This shall be part two of a four part series, discussing and detailing natural climate cycles and how these cycles combine and/or interact to contribute to and/or drive the weather that we observe, over time. Once the entire series has been completed, it will be compiled into a book with some additional details, and graphics added.
It should be noted, that much of the data used when discussing climate over this length of time or longer, is scientifically reconstructed. Actual observations can only take us back a few hundred years. Beyond that, methods such as: ice cores; ocean, lake, and river bottom sediment cores; tree rings; geologic evidence in rocks; and historical accounts can be used to get an indication of what climate factors were like in Earth’s past. Some of this data is open to interpretation, but I shall attempt to use the most logical, common sense approach to interpreting such data.
Part two of this series, shall discuss natural climate oscillations that cycle with a periodicity of 100 to 10,000 years.
Solar Cycles / Oscillations
Short term solar oscillations were discussed in part 1 of this series. However, new research indicates that longer term solar oscillations also play a significant role in our natural climate variability. Of utmost importance, is an apparent cycle of “Grand Solar Minimum” or “Solar Hibernation“, such as the “Maunder Minimum“, which appears to cycle around every 412 years on average. The “Maunder Minimum” began around the year 1615, or 400 years ago. Therefor, we can assume that if this new research is correct, then the cycle is due to repeat.
The above chart shows sunspot cycle length and Total Solar Irradiance, over the last ~420 years. TSI is a measure of energy which reaches the top of earth’s atmosphere, from the Sun. It is measured in Watts per square meter. The “Maunder Minimum” is clearly identified on this chart, near the left edge, as a sharp and long lasting dip in solar output. Also of note, is the recent rise in TSI over the latter half of the 20th Century, which is largely responsible for the period known as “Global Warming“. During this time, sunspot cycles were both shorter (peaks were closer together) and more intense.
Three crucial solar oscillations can be identified by this chart. At the top we see a ~103 year oscillation in the length of sunspot cycles from as short as 8 years, to as long as 14 years. In the bottom chart we see the ~206 year oscillation in sunspot cycle intensity. The intensity of solar irradiance should have peaked around the year 1800 AD, but in stead a period known as the “Dalton Minimum” occurred at this time, due to the sunspot cycles being longer / farther apart. The entire length of this chart, signifies the ~412 year cycle of “Solar Hibernation“. This oscillation for the most part alternates every-other trough in the ~206 year oscillation. These oscillations then also provide the basis for a cycle of warm periods, or “climate optimums” which appear to peak every 824, 1,030, or 1,236 years. These optimums are apparently dependent upon just how well these oscillations line up, and the effect of longer period oscillations, as well.
Millennial Cycle of Warm Periods
What exactly causes these cyclic warm periods, is still being intensely investigated. The most logical hypothesis, assuming that each cyclic warm period happen the same way the “Modern Warm Period” did, is that it is a cyclic peak in solar activity. A period where sunspot cycles become shorter (closer together) and more intense, as is shown above to have occurred in the late 20th Century. Some influence may also be exerted on the Sun by neighboring stars as they rotate together around the galaxy, but confirming that hypothesis will be difficult.
Looking back farther, actual temperature measurements only go back so far, especially in the Americas, where weather stations have only been in place since the mid to late 1800s. So, in order to see further into the past, scientists must reconstruct the temperatures based on the numerous methods available to us. While these temperature reconstructions are fantastic tools for seeing deep into Earth’s past climate, they are somewhat open to interpretation, and different methods sometimes lead to different and occasionally even conflicting results. Here, I shall use the most common, most trusted methods, and show what the most consistent results have been, to get a picture of natural climate cycles over the past 10,000 years, since the end of the last period of glaciation.
This chart shows the results of an ice core study from the ice sheet in northern Greenland, indicating relative temperature at that location. Here, you can see a relatively regular cycle of “warm periods“, which occur roughly every 824 to 1,236 years. At the far left of the chart is the end of the “Younger Dryas“, where this interglacial period began in the Northern Hemisphere, which is a period also known as the “Holocene“. Long range ice cores indicate that many interglacial periods have two temperature peaks, or “climate optimums“, and on this chart we see those peaks being at about 7,300 years ago, and 3,600 years ago. More information about the glacial / interglacial cycle will be available in Part 3 of this series.
The peak 7,300 to 6,000 years ago is known as the “Mid-Holocene Climate Optimum“, which is believed by many scientists to be the peak of this interglacial (between glacial advances) period. Since then, the warmest temperatures within the past 5,000 years were during what is known as the “Minoan Warm Period“, which occurred around the year 1200 BC. Next came the “Roman Warm Period“, which peaked around time of Christ, or around 20 BC. Then came the “Medieval Warm Period“, which peaked at around the year 1050 AD. Finally, the “Modern Warm Period” peaked around the year 1990 AD. At the far right of the chart, the ice core data is augmented with actual temperature observations at that location, which clearly shows the peak of the “Modern Warm Period”. The “Modern Warm Period” exactly matches the timing and scope of the Millennial warm period cycle, and also exactly matches the timing and scope of the peak in solar activity shown in Chart 2-A above.
Now, lets look back farther still, and see if these cyclic warm periods only occur during the Holocene, or if they’re a constant in climate.
This chart is formulated in much the same way as the Chart 2-B above, but goes back twice as far. Bracketed in the top right, is the size of the 10,000 year chart in relation to the 20,000 year chart. Here, we can see that these cyclic warm periods do indeed continue back well beyond the Holocene, into the coldest, deepest parts of the last period of glaciation. While larger forces are at play to drive the overall long term climate, the shorter term Millennial oscillation drives temperature fluctuations of about +/- 1.5°C from the mean temperature being driven by longer term cycles, that will be discussed in later parts of this series. Each successively shorter cycle happens within the longer term cycles.
We can also see that temperatures began to warm in the Northern Hemisphere about 15,000 years ago, but then sharply fell again. This sharp fall in temperatures around 13,000 years ago is what is known as the “Younger Dryas“. The cause for this sharp dip in temperatures is not fully understood, and did not have the same affect in the Southern Hemisphere, as Vostok ice cores do not indicate much of a dip in temperatures during this period. Some theories have been put forward to attempt to explain this. The most logical of which being that the rapid melt of the Arctic ice sheets around 14,600 years ago, caused the North Atlantic Ocean to become flooded with fresh water, which then caused a shift within the Thermohaline Current, which had the result of cooling the Arctic and growing the ice sheet once again. Some scientists have stated that this melting alone could not have caused the Younger Dryas, but perhaps some kind of asteroid impact in the Arctic may have enhanced or augmented the melting that was already underway. This would seem to be supported by the fact that the melting after the Younger Dryas happened just as quickly, yet did not have the same affect on the North Atlantic. In any case, the Younger Dryas is indicated in all ice cores throughout the Northern Hemisphere, so we know it happened. The main question is how can we explain it?
Cycle of Sharp Cold Shifts
Between the Millennial Warm Periods are often lengthy periods of substantially cooler climate. These cooler periods are likely due to reduced solar activity, as was the case during the “Little Ice Age“, which stretched from the 1200s AD to the mid/late 1800s AD, a period of around 600 years. As indicated in Chart 2-A, the Maunder Minimum largely contributed to the coolest years of the Little Ice Age.
The cooler periods tend to last significantly longer than the dips in solar activity. The most logical explanation for this, is that as Infrared and TSI as a whole decrease, and the sun’s heliosphere becomes weaker, galactic cosmic rays are more able to penetrate the sun’s heliosphere and reach the planets, including Earth. Since cosmic rays are confirmed to cause an increase inatmospheric aerosols, which then lead to an increase in low clouds around the globe, this would have the affect of reflecting sunlight, and enhancing the affect of reduced solar output, which further reduces global temperatures, over time.
The cyclic periods of solar hibernation extend back beyond the Maunder Minimum. New research suggests it is a quasi-regular oscillation of about 412 years in length. So, lets take a closer look at more recent solar activity, and see if there are clear signs of this beginning.
The above chart shows TSI at the top, and the sunspot count over the same time frame, at the bottom. Direct solar records began too late into the process of entering into the Maunder Minimum, to be able to do a direct comparison between then, and now. Therefor we must assume that if the Sun is indeed entering into a new “Solar Hibernation“, we should see solar cycles getting both weaker, and farther apart, as an indication of it. Cycle 21 and 22 were 9 1/2 years apart, which was also typical of the two cycles preceding cycle 21. Cycle 22 and 23 were about 11 1/2 years apart. Cycles 23 and 24 were 14 years apart. Clearly, the cycles are getting farther apart, and that alone is enough to cause a decline in global temperatures. The chart also indicates that each successive peak is lower than the one before it, so each is also getting weaker. This will enhance the effect of cooling our global temperatures over time.
I outlined in this article, other factors which also indicate the existence of this ~412 year solar cycle, and what the effects may be if this cycle is repeating.
Past Affects of Climate Shifts
Human history is replete with mass migrations that are a direct result of both the Millennial warm period oscillation, and the effects of “Solar Hibernation” periods, as well. One example is the “Great Wall of China“, which was largely built during the “Roman Warm Period”. It has been surmised by some who study both history and climate, that the “Great Wall” was built because the Chinese knew about this temperature oscillation, and that the climate would soon cool. The Chinese economy could not withstand being bombarded with a mass migration of the Mongols and people of the Eurasian Steppes to the north, so the “Great Wall” was built to keep them out.
The Mayan Civilization is another example. Given their knowledge of astronomy and geography, and having such an accurate calendar system, it is quite possible that they too, knew the climate was about to change to their determent. Having been founded just as the “Roman Warm Period” was ending, and likely knowing the Medieval Warm Period was soon to come, they abandon their magnificent cities in favor of cooler, wetter lands to the north.
The Harappan Civilization is yet another example. The Harappans thrived in what is now northwest India and southeast Pakistan, along the Indus River, around the same time that the Egyptians were building the pyramids at Giza. Then as the Minoan Warm Period began, the monsoon rains changed, and their entire rich civilization was lost to encroaching desert. Like the Mayans, they too were forced to migrate northward, into the Eurasian Steppes where the climate was more suitable.
There are many other examples which serve to help confirm the results of the ice core data shown above, such as the early inhabitants of Ireland migration from what is now Libya, as the Sahara changed from savannah to desert 5,500 years ago at the end of the “Mid-Holocene Climate Optimum“, but I’ll save those for the book form of this series.
Previous parts of this series….
Natural Climate Cycles Part 1 – Short Term Oscillations
Natural Climate Cycles Part 3 – Glacial Cycles and the Milankovich Cycle Theory
Natural Climate Cycles Part 4 – Deep Time Cycles
** Data source: Lean 2000, SIDC sunspots, PMOD and ACRIM Composite TSI
*** Data source: GISP2 data set.
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