Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

[Phys-L] ocean energy, thermal energy, heat



I just did some amusing order-of-magnitude calculations.

1) The Gulf Stream moves at about 1 knot (i.e. a couple of km/hour).
If you dissipated all that kinetic energy, it would raise the
temperature of the water by about 0.0004 kelvin, i.e. about
0.4 millikelvin. That's not very much.

2) That can be compared to the rate of warming due to climate
change over the last 50 years. 0.4 millikelvin corresponds
to less than one week's worth of warming along the established
trend.

3) When non-experts think of "energy" in the ocean, typically
they think of tides, waves, and swells. This is however several
orders of magnitude smaller than the KE in the ocean currents.
Hint: The swells affect only the water near the surface,
whereas the currents run deep.

==========================
Pedagogical remarks:

In a great many cases, when people talk about «heat» or «thermal
energy» it would be simpler and better to talk about the plain
old /energy/. Most of the laws of physics care about the energy,
plain and simple, and the more ways you divide it up the more
likely you are to make a mistake. The energy stored in the ocean
is an example.

However, this creates a communications dilemma.
-- If you call it /energy/ you are likely to be misunderstood,
insofar as the listener might think you are talking about the
more-obvious forms of energy (swells and perhaps currents),
even though you're not.
-- If you call it «thermal energy» you are being a bit redundant.
Not wrong, but redundant. This blocks one misunderstanding but
allows another, insofar as the listener might think you are
simply overlooking the more-obvious forms of energy.

As a possible solution to this dilemma, I am reminded of the rule
that says one idea per sentence, and one sentence per idea. You
could say that there is a tremendous amount of /energy/ stored
in the ocean, and then say in a separate sentence that virtually
all of it is thermal energy, tied up in the heat capacity of the
water. You can measure the energy content using thermometers.

As I've said before, usually it's not possible to draw a clean
distinction between «thermal» energy and any other kind of
energy. This is one of those cases where virtually all of the
energy is in thermal form, so nobody cares whether it might get
converted to/from some other form(s).

In particular, here's one operational consequence: You cannot
/directly/ solve the climate change problem by extracting energy
from the ocean, using wave- or tide-driven power plants. That
just doesn't have enough to be /directly/ useful.

In contrast, there is an /indirect/ advantage. Insofar as the
power plant uses ocean energy rather than fossil carbon, it
makes a useful contribution. Remember that the energy you
put into the environment /directly/ from burning fossil carbon
is negligible compared to the net energy that you are adding
to the environment over the next 100 years, indirectly via CO2.

One last point: It pays to think about the energy (not just
the temperature). There is a tremendous amount of energy in
the ocean. Moving a little bit of energy from the ocean to
the air can change the air temperature quite a bit, without
changing the overall system energy.

Energy helps explain the temperature, and then temperature helps
explain the impact on plants, animals, glaciers, et cetera.

This is all rather simple physics, if you think about it the
right way.