research itself, and third is the relatively straight forward statistical problem of assigning meaning to specific numbers. That third one is important for journalists and regular people to pay attention to, because the climate science denial community is already exploiting it to misrepresent this study.

This is a complex and difficult problem

We know that the vast majority of the extra heat resulting from global warming ends up in the ocean, and also, we know there is a lot of interaction between the ocean and the atmosphere, with heat that might otherwise add to the atmosphere seemingly entering the ocean on a regular basis, with some of it occasionally coming out in large quantitates during El Nino events. This relationship is expected to change over time as the ocean warms, as the transfer of heat between ocean and atmosphere depends in part on the relative difference between them. At some point it is likely that the degree to which the ocean takes up net heat will decrease if the ocean warms up beyond a certain point.

Over the medium and long term this matters a lot. Because of the ocean (and polar ice and a few other things) the effect of increasing greenhouse gasses is not instantaneous. If the Earth was a simple rock with no water, but a Nitrogen atmosphere with, say, 250ppm of CO2, the greenhouse effects of the CO2 would ensure that the atmosphere was at least a little warm. If we doubled the CO2 the atmosphere would warm further, and it would do so very quickly. A new equilibrium would be reached in a geological instant (a few years?). But with the ocean, that change is much slower slower (decades, perhaps many decades), because the ocean buffers the atmospheric change.

When heat goes into the ocean, it then moves around in the ocean because it disperses across the aqueous medium, and because water is always moving in currents or mixing. An El Nino is a change in the movement of water that has been warmed with contact with the surface, so that warm water that has been building up at depth over time changes its movement pattern and moves closer to the surface (and to a different horizontal location) where heat is released. That is one (especially large and important) example of the complex dynamic of atmosphere, ocean, and heat. Currents that move through the upper ocean then dive down to depth may move some of the surface heat to the deeper waters, especially where the currents have dived not just from cooling water (hot water would tend to go up, cold water would tend to go down) but because it is driven in “conveyor” systems which may run counter to expectations of where water should go when considering only local conditions, and especially, if the water is dropping because of an increase in salinity. Again, this is an example of the complexity of the system.

If we add a lot of CO2 to the atmosphere, the atmosphere will warm up, but because of the complexities cited above, it is hard to say how much or how long it will take. The ocean serves to slow the process down. In fact, it is quite possible that if the ocean would be so kind as to absorb a certain amount of this heat permanently, maybe global warming would be somewhat reduced. The ocean is potentially a way of stretching out the effects of global warming. But this effect is likely reduced if the abyssal ocean is not in the game.

Complexities in the research

Meanwhile the measurement of heat in the ocean has been very sparse. Over the last decade more measurements have been taken using new technology, but even that is not as good as we would like to understand what is going on at depth. So, when it comes to understanding heat in the ocean, we may sometimes feel like we are at sea. The two papers in this week’s Nature Climate Change are much more important as studies that calibrate or refine the process of measuring ocean heat dynamics under global warming than they are studies that change our view of global warming. Neither paper concludes anything unexpected, both provide important refinements to key numbers, exploiting the last decade of improved data collection.

One of the complexities is in the details of the Llovel et. al study as compared to the handful of previous related studies. One of the key numbers is the energy imbalance where the ocean absorbs extra AGW produced heat. Energy imbalance is measured in terms of Watts per m–2. The present study yields a value of 0.72. A previous study reported 0.54. Other estimates have varied in this range. Llovel et al point out, however, that these differences may be due to differences in the ocean depth considered in each study and the time periods covered. At least one earlier study measured energy imbalance for the top 1,800 meters, while Llovel et al look at the top 2,000 meters, and all the studies cover somewhat different time periods.

So, we have changing quality of data, a data set that is growing incrementally over time, studies that look at slightly different time and space parameters. And, on top of this, we have the increasingly advanced methods of figuring this all out. Both of the Nature Climate Change studies used a combination of direct measurements of temperature at various depths, a measurement of the altitude of the top of the ocean (sea level) from highly accurate satellite instruments, and measures of the mass of the water in the ocean, from the GRAIL gravity research project. If the mass of the ocean stays the same (same number of water molecules) but the surface rises, that is from heat, and that allows an estimate of energy imbalance. If the ocean goes up more than it should from heat expansion, the extra may be from glacial melting. And that is the simple version.

Statistical reasoning

The statistical part of this is not really so complex. Well, it is, but the part I want to point out is not. Llovel et al concluded “Accounting for additional possible systematic uncertainties, the ocean below 2,000 m contributes ?0.13 ± 0.72 mm yr?1 to global sea-level rise and ?0.08 ± 0.43 W m?2 to Earth’s energy balance.” Sea level rise is close to 3 mm a year, so the abyss is decreasing sea level rise by close to 4%. And, the abyss is in negative energy balance, while the upper ocean is in positive energy balance.

But look at the numbers. –0.13 plus or minus 0.72. There is actually no way to say that the abyssal ocean is contributing negatively to sea level rise. Zero (or small positive numbers) are well within the range of statistical probability. For energy imbalance, –0.08 plus or minus 0.43. Again, zero and small positive numbers are well within the statistical range for this value.

But for some reason we see various individuals, including sadly at least one climate scientist (Judith Curry: “Evidence of deep ocean cooling?“), but mostly anti-science climate trolls, crowing that the “deep ocean” is cooling therefore we are not experiencing global warming. However, the truth is that the total amount of heat that is going into the ocean, instead of the atmosphere or other places, was thought to be large, is still known to be large, and in fact is larger than we were originally thinking (from these papers and several others that have come out recently). And, the contribution of the abyss ocean to both sea level rise and energy imbalance is statistically nil. It might be negative, it might be positive, but it is tiny either way. The deep ocean, on the other hand, is in strong positive energy balance.

¹The terminology used in this discussion and some of the research papers (or reporting thereof) is a bit confused. Climate scientists have repeatedly said that heat is going into the “deep ocean” and this paper seems to say it is not. But it is. It is a matter of terminology. This is a source of confusion sometimes exploited by climate science denialists. A good way to define these terms is as follows:

Shallow ocean = 0-700 meters
Deep ocean = 700-2,000 meters
Abyss = > 2,000 meters

Hansen, J., Mki. Sato, P. Kharecha, and K. von Schuckmann, 2011: Earth’s energy imbalance and implications. Atmos. Chem. Phys., 11, 13421–13449, doi:10.5194/acp–11–13421–2011.

Llovel, W. J. K. Willis, F. W. Landerer, I. Fukumori. 2014. Deep-ocean contribution to sea level and energy budget not detectable over the past decade. Nature Climate Change, 5 October.