Much like a heated kettle of water takes some time before it comes to the boil, it seems intuitive that the world's oceans will also take some time to fully respond to global warming. Unlike a kettle, however, it's not obvious how the oceans warm.
Adding further greenhouse gases to the atmosphere warms the ocean cool skin layer, which in turn reduces the amount of heat flowing out of the ocean. Reducing the heat lost to the atmosphere allows the oceans to steadily warm over time - as has been observed over the last half century.
Sunlight penetrating the surface of the oceans is responsible for warming of the surface layers. Once heated, the ocean surface becomes warmer than the atmosphere above, and because of this heat flows from the warm ocean to the cool atmosphere above. This process is represented in the graphic below:
Figure 1 - simplified steps of ocean heating
The rate of flow of heat out of the ocean is determined by the temperature gradient in the 'cool skin layer', which resides within the thin viscous surface layer of ocean that is in contact with the atmosphere. It's so named because it is the interface where ocean heat is lost to the atmosphere, and therefore becomes cooler than the water immediately below. Despite being only 0.1 to 1mm thick on average, this skin layer is the major player in the long-term warming of the oceans.
The cool skin behaves quite differently to the water below, because it is the boundary where the ocean and air meet, and therefore turbulence (the transfer of energy/heat via large-scale motion) falls away as it approaches this boundary. No longer free to jiggle around and transfer heat via this large scale motion, water molecules in the layer are forced together and heat is only able to travel through the skin layer by way of conduction. With conduction the steepness of the temperature gradient is critical to the rate of heat transfer.
Greenhouse gases, such as carbon dioxide, trap heat in the atmosphere and direct part of this back toward the surface. This heat cannot penetrate into the ocean itself, but it does warm the cool skin layer, and the level of this warming ultimately controls the temperature gradient in the layer.
Increased warming of the cool skin layer (via increased greenhouse gases) lowers its temperature gradient (that is the temperature difference between the top and bottom of the layer), and this reduces the rate at which heat flows out of the ocean to the atmosphere. One way to think about this is to compare the gradient (steepness) of a flowing river - water flows faster the steeper the river becomes, but slows as the steepness decreases.
The same concept applies to the cool skin layer - warm the top of the layer and the gradient across it decreases, therefore reducing heat flowing out of the ocean.
An important point not be be glossed over here, is that changing the temperature gradient in the cool skin layer by way of greenhouse gas warming is a worldwide phenomenon. Once the gradient has changed, all heat leaving the ocean thereafter has to negotiate its way through the layer. With the gradient lowered, the ocean is able to steal away a little bit more from heat headed for the atmosphere. It is in this ever-present mechanism that oceans are able to undergo long-term warming (or cooling).
Obviously it's not possible to manipulate the concentration of CO2 in the air in order to carry out real world experiments, but natural changes in cloud cover provide an opportunity to test the principle. Under cloudy conditions, the cloud cover radiates more heat back down toward the ocean surface than happens under clear sky conditions. So the mechanism should cause a decline in skin temperature gradients with increased cloud cover (more downward heat radiation), and there should also be a decline in the difference between cool skin layer and ocean bulk temperatures - as less heat escapes the ocean under increased atmospheric warming.
This was observed in an experiment carried out in 2004, aboard the New Zealand research ship Tangaroa. Using intruments to simultaneously measure the 'cool skin', the ocean below, and the amount of heat (longwave radiation) reaching the ocean surface, researchers were able to confirm how greenhouse gases heat the ocean. It should be pointed out here, that the amount of change in downward heat radiation from changes in cloud cover in the experiment, are far greater than the gradual change in warming provided by human greenhouse gas emissions, but the relationship was nevertheless established.
Figure 2 -The change in the skin temperature to bulk temperature difference as a function of the net longwave (heat) radiation. The net forcing is negative as the atmosphere is cooler than the ocean skin layer, but approaches zero under cloudy conditions. See Real Climate post "Why Greenhouse Gases Heat The Ocean" by Professor Peter Minnett.
The effect of greenhouse gases on ocean heat isn't confined to daylight hours however, they toil away around the clock. The warming of the oceans by sunlight, makes the daytime surface waters more bouyant than the cooler waters below and this leads to stratification - a situation where the warmer water floats atop cooler waters underneath, and is less inclined to mix. At night much of the heat accumulated during the day is lost back to the atmosphere (the overling air still being cooler than the ocean), and this cooling leads to the stratified surface layers sinking and mixing with lower layers. This allows the remaining heat to be transported down deeper into the ocean, causing an increase in ocean heat content over the long-term. The typical diurnal (day/night) cycle is seen in the figure below:
Figure 3 - Schematic showing the upper ocean temperature profiles during the (A) nighttime or well mixed daytime and (B) daytime during conditions conducive to the formation of a diurnal warm layer. Image from Gentemann & Minnett (2008)
Given the atmospheric lifetime of carbon dioxide is many hundreds to thousands of years, we can now understand that long-lived greenhouses will also continue to exert a warming influence on the worlds oceans for a very long time. Indeed, climate models suggest that ocean warming will continue for at least a thousand years even if CO2 emissions were to completely stop. See below:
Fig 5 - Time series of the (modeled) climate response to a cessation of CO2 emissions. a) global mean thermosteric sea level anomaly (b) and zonal mean ocean temperature at 792.5mtrs, 66 S (the Southern Ocean). Green line = cessation of CO2 at 2010 & red line = cessation at 2100. From Gillett (2011).
Because of their effect on lowering the temperature gradient of the cool skin layer, increased levels of greenhouse gases lead to more heat being stored in the oceans over the long-term. This ocean warming mechanism has been observed experimentally, and is also supported by numerical modeling.
So although greenhouse gases, such as carbon dioxide, don't directly warm the oceans by channeling heat down into the oceans, they still do indeed heat the oceans, and are likely to do so for a very long time.
Posted by Rob Painting on Tuesday, 18 October, 2011
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