I’m trying to understand these terms in the context of climate change and haven’t had any luck Googling my question.
I see that NOAA talks a lot about ocean heat content (OHC) as opposed to ocean temperature. OHC seems to have risen 30-somethings (joules?), but the temperature is only up less than 1 degree.
What does OHC quantify?
Thank you.
Pretty much what it sounds like–a measure of heat, which is a measure of energy. Different materials can hold different amounts of heat. For example, if a given volume of air has the same amount of heat as the same volume of water, the air will have a hotter temperature (see heat capacity). Water with different composition (salt vs. fresh) may have different temperatures if they contain the same amount of heat.
By measuring heat instead of temperature, you are eliminating the variable of what kind of substance you are measuring.
Ocean heat content deals with the overall energy stored in the oceans. Temperature, on the other hand, tends to be related to surface ocean temperature.
Recent studies seem to show that increased winds and currents are mixing the top and bottom ocean layers faster than before. So while the top layer may not be increasing in temperature quite as fast you think it should, the overall heat content of the ocean is growing quickly.
Thank you but I’m not getting it, I wish I understood what you mean by that.
I get that things “feel” different, like a hot rock feels hotter than a hot blanket, even if they’re roughly the same temperature. And I know that a hot wet blanket feels hotter than a hot dry one. And I realize I’m talking about conductivity now 
I don’t understand how to say the oceans have a lot more energy without them also having a lot more heat. Is the lot more energy expressed as motion?
Okay, I see Grey answered me – so it IS lots more movement
In the context of climate change, temperature is directly proportional to energy content, and the constant of proportionality is different depending on the substance in question. For some rough examples, it take twice as much energy to raise a gram of water one degree C than it does to raise a gram of air one degree. Note well this is per unit mass and that there are other ways to express these values.
Thank you watchwolf49.
Thanks CookingWithGas
I don’t want to speak on behalf of Grey but I think you may have misinterpreted what Grey described. The additional energy is not the kinetic energy of movement. The movement means you can’t just take the temperature of the top layer and assume that represents the heat in the whole depth of the ocean. My explanation was more of a simple physics lesson in heat, but Grey’s answer is much more responsive to what is probably your underlying interest in measuring global warming.
The difference between a hot dry blanket and a hot wet one is not due to conductivity but to heat capacity. The wet blanket at the same temperature is holding a lot more heat, which is available to be transferred to your hand when you touch it, therefore makes your hand feel warmer. The same thing is happening in reverse when you go out in air with a temperature of 60F vs. jumping in water that is 60F. The water has a greater ability to suck the heat out of your body so it makes your body feel much colder than air at the same temperature.
Yeah I’m still lost.
So are you saying that “temperature” isn’t a great way to measure the ocean because it’s so much more dense than air?
It just seems like if the energy contained in the ocean has increased dramatically, then it would be a lot warmer. Or more turbulent. Or something. I get that heat = energy, that’s why hot water eventually boils. That’s why climate change = more dramatic storms. And I believe that the oceans have absorbed 90% of our carbon dioxide. I just don’t understand why the oceans aren’t a lot warmer.
Water has a high specific heat capacity. In simple terms, that means it takes a lot of energy to raise its temperature. That is why the oceans can have a huge amount of energy added to them and not raise their temperature much. Air, on the other hand, heats up rapidly with little energy added, compared to water.
I used to explain it to ninth graders this way. You have cookies in the oven. The air temperature there is 400 F. You have on short sleeves, open the oven, use mitts to protect your hands, and remove the cookies. While you do that, hot air of nearly 400 degrees touches your arms briefly. You are unharmed. Upon the top of the stove, you have water boiling. It temperature is only 212 F. You just stuck your arms in 400 F air, but you would never dip your hands in 212 F water! Why? Heat (energy) and temperature are obviously not the same thing, since a few seconds in 400 F air is harmless while a few seconds in 212 F water will take your skin off. The water has much more energy in it than the air does.
Fantastic, thank you.
Temperature is the best way to measure energy content, just make sure you use the proper heat capacity. Also remember that temperature in the ocean can change dramatically with depth, so simply testing the surface is not a good way to measure the energy content of the whole water column.
Also, I mis-spoke above, liquid water take four times as much energy per gram per ºC as air.
CookingWithGas pretty much covered it but let me try anyway.
The ocean column is huge - we’re talking about, on average, a depth of 3700 m. Now consider just a huge tube of sea water 1m x 1m x 3700m. The top of the colum might be ~15C while at the bottom it’s 3C and has an overall heat content of X (since I have no idea right now).
So imagine you increase the temperature at the base of the column from 3C to 5C while leaving the surface temperature at 15C. You would now have a large increase in heat content in the column with no corresponding increase in surface temperature.
In reality our ocean surface layer seems to be cycling over down into the deeps faster than expected. That means cooler water is coming up and minimizing the change in surface temperture change. The overall heat content of the water though continues to increase.
This page has a graph of ocean heat content over time from several data sources. The scale is in 10e21 joules - huge numbers, as you’d expect. 30 joules is a minuscule amount of energy compared with that of the world’s oceans.
The average temperature of the oceans would not be as good a measure of energy absorbed by the oceans because the specific heat of the oceans isn’t uniform. Some places are saltier than others, for instance, and saltier water has more energy per degree Celsius. They calculate ocean heat content by measuring the temperature and specific heat at many different locations and depths all over the world, and combining the data to estimate the total heat energy.
BTW, it’s not just the amount of heat energy in an object that determines its capacity to burn you when touched. There’s a unit called thermal effusivity that measures a material’s ability to exchange heat energy with its surroundings, that takes both volumetric heat capacity and thermal conductivity into account. That is, it depends on how much heat energy there is per unit volume, and how quickly it can conduct heat.
Think about it this way. You want to keep a room cool but you have to bring one of the following into the room:
1/ a one foot across round tray of water with the barest skim of water across it - but the water is at near boiling point
2/ a drum that is a foot across but say ten feet deep, full of water that is about 20 degrees above room temperature.
Which is going to increase the room temp more? Answer: the drum. The tray of water is hot but there’s so little of it that it’s not going to have much effect. The drum of water is only warm but there’s enough of it that it’s actually going to make a difference to the room. The tray has a high temperature but, considered overall, doesn’t actually have much heat content. The drum has a modest temperature but has a lot of heat content because there’s so much of it.
With GW the issue is that surface temperature may not mean too much because (on the scale of the planet) there isn’t that much surface water. But if the whole depth of all the oceans goes up in temperature by a very small amount that represents a hella lot of energy.
Good point. If the ocean were homogenous, and if the mass of seawater is constant, then average temperature would be an equivalent measure, and we could talk about “average temp” and “surface temp.” As it is, we can talk about heat content and assume that “temperature” means “surface temperature” (which also has climate significance.)
Also, when doing the math, it’s the heat content that really matters, not the temperature. It’s a reservoir, and heat content is the amount of heat in that reservoir. Given average temperature, we’d have to convert to heat content per unit mass and multiply by mass, to get the quantity that’s meaningful from a climate perspective. So there’s really no point in using average temperature.
The reason heat content is a big deal is because it explains where the energy coming from the sun is going. If not for the fact that ocean heat content is rising, we’d see higher temperature changes due to increased CO2 levels. It’s a “that’s good! …no, that’s bad” scenario. While it’s good that the ocean buffers us, it’s also storing up energy, so that if/when we do “fix” the source problem, the effects of the sins of the past will continue to haunt us longer (and it takes longer for us to convince everyone that yes, it really really really is getting hotter!)