Guest essay by David Archibald’

Many good things come to an end and that includes the Modern Warm Period. Mild winters and early springs are now spoken of in the past tense. The peak of the Modern Warm Period was 2006 as shown by the oceanic lead indicator, the Gulf Stream, also called the North Atlantic Current. From Professor Humlum’s site, Figure 1 following shows the Argo data from a transect south of Iceland from 2004:

Figure 1: North Atlantic Current temperature 0 – 800 metres depth 2004 – 2018

The North Atlantic Current has cooled by 1.0°C to date since the peak in 2006. Figure 2 following incorporates data from the same region back to 1955:

Figure 2: North Atlantic Heat Content Anomaly 1955 – 2017

This figure shows that the North Atlantic Current has to date lost half the heat content it gained from the early 1980s.

Figure 3: Central England Temperature 1659 – 2017

This is the longest thermometer record on the planet with nearly 360 years of data. There is not much evidence for the Modern Warm Period on this graph with temperatures of the late 20th century only slightly above those of early 18th century, 300 years ago. The running average from Figure 2 is plotted in red (data courtesy of Professor Humlum). It is evident that the heat content from the North Atlantic Current caused the milder winters of recent memory. The R2 of the relationship is 0.3935.

What caused the Modern Warm Period to begin with? Cliver noted in the 1998 that “During the past ~120 years, Earth’s surface temperature is correlated with both decadal averages and solar cycle minimum values of the geomagnetic aa index. The correlation with aa minimum values suggests the existence of a long-term (low-frequency) component of solar irradiance that underlies the 11-year cyclic component.”

The aa Index starts in 1868 as shown by Figure 4:

Figure 4: aa Index 1868 – 2018

The end of the Little Ice Age is evident in 1900 as that was the year that aa Index started rising until the 1950s. From the 1950s there was a slight uptrend culminating in the peak of 2003. The aa Index then plunged back to levels characteristic of the Little Ice Age. The end of the Modern Warm Period was in 2009 at the Solar Cycle 24/25 minimum.

There is a lag between the aa Index and temperature of the atmosphere due to the damping effect of the thermal inertia of the oceans. Plotting the aa Index against the NOAA Northern Hemisphere temperature anomaly, peak correlation occurs with a lag of six years as shown by Figure 5:

Figure 5: aa Index plotted against Northern Hemisphere temperature anomaly lagged six years 1880 – 1995

The aa Index during the first half of Solar Cycle 24 averaged 13.1. The three years up to 2018 were higher at an average of 20.6. According to the relationship shown by Figure 5, the pulse of heat from the higher aa Index of the last three years will cause higher temperatures out to 2023.

So far in 2018 the aa Index has averaged 13.0 and should continue to weaken into the Solar Cycle 24/24 minimum expected in 2020. From the beginning of the aa Index record in 1868 to the end of the Little Ice Age in 1900, the aa Index averaged 15.4. It averaged 20.9 for the Modern Warm Period. For the Modern Warm Period not to be over, the aa Index would have to average over 20.0. That eventuality has a low probability given the way the Sun is behaving.

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

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