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Saturday, April 21, 2018

Global Heat Budget #2: Oceans


The world’s oceans are warming.  Ocean warming is the strongest confirmation that greenhouse gases are warming the planet. 

The heat capacity of water is among the highest of common substances.  That means that water can absorb a large amount of heat while its temperature changes only slightly.  The measurable warming of the world’s oceans indicates that a very large amount of heat has come from somewhere.  The only credible source for so much heat is the retention of heat by atmospheric greenhouse gases.  Let’s look at the source of the data, and the numbers.

ARGO Oceanographic Program
Rising ocean temperatures have been measured by oceanographic surveys since the 1970s.  However, these ocean surveys were limited in geographic coverage and continuity of data acquisition.  A more comprehensive system, ARGO, was put in place beginning in the early 2000s, with improvements and new deployments continuing today.  Today, ARGO consists of nearly 4000 floats which continuously measure ocean temperature, salinity, density and currents from the surface to 2000 meters. 
ARGO floats measure temperature to an accuracy of two-thousands (0.002) of a degree Celsius.  The floats are “parked” at 1000 meters, and every ten days submerge to 2000 meters and return to the surface, where data is broadcast to satellite receivers.  The system provides comprehensive coverage worldwide except for polar latitudes, and continuous measurements.

Ocean *Weather*
Like the atmosphere, ocean temperatures are seasonal, cyclic, variable, and turbulent.  The large number of ARGO floats was designed to adequately measure and characterize the variable temperatures of the ocean.  The volume of data acquired allow scientists to make maps of the changing water temperature and calculate the total heat content in the ocean.
Observations
Surface temperatures are warming the fastest.  NOAA presents charts of average ocean temperature and ocean heat content according to water depth, based on ARGO observations and earlier oceanographic studies.

Surface waters (0 – 100 m) have warmed by about 0.6 degrees C on average since the late 1960s. 
Intermediate waters (0 – 700 m) have warmed by a little over 0.2 degrees C on average since the late 1960s. 
Relatively deep waters (0 – 2000 m) have warmed by about 0.1 degree C on average, since the late 1960s.
Over all depth increments observed, the rate of warming seems to be slightly increasing.

Heat Content
The changing heat content of the ocean is a simple function of the change in temperature.  The heat capacity (or specific heat) of water represents the amount of heat required to change the temperature of a given volume of water.  From an observed change in temperature, we can back-calculate the amount of heat that has entered the ocean.  The density and heat capacity of water change slightly with pressure (and water depth).  NOAA has calculated the heat content of the ocean over various depth intervals from the temperature data and heat capacity.  

The heat content of the ocean at intermediate depths (0 – 700 m) has increased by about 2 x 1023 joules since the late 1960s. 
The heat content from the surface to 2000 meters (0 – 2000 m) has increased by about 3 x 1023 joules since the late 1960s.  This means that the heat content over the interval from 700 m to 2000 m has increased by about 1 x 1023 joules, about half of the increase in heat content at intermediate water depths.
Source of Increasing Heat
NOAA unfortunately did not report temperature change or heat content in separate depth intervals, but only in overlapping intervals of 0 – 100 , 0 – 700, and 0 – 2000 meters.  Starting from the average change of temperature for each interval, I calculated the heat content for 0 – 100 m, 100 – 700 m, and 700 – 2000 m.  My figure for total heat content calculated from temperature change exceeds the heat content reported by NOAA by 14%, probably due to errors in my single-point values for temperature or heat capacity over these depth intervals.
Temp Rise (C)
Volume (km3)
Density (g/cc)
Mass (kg)


Heat Capacity (J/kg-C)
Change in Heat Content (J)
0 - 100 m
0.6
5.23E+07
1.025
5.10E+19


3928.00
1.20E+23
0 - 700 m
0.2
3.69E+08
1.034
3.57E+20


3421.50
2.44E+23
0 - 2000 m
0.1
1.36E+09
1.329
1.02E+21


3339.04
3.41E+23
Intervals
Change in Heat Content
Percent of Heat Change
Change in Heat Content per 100 m
0 - 100 m
1.2E+23
35%
1.2E+23
100 m - 700 m
1.2E+23
36%
2.1E+22
700 m - 2000 m
9.6E+22
28%
7.4E+21

There is a large difference between the heat gained in the upper 100 meters of the ocean and the heat gained at deeper levels by equivalent volume.  The ocean is clearly heating from the surface downward.  About 35% of the total heat increase has occurred in the upper 100 meters of the ocean, about 36% in the next 600 meters, and about 28% in the next 1300 meters.  Research on deep ocean currents shows that heat is also being introduced into the deep ocean by currents, rather than by conduction. 

The geographic distribution of ocean heating also shows atmospheric influence.  The ARGO ocean data shows distinct heating anomalies between 30 and 40 degrees of latitude, north and south.  These are the down-welling points of large atmospheric convection cells termed Hadley cells.  You can see atmospheric circulation in observations of ocean warming.


Conclusion
The first post in this series quantified anthropogenic heating and cooling, primarily from greenhouse gases, particularly CO2.  This post looked at the largest heat sink on earth – the oceans.  

Net Anthropogenic heat absorbed by the planet from 1970 to 2016 was about 3.4 x 1023 joules.  Over the same period, the heat content of the oceans has increased by about 3.0 x 1023 joules.  Anthropogenic heat is the only credible source for the heat appearing in the ocean, and the warming oceans confirm that greenhouse gases are, in fact, warming the planet.  
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References
Global Heat Budget #1: Anthropogenic Heat



Ocean heat content figures.

Ocean temperature figures.

Gridded temperature data in map view.

The Oceans Their Physics, Chemistry, and General Biology, UC Press E-Books Collection, 1982-2004, University of California Press 
Physical properties of sea water.

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