News Brief by Kip Hansen — 30 November 2019
A recent study in Oceanography, the Official Magazine of The Oceanography Society, titled “Atlantic warming since the Little Ice Age” [.pdf here], is interesting in its entirety, with an Abstract as follows:
“Radiocarbon observations suggest that the deep Atlantic Ocean takes up to several centuries to fully respond to changes at the sea surface. Thus, the ocean’s memory is longer than the modern instrumental period of oceanography, and the determination of modern warming of the subsurface Atlantic requires information from paleoceanographic data sets. In particular, paleoceanographic proxy data compiled by the Ocean2k project indicate that there was a global cooling from the Medieval Warm Period to the Little Ice Age over the years 900−1800, followed by modern warming that began around 1850. An ocean simulation that is forced by a combined instrumental-proxy reconstruction of surface temperatures over the last 2,000 years shows that the deep Atlantic continues to cool even after the surface starts warming. As a consequence of the multicentury surface climate history, the ocean simulation suggests that the deep Atlantic doesn’t take up as much heat during the modern warming era as the case where the ocean was in equilibrium at 1750. Both historical hydrographic observations and proxy records of the subsurface Atlantic are needed to determine whether the effects of the Little Ice Age did indeed persist well after the surface climate had already shifted to warmer conditions.”
Those interested in the relationships between deep Atlantic Ocean water temperature, surface Atlantic water temperature, and possible effect on climate — and, of course, effects of atmospheric climate on deep Atlantic Ocean temperature and the ocean’s heat uptake and release — should read the whole paper.
Of particular interest for today’s Climate Debate are these three graphs — the first two of which are simply copied from the paper itself, including their captions:
This first graph shows the Ocean2K reconstruction of global surface temperatures (anomaly with baseline 15 CE — broad black trace) and on the right-hand side, various modern regional Atlantic Ocean surface water temperatures.
The second graph:
This is the view we are accustomed to seeing in the Climate Debate — the global surface temperature anomaly (some baseline — in this case 15 CE) with a starting point around 1850 (some start a bit later, 1890). Note that it is “Identical to Figure 1, but restricted to the years 1850−2015.” The starting point is picked to represent “the start of the Modern Industrial Era”.
Now an annotated version of the second graph:
Here we have the second graph 1850-2015, with the global Average Surface Temperature anomaly (again — baseline 15 CE) but I have dropped in a smaller window, on the left, bringing forward the Roman Warm Period and the Medieval Warm Period (MWP) — the years 100-800 CE (same scale) — to illustrate the difference between the peak Global Average Surface Temperature (GAST) of the Medieval Warm Period to the most current GAST on the graph (2015).
This exposes the ubiquitous trick of the Climate Debate, in which Global Temperatures are [almost] always shown only from the depths of the Little Ice Age (clearly marked on the first graph by Gebbie), resulting in images similar to Gebbie’s Figure 2 — despite the fact that most 2 millennia reconstructions clearly show the Roman and Medieval Warm Periods as generally in the same range as the Modern Warm Period. Given the acknowledged range of error in any temperature reconstruction and in modern estimates of global surface temperatures (today, in absolute temperatrures, around +/- 0.5ºC or a range of 1ºC) — there may be little, if any, significant-to-the-global-environment difference between the two periods.
The Medieval Warm period did not result in a “Climate Catastrophe” and the [iffy] little additional 0.2°C seen today is very unlikely to spark a modern Climate Catastrophe either.
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A great deal of the polarization in the Climate Debate is based on this little trick of data presentation — using a starting date known to represent a low point in some data set of a measurement which the author wishes to show has increased to a present high. Failure to show the full context of the data is a type of data falsification.
Kudos to Geoffrey Gebbie for including both graphs in his paper.
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