NOAA = (US) National Oceanic and Atmospheric Administration (Dept. of Commerce)
NASA = (US) National Aeronautics and Space Administration
UKMO = (UK) United Kingdom Meteorological Office
I think it's safe to say that measuring the surface temperature of any ocean is difficult.
That means compiling a sea surface temperature global average results in a very rough temperature estimate for almost 70% of the surface of our planet.
Even worse, there are sea surface temperature "adjustments" made in secret, without public justification, and they are extremely subjective.
It is very unlikely that successive revisions to the very same data would lead to ever greater global warming trends ... but that's exactly what happens!
The "adjustments" to surface temperature data explain an accelerating gap between surface measurements and weather satellite measurements in the past decade.
Weather satellite data reflect significantly less warming than surface measurements, and they show 1998 as the warmest year since they were launched in 1979, not 2015.
Climate models have consistently overestimated global warming, often by a large margin, for almost four decades.
Actual temperature data from weather satellites indicate sea surface temperatures of the tropical oceans warmed at a not-very-alarming rate of about +0.8 degrees C. per century ...
... but Global Climate Models had predicted greenhouse gasses should have warmed tropical ocean surfaces by much more: +1.9 degrees C. per century.
That's a pretty big forecasting error, considering that oceans cover 76% of the tropics (24S-24N), and tropical oceans are about 46% of all oceans.
Statistical compilations of average sea temperatures are based on data from ships, buoys, and in some cases satellites.
The two data suppliers (NOAA and UKMO) estimate weekly and daily values of sea surface temperatures in ice-free oceans.
Sea surface temperatures
have been measured several ways:
(1) Buckets of different types thrown over the sides of ships, hauled back aboard filled with water, and then sailors would place thermometers in them.
Buckets were the sole method used before the 1930s.
(2) Then ship-based engine room cooling water inlets (ERI) were used.
There was a mix of buckets and ship-inlets from the 1930s to the 1970s.
(3) Buoys have been used since the 1970s.
There was a very large increase in drifting buoy-based observations starting in the early 2000s.
Today there is a mix of buoys, ship inlets and buckets.
Data suppliers make "adjustments" to the raw data.
Researchers found differences between measurements at ship engine inlets vs. measurements by buoys.
In recent years the "global warmunists" became desperate for excuses for the flat average temperature of Earth since the early 2000s.
After the fact, they decided to blame a ship inlet-buoy bias and re-wrote history.
They "adjusted" sea surface temperature data to show more global warming.
These "adjustments" turned a flat temperature trend from the early 2000s to mid-2015 into a slightly rising trend.
The "adjustments" must be arbitrary because they contradict temperature data from weather satellites, weather balloons, and ARGO floats.
The most recent "adjustment" by NOAA's ERSST.v4 was nicknamed "the pause-buster” by climate change skeptics.
NASA:
National Aeronautics and Space Administration
NASA provides the GISS LOTI monthly land + sea surface temperature data compilation.
Goddard Institute for Space Studies (GISS) ignores any available sea surface temperature data near the poles.
For areas where seasonal sea ice has existed, they arbitrarily extend land surface temperature data out over the oceans, even when actual sea surface temperature observations are available.
GISS switched to the NOAA's new “pause-buster” ERSST.v4 sea surface temperature reconstruction with their July 2015 update.
NOAA:
National Oceanic and Atmospheric Administration
NOAA provides the National Centers for Environmental Information (NCEI) monthly surface land + sea temperature data compilation.
NCEI provides the new Extended Reconstructed Sea Surface Temperature version 4 (ERSST.v4)
ERSST.v4 infills (uses wild guesses) for surface areas (grids) without actual temperature data in a given month.
NOAA blatantly manipulated their sea surface temperature model to produce the highest annual temperature on record (since 1880) in 2015 for political reasons.
"Adjustments" to the ERSST reconstruction can not be justified for scientific reasons.
Note: The National Centers for Environmental Information (NCEI) was formerly known as the National Climatic Data Center (NCDC).
UKMO:
United Kingdom Meteorological Office
UKMO provides the HADCRUT4 monthly land + sea surface temperature data compilation.
HADCRUT4 uses the HadSST3 sea-surface temperature (SST) reconstruction, which is from the Hadley Centre.
Unlike the NASA-GISS and NOAA-NCEI sea-surface reconstructions, the UKMO-HadSST3 surface areas (grids) with no actual temperature data for a given month are NOT infilled (wild guessed).
If a 5-degree latitude by 5-degree longitude area of the surface (aka "grid") of our planet does not have actual temperature data in a given month, it is left blank (ignored).
Blank grids are simply assigned the average value for their hemisphere, so will have no effect on their hemisphere's average temperature.
UKMO sea surface temperature data are most often used in research papers.
ARGO:
"Argo is a global array of more than 3,000 free-drifting profiling floats that measures the temperature and salinity of the upper 2000 m of the ocean.
This allows, for the first time, continuous monitoring of the temperature, salinity, and velocity of the upper ocean, with all data being relayed and made publicly available within hours after collection.
ARGO ocean floats were supposed improve sea temperature measurement data."
Source of quote: http://www.argo.ucsd.edu/
The 3600+ ARGO buoys reflect ocean warming at a rate of just 0.02 Cº per decade (+0.23 Cº per century, or +1 C° in 430 years!)
An honest chart showing these data would look like a straight line.
There's a discrepancy between NOAA and ARGO data.
From 2004 to 2014, the NOAA data say oceans are warming at +0.05 Cº decade (+0.5 Cº century), more than double the rate shown by ARGO (+0.23 Cº per century).
Note: ARGO has not provided new ocean data since December 2014 - reason unknown
ARGO has better quality data, but the resolution (number of measurements) of ocean data are very low.
Each of the 3600 automated ARGO bathythermograph buoys takes just three measurements a month in 200,000 cubic kilometres of ocean (roughly a 100,000-square-mile box, more than 316 km square and 2 km deep).
Climate researcher Willis Eschenbach said the ARGO resolution is like taking a single temperature at a single point in Lake Superior less than once a year (and using it to define the average temperature of Lake Superior).