Buckets of questions

A new version of sea-surface temperature data corrects some bias due to the different methods to measure water temperature, either with non-insulated buckets, insulated buckets and engine intakes. However, the global sea-surface temperature history still raises many questions.




The reader here will be disappointed if he/she expects an extension of yet another battle between the realclimate, climateaudit and Pielke Jr.'s blog. Those thirsty for real bloggospheric action are kindly invited to go there. Here, you will read some open thoughts about the so far best curve that describes the long-term history of global average sea-surface-temperature (SST) measurements. The relevant paper by Kennedy et al has been just published, and I reproduce the main Figure from that paper below:

Figure from Kennedy et al. "Reassessing biases and other uncertainties in sea-surface
temperature observations measured in situ since 1850, part 2:
biases and homogenisation".Global-average SST anomaly (relative to
1961-1990) showing the cumulative effect of adding different error components (coloured areas): median HadSST3 value (black) 2-sigma uncertainty arising from assumptions in bias adjustments (purple); measurement and sampling error, assuming these are uncorrelated between grid boxes (red); and the uncertainty including the intergrid box correlations (blue) and the total uncertainty including all the above terms and the coverage uncertainty (orange). The lower panel shows the sizes of the individual components as 2-sigma uncertainties.



1) The sea-surface temperature has clearly risen from 1900 until today by roughly 0.9 K. The main question here that any scientist would like to answer is what are the factors or combination of factors that have caused this warming. Note that even if temperatures had been much higher in , say 1800, even much higher than today -which I doubt - this question would remain. We see a change and we have to find an explanation for that change. Now imagine that temperatures had remained constant at 17 C over the whole 20th century. The question would then be why global mean SST are 17 C. In both cases there is a question to answer, and the explanation should involve known physical processes. By this I mean among other things that, for instance, 'recovery from the Little ice ' is not a known physical process that is described by any known equation. Also, natural oscillations are not a known physical phenomenon per se. If there is a 'natural oscillation' there is something that oscillates and for some reason. What is that and what makes it (quasi) oscillate ? Neither is an answer of the type 'it was warmer in the Medieval Warm period, so I dont care' permissible.
I think t we all should require physically consistent explanation from any theory of climate change, and not only from the anthropogenic greenhouse effect .

2) The increase has not been monotonous monotonic though. If we would set up a poll among our readers to define different phases in the history of SST, I am pretty sure that all would come up with the same ,o r with a very similar answer: cooling up to 1910, warming up to 1945, cooling up to 1975, warming up to now (here some would argue that the last 10 years SST has been stable, but we want to focus here on the 20th century development). This is a second question that also needs an explanation. There are some around, involving a combination of external climate forcings and internal quasi-oscillations. But what it is really striking - I think- is the fact that these warming and cooling phases are so sharply defined. The figure above shows annual mean values, but I venture to think that if we had the monthly mean values we would even be able to pin down the months in which the trends flip. To state it provocatively, we could perhaps identify the month in the year 1910 in which global warming began. Considering that these are globally averaged data of a quite complex geophysical system, measured with some error range, I find this truly remarkable. All of us would tend to think that at these large spatial scales, nature should behave more more smoothly.

3) The warming trend between 1910 and 1945 is quite similar to the warming trend between 1975 and today, 0.6 K in 30 years, although the external forcings have been qualitatively and quantitatively quite different. Maybe it is just a fluke, or maybe SST for some reason can only increase at a maximum pace, shovelling all heat flux above a certain threshold to the deeper ocean. Anyway, adopting an agnostic uninformed point of view, a simple extrapolation of these trends in the 20th century into the future would already result in 1.8 K warming of SST between 2000 and 2100.