Monday, 29 April 2013

Ocean Heat Uptake?

"When people say 'no warming in 15 years', they're cherry picking the timeframe to begin in an abnormally hot year. It's like arguing that your car must have broken down because it hasn't moved in the 15 seconds while you've been stopped at a red light."
So writes John Abraham and Dana Nucitelli in a Guardian article brought to my attention by Flora Faun in one of her usual displays of intellect; copying and pasting a Guardian URL into a sentence-long swear word.

It's a false argument for the simple reason that 1998 is one year in fifteen, and therefore the trend line looks flat even if you remove the data point for 1998 (the second data point in the chart below...)

Yet it is interesting that Nucitelli attempts this ridiculous parry in the first place, rather than accept the point that - contrary to the majority of IPCC models - there has been no statistically significant surface warming for fifteen years and to then consider the implications. The reason why it is interesting is because he makes no serious attempt to defend the failure of climate model predictions and insteads shifts to a claim about how much heat has been absorbed by the oceans...
"The confusion on this subject lies in the fact that only about 2 percent of global warming is used in heating air, whereas about 90 percent of global warming goes into heating the oceans..."
Yes the oceans are massive and absorb a lot of heat. However, ocean heat content has only been measured systematically for about a decade (using what is called the "ARGO" system of floats that began in the early 1990s), with prior measurements being too sparse in the range and frequency of coverage. The different sets of data (pre-Argo [prior to 2000], early-Argo [prior to 2003] and late-Argo [post-2003]) were recently spliced together by the NOAA showing an aggregate for ocean heat content over the past two decades (this splicing is made obvious by the very large "jumps" in the data). Yet rather than showing a continuous warming trend, this data also shows a relative standstill in the overall trend, but over the last ten years (2003-2013) rather than the last fifteen...

However, that spliced data-set only covers ocean heat content in the "shallows" where one might expect to find it (i.e. to a depth of 700 meters). Yet Nucitelli's Guardian contains a further claim supported by a recently published paper (Balmaseda et al 2013) which reanalysed the ARGO data...
"Just recently, another paper published in the journal Geophysical Research Letters found that the warming of the oceans since the turn of the century has been the most sustained in the past 50 years. They also found that, consistent with my team's research, about 30% of overall global warming has gone into the deep oceans below 700 meters due to changing wind patterns and ocean currents."
The claim is that shifting wind patterns and La Nina events have driven more of the ocean's heat content to depths below 700 meters, and this is illustrated in the following chart...

The empurpled curve shows the greatest heat content for "total depth", although it may be likely that the additional heat content below the 700 meter depth occurs at just slightly greater depths, rather than all the way down to 2000 meters and below. By comparing the empurpled curve to the blue curve immediately below (upper 700 meters) we can see that the additional heat content attributed to the sub-700 meter depths since 2004 is approximately 50 to the power of 22, which does seem to be significantly large and almost the same as the difference in heat content between the 300 meter depth (grey curve) and the 700 meter depth (blue curve). However, it is difficult to judge the significance of that heat value given that the data set is so limited - i.e. there isn't good data going back over a multidecadal timescale to compare changes in ocean heat uptake. It may be that the additional heat can be explained as occuring due to the natural La Nina events without any human-induced warming as Bob Tisdale explains at some length in this post.

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