The Neoglacial Period (also called neoglaciation or the Neoglacial) refers to a long-term cooling trend in Earth’s climate during the Holocene epoch. It describes the “renewed glaciation” or gradual return to cooler, wetter conditions following the warmest phase of the Holocene.

From Watts Up With That?

By Andy May

Most agree that the Milankovitch cycles of eccentricity, obliquity, and precession drive long-term global and hemispheric climate changes, see figure 4 in this post for a brief description of them. The modern climate debate is about short-term climate change. The “consensus” says that human emissions have caused “the most rapid change” or “temperatures are the warmest in X years” (Lecavalier et al., 2017) and (IPCC, 2021, p. 8) with X varying from one thousand years to over 100,000 years. Obviously, we only have global instrumental data for the past 170 years or so, so any global or hemispheric data before then is either local or proxy temperature data.

The mainstream view is to ignore inconvenient data that shows CO₂ and methane air concentrations do not correlate with temperature during the Holocene Epoch, or the past 12,000 years as shown in figure 4 here. Correlation is not causation, but the lack of correlation normally precludes causation. If changes in heat storage in the climate system are ignored, as is often done, then only outside forcing can cause climate change. Since recent climate changes (since 1950) have been too rapid to be caused by the Milankovitch orbital cycles, the only outside forces left are the Sun and greenhouse gases (GHGs). Since the oceans and atmosphere change the amount of heat they store, as opposed to emit to space, climate changes as climatic heat storage changes (Irvine, 2014). We can observe this in the 60-70-year climate or ocean oscillations, like the Atlantic Multidecadal Oscillation (AMO, see here and here).

The Sun and GHGs work differently. Solar radiation penetrates the ocean surface and warms the water at depth, storing heat in the ocean. GHGs absorb radiation emitted by Earth and mostly use it to warm gas molecules near them. Infrared emissions from GHGs cannot penetrate the ocean surface (Wong & Minnett, 2018). By warming the surface, they stimulate more evaporation sending a lot of the energy back to the atmosphere, although a small portion is circulated deeper by convection. Heat stored in the oceans stays in the climate system longer than storage in the atmosphere and change climate on decadal and century time scales (Irvine, 2014). This is seen in ocean oscillations. Changes in GHGs only affect the atmosphere and can only affect climate in the very short term. Theoretically temperature changes from the top of one 11-year solar cycle to the bottom is only about 0.02°C due to the direct change in radiation reaching the Earth. However, the actual observed change is five times higher or 0.1°C and the increase in the upper atmosphere is 0.3°C. This is discussed in more detail here and (Hoyt & Schatten, 1997), (Lean, 2017), and (Haigh, 2011).

The Neoglacial

The Neoglacial period is not formally defined and its onset (that is the beginning of glacial growth in the late Holocene) varies with location. It is a global glacial advance (with the possible exception of Antarctica, see figure 1 and here) from the end of the Holocene Climatic Optimum. The important point is that the Neoglacial onset is not synchronous around the world (McKay et al., 2018). Here we begin the Northern Hemisphere Neoglacial at roughly 3,800BC, just after the Mid-Holocene Transition (MHT) from the Holocene Climatic Optimum (HCO). The literature has the Neoglacial starting anywhere from 2,000BC to 3,000BC, so my pick is a little early, but not excessively so.

When the chosen Neoglacial period begins the average latitude of the Intertropical Convergence Zone (ITCZ) shifts dramatically southward which initiates the desertification of the Sahara. The average ITCZ latitude responds strongly to interhemispheric temperature gradients, and it shifts southward when the Northern Hemisphere cools as it did around 4,700BC (see figure 1 here). For more on this dramatic change in climate see here, as well as (Wanner & Brönnimann, 2012) and (DeMenocal et al., 2000).

Regardless of complaints from the “consensus,” the Neoglacial period may not have ended yet, it is still much colder today in the Northern Hemisphere than most of the Holocene as shown in the featured image of my previous post. Given the length, strength, and depth of the Neoglacial; it is hard to call its end after only seventy years of on-and-off warming. It will take another 100 to 200 years of warming to be sure we have really and truly ended the Neoglacial.

The first part of the Holocene, until around 4,000BC was quite warm, at least according to my favorite Northern Hemisphere proxies, the Vinther air temperature proxy (shown black in figure 1) and the Rosenthal Makassar Strait 500-meter proxy of Northern Pacific sea surface temperatures (SST, shown in blue). The lower part of figure 1 shows the number of global glacial advances from (Solomina et al., 2015) as a blue line and the central time and duration of solar grand minima (SGM, black dots) and solar grand maxima (SGMx, orange dots) from (Usoskin, 2017). Before the earliest SGMx shown (at 3860BC) there is not another until 6,120BC. Thus, the Holocene Climatic Optimum (HCO) is likely due to orbital cycles as long assumed and not due to solar maxima.

The Mid-Holocene Transition and the Neoglacial begin with a cluster of six SGMs and four SGMxs, so the sun was highly variable then. It might have played a role, along with the obvious orbital insolation forcings in the initiation of the Northern Hemispheric cooling, but the pattern is ambiguous as to its net effect on climate. Likewise, the 4.2 ka (thousand years ago) climate event does not correlate with the SGM before it or the SGMx after it, it must have other causes.

The overall picture of figure 1 shows a general decline in temperature as the number of glacial advances increases. The period from 2000BC to 500BC has little solar variability, but a strong 120-year SGM at 750BC and a dramatic increase in glacial advances. Right in the middle of these advances is the Bronze Age collapse in the eastern Mediterranean. The strongest SGM is centered on 1470AD, and it is accompanied with the maximum number of glacial advances, these are both just before the deepest part of the Little Ice Age from 1500 to 1750AD.

The spectacular 1177BC Bronze Age collapse (Cline, 2014) follows the 70-year SGM at 1385BC by almost 200 years, so it is unclear how much influence it might have had on that climate catastrophe. The Bronze Age collapse of eastern Mediterranean civilization led to the Greek Dark Age, and it did not end until the Roman Warm Period (RWP) began around 300BC, (some place the beginning later around 250BC). The RWP does not end until 400-500AD.

The Roman Warm Period saw the rise of the Roman Empire, the unification of China by Ch’in in 221BC, and it began just after the death of Alexander the Great. It also saw the rise of India’s greatest ancient emperor, Ashoka the Great who unified India for the first time around 266BC. Ashoka converted to Buddhism and promoted the spread of the religion. This period also includes the life of Jesus of Nazareth and the rise of Christianity.

The Roman Warm Period is notable because it coincides with three consecutive solar grand maxima and contains no solar grand minima. It is also within one of the longest periods in the Holocene without an SGM. The other such long gap, from 1385 to 2450BC, essentially marks the peak of the Bronze Age.

The European Dark Age can be identified by lower temperatures in the Vinther record between 500 and 800AD and an SGM at 690AD. The Medieval Warm Period exists between the SGM at 690AD and one at 1030AD and is more of a transitional period into the Little Ice Age than a true warm period like the RWP. The Little Ice Age has no SGMxs and four SGMs, with an exceptionally long one at 1470AD. The Vinther record reaches its coldest point at 1700AD and the North Pacific Rosenthal record is coldest at about 1810AD, so 1750AD is a reasonable Northern Hemisphere date for the modern warm period to begin. The last SGM is centered on 1680AD and it lasts from 1640 to 1720AD.

Once the last SGM is done, the next solar event is the Modern Solar Maximum centered on 1970 and from 1930 to 2010. It is the longest solar maximum since 3,170BC and the first solar maximum since 505AD.

Discussion

I am definitely not saying that solar variability is the only thing causing climate change and I do not think it is stronger than the Milankovitch orbital cycles (see figure 4 here). But, when we came out of the Little Ice Age, the coldest period in the entire Holocene Epoch, and a period with no solar grand maxima and four solar grand minima, including the strongest SGM (as measured by duration) in the Holocene, one has to consider that solar variability contributed to the Little Ice Age.

Then we must consider the Modern Warming Period. It coincides with the first solar grand maximum in 1,465 years and the strongest in 5,140 years. It seems quite reasonable to conclude that the modern solar grand maximum contributed to the observed recent climate changes. Climate change is a complex combination of the Milankovitch cycles, solar cycles, and (maybe) anthropogenic factors. It does not have just one cause at any time scale.

Download the bibliography here.