Genevieve Wanucha
Oceans at MIT

Carl Wunsch (MIT PhD ’67), Cecil and Ida Green Professor Emeritus of Physical Oceanography at MIT, has spent an entire career investigating the ocean’s role in climate, from both observational and theoretical angles. Early in his career, he spent many months working at sea. He helped organize the World Ocean Circulation Experiment, which collected the most comprehensive data set from the global ocean, and chaired the science committees leading to the flight of altimetric satellites. He has provided analysis of the oceanic general circulation and its climate influences. Mathematical methods, such as inverse theory and state estimation, form his understanding of ocean circulation and climate.

Because the ocean has an enormous ability to absorb heat from the atmosphere, it’s only logical that the ocean has warmed, and will continue to warm, as increasing amounts of greenhouse gases trap heat in the atmosphere. But what is the observational evidence that the ocean has warmed in recent years and how difficult is it to quantify? Oceans at MIT asked Wunsch about the limits of our knowledge.

1. Is there evidence that the ocean has been warming?

Overall, the ocean does appear to have warmed over the last 20 years when, relatively speaking, there are enough data to do a reasonably accurate calculation. These recent observations come from Argo floats, altimetric satellite measurements, measurements made using elephant seals tagged with instruments that monitor ocean conditions, the World Ocean Circulation Experiment (WOCE) era and follow-up repeated high resolution shipboard data, and better meteorological estimates. Before 20 years ago, so few data exist that calculations remain highly uncertain.

It’s plausible that modern ocean temperature changes are proceeding faster than earlier in the Holocene Epoch (a relatively warm period in Earth’s history that began about 11,750 years ago at the close of the last major ice age), but that signal would be lost from the record if it had been present. Paleoceanographic data, which come from “proxies” such as planktonic shell measurements of oxygen isotope ratios, are very interesting sources of information about ocean temperatures thousands of years ago, but (A), they are so sparse that no one can compute an accurate global average, and (B) interpretation of these data means that the conversion to temperature makes the numerical values very uncertain. Furthermore, the time-spacing of those data that do exist make it almost impossible to detect rapid time changes in temperature. It’s a different problem altogether from the modern one.

2. You and MIT physical oceanographer Patrick Heimbach recently published a paper in the Journal of Physical Oceanography on the mathematical modeling of the temperature changes in the deep ocean. You found that parts of the ocean actually cooled in your model. What mechanism could be responsible for the cooling?

We estimated the change in heat content of the ocean over a 20-year period using all of the many different kinds of data that are available in the interval from 1992-2011. Overall, we estimated that the ocean warmed over the past 20 years. The year-to-year variations in any given region are very large, sometimes warming, sometimes cooling, making it hard to compute an accurate average top-to-bottom. Over the 20 years, parts of the ocean appear to have cooled somewhat—despite the fact that on average we found a net warming. No contradiction exists because the ocean can take a long time to respond to heating and cooling, and parts of it are likely just beginning to change owing to atmospheric conditions from hundreds of years ago.

An analogue is the way a large house responds to outside air temperatures: if the outside temperatures go up, some parts of the house will tend to heat up quickly, particularly if the sun shines directly through the windows. Other rooms may take hours or days to start to warm. If there is a deep basement, that might stay cool for weeks despite the rest of the house having warmed up, even when the attic is stifling. When the outside temperatures drop, parts of the house may still be warming up even as the more responsive rooms have started to cool down again. The ocean has many “rooms,” many far removed from where the sun is directly shining.

3. Do you believe that the observed warming is due to anthropogenic forcing from greenhouse gases?

That seems very likely—based upon some simple physical principles. If you add greenhouse gases to the atmosphere, it will warm up, and there is then a very powerful tendency for the ocean to warm as well, more slowly. Think of the house analogy again: if I turn on a heater in one room it’s difficult to see why the adjoining rooms would not slowly warm up, and then the rooms next to them, etc. If the heated room were perfectly insulated, that wouldn’t happen, but there’s no such thing as perfect insulation, and the ocean has no analogue of that anyway.

We know the atmosphere has warmed, although exactly how much and where is subject to some uncertainty, and so the most reasonable inference is that the ocean did too–although again, exactly where, and by how much, is a serious research question. If the atmosphere is in a state to warm the ocean, everyone would agree that the upper ocean will warm first. But, there are a lot of ifs and buts. If there were a natural warming of the atmosphere, maybe because cloud cover changed, the upper ocean would also warm first. So how does that tell you if the cause is natural or artificial? Also, the upper ocean exchanges water with the deeper ocean. One question we must consider is whether that exchange is leading to a warming or cooling of the upper ocean.