Sea level rises.
Sea level falls.
Land level rises.
Land level falls.
Ocean basins expand.
Ocean basins shrink.
The changes listed above make measuring sea level very difficult.
The average temperature rises.
The average temperature falls.
CO2 levels in the air increase.
CO2 levels in the air decrease.
We live on a constantly changing planet.
Earth's surface is more than 70% water.
Oceans contain a large amount of dissolved carbon dioxide (CO2).
When oceans warm, they release CO2 into the air, just like a glass of cold soda does when it warms.
When oceans cool, they absorb more CO2 from the air.
Based on studies of Antarctica and Greenland ice cores, there is a relationship between rough estimates of CO2 levels and average temperature.
But not the relationship the "warmunists" had expected.
The ice core data show that temperature peaks lead to CO2 level peaks roughly 500 to 1,000 years later.
So, whatever caused the oceans to warm in the past 800,000 years, led to the oceans GRADUALLY releasing some of their dissolved CO2 into the air.
If you thought increasing amounts of CO2 in the air is the primary cause of global warming, you would expect:
(1) If warming oceans release more CO2 into the air,
(2) Then more CO2 in the air would warm the atmosphere even more,
(3) And the warmer atmosphere would warm the oceans ... and then (1) through (3) would repeat ... eventually leading to runaway warming.
Interesting theory -- wrong, but interesting -- there's no evidence our planet has EVER had runaway warming at any time in the past 4.5 billion years, even with CO2 levels peaking at an estimated 10x to 20x higher than today.
Ocean currents transfer large amounts of heat around the planet.
El Nino events, roughly every five years, transfer heat from a portion of the Pacific Ocean into the atmosphere.
Ocean surface temperature is difficult to measure.
Before World War II (WWII) the primary ocean temperature measurement methodology was sailors throwing buckets over the sides of their ships and sticking thermometers in the water.
Those voluntary measurements were made almost entirely in major sea lanes in the Northern Hemisphere.
There were few measurements in the Southern Hemisphere, which happens to be about 80% oceans and 20% land.
A perfect individual bucket over the side measurement could be no better than +/- 0.5 degrees C., assuming the thermometers were denominated in degrees C.
With most of the oceans unmeasured before WWII, the margin of error for the estimates of the GLOBAL average ocean temperature was likely to be up to +/- 2 degrees C.
A switch from wooden buckets to canvas buckets was part of the margin of error.
After 1950, the temperature of engine cooling water, coming in from ship hull water intakes, became the primary measurement methodology.
Now the ocean averages use ARGO floats and the water intakes of ships.
The relatively accurate ARGO float data were, for no logical reason, adjusted to match the ship water intake measurements.
The adjustment was to add +0.12 degrees C. to each ARGO measurement.
As more ARGO floats are deployed, the “average” ocean surface temperature goes up from that automatic +0.12 degrees C. adjustment to each ARGO measurement.
NASA reports the average temperature of the entire surface of our planet in hundredths of a degree C.
That makes no sense, because they also claim a margin of error of +/- 0.1 degrees C.
I believe estimates of the average surface temperature of the oceans are likely to have a margin or error of at least +/- 0.5 degrees C., and more likely +/- 1.0 degrees C. -- better than the old water bucket methodology, but not very precise.
The NASA - GISS claim that the average surface temperature of our planet has been calculated with a margin of error of only +/- 0.1 degrees C. is wishful thinking that is not even close to reality.