In an earlier post – Why Global Mean Surface Temperature Should be Relegated, Or Mostly Ignored – I commented:
There’s a huge amount of attention paid to the air temperature 6ft off the ground all around the continents of the world. And there’s an army of bloggers busy re-analyzing the data.
It seems like one big accident of history. We had them, so we used them, then analyzed them, homogenized them, area-weighted them, re-analyzed them, wrote papers about them and in so doing gave them much more significance than they deserve. Consequently, many people are legitimately confused about whether the earth is warming up.
Then we looked at some of the problems of measuring the surface temperature of the earth via the temperature of a light ephemeral substance approximately 6ft off the ground.
In Warming of the World Ocean 1955-2003, Levitus (2005) shows an interesting comparison of estimates of absorbed heat over almost half a century:
Once you find out that the oceans have around 1000x the heat capacity of the atmosphere, the above chart won’t be surprising.
For those who haven’t considered this relative difference in heat capacity before:
- if the oceans cooled down by a tiny 0.1°, transferring their heat to the atmosphere, the atmosphere would heat up by 100°C (it wouldn’t happen like this but it gives an idea of the relative energy in both)
- if the atmosphere transferred so much heat to the oceans that the air temperature went from an average of 15°C to a freezing -15°C, the oceans would heat up by a tiny, almost unnoticeable 0.03°C
So if we want to understand the energy in the climate system, if we want to understand whether the earth is warming up, we need to measure the energy in the oceans.
An Accident of History
Measuring the temperature of the earth’s surface by measuring the highly mobile atmosphere 6ft off the ground is a problem. By contrast, measuring ocean heat is simple..
Except we didn’t start until much later. Sea surface temperatures date back to the 19th century, but that doesn’t tell us much. We want to know the temperature down into the deep all around the world.
Here is a typical sample. Unlike the atmosphere, the oceans are more “stratified” – see Why Global Mean Surface Temperature Should be Relegated, Or Mostly Ignored for more on the basic physics of why the ocean is warmer at the surface. However, the oceans have complex global currents so we need to take a lot of measurements.
Measurements of the temperature down into the ocean depths didn’t really start until the 1940s and progressed very slowly since then. Levitus says:
Most of the data from the deep ocean are from research expeditions. The amount of data at intermediate and deep depths decreases as we go back further in time.
Fast forward to 2000 and the Argo project began to be deployed. By early 2010, over 3300 sensors have been moved into place around the world’s oceans. The Argo sensors drop to 2km in depth every 10 days and automatically measure temperature and salinity from the surface to this 2km depth:
Why salinity? Salinity is the other major factor apart from temperature which affects ocean density and therefore controls the ocean currents. See Predictability? With a Pinch of Salt please.. for more..
As we go back from 2010 there is progressively less data available. Even during the last 10 years measurement issues have created waves. But more on that later..
It’s often best to step back a little to understand a subject better.
In 2000, Science published the paper Warming of the World Ocean by Sydney Levitus and a few co-workers. The paper has a thorough analysis of the previous 50 years of ocean history.
Now and again the large number of joules (unit of energy) are turned into a comparison W/m2 absorbed for the time period in question. 1W/m2 for a year (averaged over the entire surface of the earth) translates into 1.6×1022J.
But it’s better to get used to the idea that change in energy in the oceans is usually expressed as 1022J.
The graphs above show a lot of variability between oceans but still they all demonstrate the similar warming pattern.
Here is the data shown (from left to right) as the energy change in the deeper 3000m, 800m and 300m.
We are used to seeing temperature graphs, even sea surface temperature graphs that go up and down from year to year. Of course we want to understand exactly why, for example see Is climate more than weather? Is weather just noise? It’s easy to think of reasons why that might happen, even in a warming world (or a cooling world) – with one of the main reasons being that heat has moved around in the oceans.
For example, due to ocean currents colder water has been brought to the surface. The measured sea surface temperature would be significantly lower but the total heat hasn’t necessarily changed – because we are only measuring the temperature at one vertical location (the top).
So we wouldn’t expect to see a big yearly decline in total energy.. not if the planet was “warming up”.
So this is quite surprising! See the change downward in the 1980’s:
What caused this drop?
Here’s a another fascinating look into the depths that we don’t usually get to see:
Here we see changes in the deeper North Atlantic in two comparison periods about 15 years apart. (As a minor note the reason for the comparisons of averaged 5-year periods is the sparsity of data below the surface of the oceans).
See how the 1990 period has cooled from 15 years earlier.
Levitus, Antonov and Boyer updated their paper in 2005 (reference below).
Here we present new yearly estimates for the 1955– 2003 period for the upper 300 m and 700 m layers and pentadal (5-year) estimates for the 1955–1959 through 1994–1998 period for the upper 3000 m of the world ocean.
The heat content estimates we present are based on an additional 1.7 million temperature profiles that have become available as part of the World Ocean Database 2001.
Also, we have processed approximately 310,000 additional temperature profiles since the release of WOD01 and include these in our analyses.
(My emphasis added). Think re-doing GISS and CRU is challenging? And for those who like to know where the data lives, check out the World Ocean Database and World Ocean Atlas Series
Here’s a handy comparison of the changing heat when we look at progressively deeper sections of the ocean with the more up-to-date data.
The actual numbers (change in energy) from 1955-1998 were calculated to be:
- 0-300m: 7×1022J
- 0-700m: 11×1022J
- 0-3000m: 15×1022J
- 1000-3000m: 1.3×1022J
So the oceans below 1000m only accounted for 9% of the change. This gives an idea of the relative importance of measuring the temperatures as we go deeper.
In their 2005 paper they comment on the question of the early 80’s cooling:
One dominant feature .. is the large decrease in ocean heat content beginning around 1980. The 0–700 m layer exhibits a decrease of approximately 6 x 1022 J between 1980 and 1983. This corresponds to a cooling rate of 1.2 Wm2 (per unit area of Earth’s total surface).
Most of this decrease occurs in the Pacific Ocean.. Most of the net decrease occurred at 5°S, 20°N, and 40°N. Gregory et al.  have cast doubt on the reality of this decrease but we disagree. Inspection of pentadal data distributions at 400 m depth (not shown here) indicates excellent data coverage for these two pentads.
And they also comment:
However, the large decrease in ocean heat content starting around 1980 suggests that internal variability of the Earth system significantly affects Earth’s heat balance on decadal time-scales.
So far so interesting, but as the article is already long enough we will come back to the subject in a later post with the follow up:
How Big Should Error Bars be and the Sad Case of the Expendable Bathythermographs.
And for one reader, in anticipation:
Warming of the World Ocean, Levitus et al, Science (2000)
Warming of the World Ocean 1955-2003, Levitus et al, GRL (2005)