More on Trenberth’s Missing Heat
While I don’t necessarily buy Trenberth’s latest evidence for a lack of recent surface warming, I feel I need to first explain why Trenberth is correct that it is possible for the deep ocean to warm while surface warming is seemingly by-passed in the process.
Then I will follow up with observations which run counter to his (and his co-authors’) claim that an increase in ocean surface wind-driven mixing has caused the recent lack of global warming.
Can Deep Ocean Warming Bypass the Surface?
It depends on what one means by “warming”. A temperature change is the net result of multiple processes adding and subtracting heat. Warming of the deep ocean originally caused by radiative forcing of the climate system cannot literally bypass the surface without some effect on temperature. But that effect might be to keep some cooling process from causing an even steeper dive in temperature.
It’s like adding a pint of warm water, and a gallon of cold water, to a sink full of room temperature water. Did adding the pint of warm water cause the temperature in the sink to rise?
To appreciate this, we first need to understand the basic processes which maintain the vertical temperature distribution in the global oceans. The following cartoon shows a North-South cross section of measured ocean temperatures in the Atlantic.
(1) surface heating by the sun (mitigated by surface evaporation and infrared radiative loss) which warms the relatively shallow ocean mixed layer;
(2) cold deepwater formation at high latitudes, which slowly sinks and fills up the oceans on time scales of centuries to millennia, and
(3) vertical mixing from wind-driven waves, the thermohaline circulation, and turbulence generated by flow over ocean bottom topography (the latter being partly driven by tidal forces).
The key thing to understand is that while processes (1) and (2) continuously act to INCREASE the temperature difference between the warm mixed layer and the cold deep ocean, the vertical mixing processes in (3) continuously act to DECREASE the temperature difference, that is, make the ocean more vertically uniform in temperature.
The average temperature distribution we see is the net result of these different, competing processes. And so, a change in ANY of these processes can cause surface warming or cooling, without any radiative forcing of the climate system whatsoever.
So, let’s look at a few ocean mixing scenarios in response to radiative forcing of the climate system (e.g. from increasing CO2, increasing sunlight, etc.), all theoretical:
Scenario 1) Warming with NO change in ocean mixing: It this case, surface warming is gradually mixed downward in the ocean, leading to warming trends that are a maximum at the ocean surface, but which decrease exponentially with depth.
Scenario 2) Warming with a SMALL increase in ocean mixing. This case will result in weaker surface warming, and slightly stronger warming of the deep ocean, both compared to Scenario 1. The warming still might decrease exponentially with depth.
Scenario 3) Warming with a LARGER increase in ocean mixing. This case could lead to an actual surface temperature decrease, but warming of the deep ocean, similar to what I believe Trenberth is claiming.
Yes, the surface waters “warmed” before the deep ocean in Scenario 3, but it was in the form of a weaker temperature drop than would have otherwise occurred.
Because of the immense heat capacity of the deep ocean, the magnitude of deep warming in Scenario 3 might only be thousandths of a degree. Whether we can measure such tiny levels of warming on the time scales of decades or longer is very questionable, and the new study co-authored by Trenberth is not entirely based upon observations, anyway.
I only bring this issue up because I think there are enough legitimate problems with global warming theory to not get distracted by arguing over issues which are reasonably well understood. It takes the removal of only one card to cause a house of cards to fall.
But it also points out how global warming or cooling can occur naturally, at least theoretically, from natural chaotic variations in the ocean circulation on long time scales. Maybe Trenberth believes the speedup in the ocean circulation is due to our driving SUVs and flipping on light switches. He has already stated that more frequent El Ninos are caused by anthropogenic global warming. (Except now they are less frequent — go figure).
In some sense, natural global warming and cooling events are made possible by the fact that we live within an exceedingly thin warm surface “skin” of a climate system in which most of the mass (the deep ocean) is exceedingly cold. Any variations in the heat exchange between those two temperature worlds (such as during El Nino with decreased mixing, or La Nina with increased mixing) can cause large changes in our thin-skinned world. It than sense, Trenberth is helping to point out a reason why climate can change naturally.
Have Ocean Winds Increased Recently?
Trenberth and co-authors claim that their modeling study suggests an increase in ocean surface winds since 2004 has led to greater mixing of heat down into the ocean, limiting surface warming.
Fortunately, we can examine this claim with satellite observations. We have daily global measurements of ocean surface roughness and foam generation, calibrated in terms of an equivalent 10 meter height wind speed, from AMSR-E:
I don’t know about you, but I don’t see an increase in surface winds since 2004 in the above plot. This plot, which is based upon wind retrievals that have been compared to (as I recall) close to 1 million buoy observations, really needs to be extended back in time with SSM/I and SSMIS data, which would take it back to mid-1987. That’s on my to-do list.
So far, I would say that the so-called missing heat problem is not yet solved. I have argued before that I don’t think it actually exists, since the “missing heat” argument assumes that feedbacks in the climate system are positive and that radiative energy is accumulating in the system faster than surface warming would seem to support.
For the reasons outlined above, Trenberth’s view of deep ocean storage of the missing heat is still theoretically possible since increased vertical ocean mixing doesn’t have to be wind-driven. But I remain unconvinced by arguments that depend upon global deep ocean temperature changes being measured to an accuracy of hundredths or even thousandths of a degree.
Finally, as I have mentioned before, even if increased rate of mixing of heat downward is to blame for a recent lack of surface warming, the total energy involved in the warming of the deep oceans is smaller than that expected for a “sensitive” climate system. Plots of changes in ocean heat content since the 1950′s might look dramatic with an accumulation of gazillions of Joules, but the energy involved is only 1 part in 1,000 of the average energy flows in and out of the climate system. To believe this tiny energy imbalance is entirely manmade, and has never happened before, requires too much faith for even me to muster.