In the mid-18th century,
some researchers
began measurements
of the water temperature
at the ocean’s surface
while on scientific
oceanic voyages.
Unlike the temperature record
for a land-based weather station,
the ships on these voyages
were continuously moving,
and the voyages only lasted
a few weeks, or months, each.
So early sea surface temperatures
( SST's )
didn’t provide a continuous record
for any individual location.
On the other hand,
ocean temperatures tend to be
much more uniform than
land temperatures.
For that reason, some researchers
suggested averaging together
all the available measurements,
from different voyages,
that went through an area
in a given month.
The UK Hadley Centre
has done this
for their HadSST3 dataset.
They claim to have found
a long-term global warming
trend since the mid-19th century,
similar to the weather-station
estimates.
The similarity between
these different estimates
convinced many people
that both estimates
were accurate.
In fact, the SST data coverage
before 1950 was quite limited,
especially for the
Southern Hemisphere.
The shipping routes which
contributed measurements,
varied from year to year,
depending on:
a) which nations were involved;
b) which ships & captains were involved;
c) weather conditions; and
d) national and international
territory agreements;
In the 1960s,
all SST measurements
were ship-based.
Since the 1990s,
most measurements
have been made
by weather buoys.
Several groups incorporate
satellite measurements into
their SST estimates,
from 1979 onwards.
As a result, directly comparing the
SST data from the early 20th century
to the current SST data is like
“comparing apples and oranges”.
There's some SST data for the
19th century, but a lot
of uncertainties about
how to compare earlier data
with modern data.
Most researchers recognized
if you had to make comparisons
of 1800s SSTs with post-1950 SSTs,
you'd have to apply data adjustments
to account for measurement changes,
but it was very unclear what those
adjustments should be:
Sometimes SST adjustments
were made so SSTs better matched
land-based temperature records.
Most of the changes in SST
data sources are unknown,
because they weren't documented,
or the documentation was lost.
If SST data were adjusted
to better match land station data,
then you can’t use that
adjusted data to claim the
land station data are reliable !
In 2000, the Argo network
of weather buoys began
deployment.
Data for pre-World War II
SSTs are probably much less
reliable than land station data
in that era -- unfortunately,
the oceans are 71% of our planet.
Ocean temperature
data are presented
in small fractions
of a degrees.
The data collection
most likely has
much larger errors
than that, from
repeated changes
in measurement
methodology.
For example, a 1940’s
cooling trend started
with a drastic reversal
from the previous
warming trend.
But oceans are supposed to
change temperature
much more slowly than land.
So what happened?
Most likely the fast change
from cooling to warming
was from the measurement
methodology change,
from dipping buckets
overboard,
to reading engine
cooling water
inlet temperatures.
Engine room
water intake
measurements
are generally
warmer than
uninsulated
bucket
measurements.
Summary of Different
Ocean Measurement
Methodologies:
(1)
First there were
the wooden buckets
thrown from a moving ship,
and then hauled up,
so a bulb thermometer
could be dipped in the bucket
to get the water temperature.
(2)
Then there were
canvas buckets.
(3)
Then there were
insulated canvas buckets.
(4)
Then there were
engine cooling water
intake temperature
readings.
-- A ship’s diesel
engine cooling water
has its temperature
measured via gage
(the gauge is typically
1 degree C accuracy,
and never re-calibrated).
(5)
Then there were
XBTs, or Expendable
Bathythermograph Sondes:
-- Launched from
moving ships
and submarines.
-- Data sent to the ship
via unspooling of
a thin copper wire
from the probe.
(6)
Then there were
ARGO Floating Buoys.
-- 4,000 submersed buoys
record water data,
while drifting at
different ocean depths.
-- Buoys surface
periodically to
transmit stored time,
depth, salinity
and temperature data
via satellite
to ground stations
for interpretation.
With all these different
measurement
methodologies,
in the past century,
we need to know
if a methodology change
inadvertently caused
warming or cooling.
But, there has never been
a scientific experiment,
with all the different
methodologies,
used at the same time,
to measure SSTs
in the same location,
and see how
the results differ !