Robert Balic writes:
I recently read the Willis Eschenbach article Argo, Temperature, and OHC (http://wattsupwiththat.com/2014/03/02/argo-temperature-and-ohc/) which reported the trend in the global ocean temperatures as 0.022 ± 0.002 deg C /decade and Steven Casey asked
“Can we believe we have that much precision to 0.002 deg C/decade? And we have not yet measured a full decade.”
Also, there was a reply to a comment of mine on The Conversation mentioning the uncertainty which stated “The temperatures in the Argo profiles are accurate to ± 0.005°C http://www.argo.ucsd.edu/FAQ.html#accurate“.
I checked the website http://www.argo.ucsd.edu/How_Argo_floats.html and found that
“The SBE temperature/salinity sensor suites is now used almost exclusively. In the beginning, the FSI sensor was also used. The temperature data are accurate to a few millidegrees over the float lifetime,” and “The temperatures in the Argo profiles are accurate to ± 0.002°C”.
The temperature profiles might be accurate to ± 0.002°C now, but weren’t the measurements made to the nearest 0.1°C previously? I looked up the accuracy of their thermistors earlier this year and it was written as 0.1°C. A high precision commercial instrument usually has a claimed ± 0.05°C accuracy so they most likely did record to the nearest 0.1°C until they installed the new units. They can’t now insist that the smaller error in the previous trend remains uncorrected because they have new instruments this year.
Why is it relevant that the temperature measurements were taken to the nearest 0.1°C if they looked at the average of over 100 measurements? Well if you take my height for example and measure me to the nearest centimeter 100 times, then the average would probably come out to be 183cm with a standard deviation of 0. Perfect!
If you had recorded my height to the nearest millimeter having taken 50 measurements of 1825mm and 50 measurements of 1835mm, you would get an average of 1830mm with a standard deviation of 5mm or 0.5cm. A random spread of measurements over that range would bring the SD down to about a quarter of a centimeter and the error estimate is usually twice this value.
The rule of thumb that I was once taught is that your minimum error is plus or minus the value of the increment that the measurements were made with (eg. 1 cm) where the number of measurements are a few, or half this value when there are a large number of measurements (eg.± 0.5cm). So if the Argo floats only measured in increments of 0.1°C then the uncertainty in the mean of many measurements is at least ± 0.05°C. Hence, a trend of 0.02°C/decade measured over less than a decade is utterly meaningless.
Someone should also have a word in the ears of those at The University of Washington.
“Because total Arctic sea ice volume from PIOMAS is computed as an average over many grid points, the random error (scatter in above figures) doesn’t affect the uncertainty in the total ice volume and trend very much.”
This is the excuse to ignore the large errors implied by this plot.
Where the model predicts a 4m thickness the submarine data is spread evenly between 2.5 and 6 m. The range is nearly 0 to almost 3.5m where the estimate from the model is 1m, that is over 100% uncertainty in the thickness yet they are absolutely sure that the ice is in a death spiral.