The Modern Warm Period Delimited

The Modern Warm Period Delimited

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Guest essay by David Archibald

This recent post discussed the end of the Modern Warm Period and the year that global cooling began. That post was inspired by a comment to a post on WUWT six to eight years ago to the effect that climate is controlled by the Sun’s magnetic flux – no need to worry about much else. The comment seemed to come from a warmer scientist – they are well funded, have plenty of time on their hands, some are smart and idle curiosity would get a few looking into what controls climate. The results would not be published of course. To paraphrase Mussolini, everything within the narrative, nothing outside the narrative, nothing against the narrative. If the Sun’s magnetic flux controls climate, you don’t have to worry about what goes on under the hood – the effect of EUV on the NAO, the GCR flux, the F10.7 flux, any other flux apart from the magnetic flux.

A comment by Bellman on that recent post inspired me to process the aa Index data a bit further. This is what the aa Index looks like from the beginning of recording in 1868:

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Figure 1: aa Index 1868 – 2018

The end of the Modern Warm Period remains December 2008 which is the month of the Solar Cycle 24/25 minimum. The beginning of the Modern Warm Period is now September 1933 which is the month of the Solar Cycle 16/17 minimum. That is the true beginning of the Modern Warm Period because of what is evident from this graph:

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Figure 2: Cumulative aa Index against long term average 1868 – 2017

It is evident from Figure 2 that 1933 marked the reversal in the trend of the cumulative aa Index plotted against its average. The Sun was running a lot hotter from 1933.

That interpretation is supported by the longer term Open Solar Flux data set by Lockwood:

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Figure 3: Open Solar Flux 1676 – 2011

The Maunder Minimum, the Dalton Minimum and the 1970s cooling period are evident from Figure 3. The lows in Lockwood’s flux during the Modern Warm Period remained higher than the average of the period from the Dalton Minimum that preceded it. Once again, plotting up the cumulative data of this series makes the break in trend evident:

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Figure 4: Cumulative Open Solar Flux 1676 – 2011

Confirmation that 1933 is the beginning of the Modern Warm Period comes from the climatic response, starting with glaciers:

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Figure 5: Curves showing the variations of glacier termini

Figure 5 is from page 15 of Ahlmann’s 1953 report to the American Geographical Society entitled Glacier Variations and Climatic Fluctuations. Glaciers in Sweden, Norway and Iceland started retreating a lot faster from about 1933. So did glaciers on the opposite side of the planet:

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Figure 6: Glacier length South Island of New Zealand

Figure 6 shows that three of the four glaciers on New Zealand’s South Island started retreating about 1933. If a lot of glaciers started retreating around 1933 that should be evident in the rate of sea level change. And so it is:

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Figure 7: Global Sea Level 1700 – 2002

Figure 7 shows that sea level was effectively flat from around 1900 to 1933 then took off from 1933. Now that the Modern Warm Period is over, glaciers should have stopped retreating and should now be bulking up. There is evidence for that starting with the Greenland Ice Sheet:

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Figure 8: Accumulated Surface Mass Balance for the Greenland Ice Sheet

The blue line is the 2016 – 2017 season which was at the upper bound of accumulation from 1981 to 2010. The Greenland Ice Sheet is back to putting on weight and that is coming out of the oceans. Figure 9 shows where the weight went on, all 544 billion tonnes of it:

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Figure 9: Map of accumulated anomaly since September 1, 2016.

The readouts are in the blue – Greenland, on balance, is now a story of ice accumulation. Other climatic evidence for the end of the Modern Warm Period is the plethora of cold temperature records set in the United State and Europe in early 2018, including sharks freezing to death.


David Archibald’s latest book is American Gripen: The Solution to the F-35 Nightmare

Superforest,Climate Change

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